73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: bswilson80 on May 21, 2013, 06:31:30 pm
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Please excuse my novice understandings. I just bought a 1974 C10 with a 350/Th350 in order to learn about maintenance. . Everything seems stock except for the Edelbrock 1406 Carb, and maybe the HEI dizzy..
I have really rough idle and very little response at part-throttle. Once the truck is moving it rides nicely.
Here's where I stand:
-I have to prime the fuel pump by turning the key to ON twice before ignition otherwise the truck will just die.
-Timing is set to 10 degrees.
-Mix screws are 2 full turns from tight.
-Compression is 120-130 on all cylinders.
-No milkshake in oil
-Spark plugs were all dry with a little carbon on each
-Vacuum test shows needle shaking quickly between 14-16
-blew smoke into vacuum system and it only seemed to come out of a small hole right below the carb choke piston (covered hole and no more smoke anywhere else)
-Small backfires (like popcorn) when slowing down, usually at about 10 mph (seems to coincide with auto-downshift)
Anything else I can tell you, let me know. Again apologies for amateur hour, but I'm trying to learn.
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Not sure what you mean by priming the fuel pump just by turning the key ON twice. It should be a mechanical fuel pump. Is it electric? Do you have any holes in the exhaust near the manifolds?
Are you sure about the spark plugs wires being in the correct order? I guess if it rides nicely it must smooth out once its off the idle circuit.
Could be sticking valves if the needle flicks 2-3 inches of vacuum.
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a miss will smooth out once the idle picks up. make sure your firing on all cylinders
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Check all the spark plug wires. One way is to clip the timing light to each wire in turn. The bad wire won't kick the light.
12* BTDC is a better timing setting. You should be able to set it to at least 16* BTDC. Earlier is better unless you get knock.
Which metering rods are in the carb? If you pull one of the rods, the size is stamped on the rod body, like "7052".
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Also, that popcorn sound could be pinging.
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Please excuse my novice understandings. I just bought a 1974 C10 with a 350/Th350 in order to learn about maintenance. . Everything seems stock except for the Edelbrock 1406 Carb, and maybe the HEI dizzy..
I have really rough idle and very little response at part-throttle. Once the truck is moving it rides nicely.
Here's where I stand:
-I have to prime the fuel pump by turning the key to ON twice before ignition otherwise the truck will just die.
-Timing is set to 10 degrees.
-Mix screws are 2 full turns from tight.
-Compression is 120-130 on all cylinders.
-No milkshake in oil
-Spark plugs were all dry with a little carbon on each
-Vacuum test shows needle shaking quickly between 14-16
-blew smoke into vacuum system and it only seemed to come out of a small hole right below the carb choke piston (covered hole and no more smoke anywhere else)
-Small backfires (like popcorn) when slowing down, usually at about 10 mph (seems to coincide with auto-downshift)
Anything else I can tell you, let me know. Again apologies for amateur hour, but I'm trying to learn.
You should have a mechanical fuel pump. You should have to pump the gas pedal a couple of times to get an immediate fire when you crank.
Make sure you are getting fire on all cylinders before you time the engine.
You really should blow smoke all around the engine. The engine sucks in, not blows out. So you will see the smoke get sucked in if it has a vacuum leak.
Set your timing to about 12-14 degrees BTDC (engine fully warmed).
Counterclockwise advances the dizzy, clockwise retards the dizzy.
Obviously you must plug the vacuum advance before timing the engine. Hook it back up once you have timed it.
Small backfire indicates that your carb is out of tune.
Take your vacuum gauge and hook it up to full manifold vacuum. If you have a tach, set your RPMs at idle to 750 in park with the engine at operating temp. Turn the mixture screws in until they seat lightly. Turn them back out 2 turns (your original position). fire the truck up and take note of reading. Turn each mixture screw a half turn in either direction, one at a time the same exact amount.
Take note if the vacuum reading goes up or down. If you turn the screw to the right, you are leaning it out: turning it left enriches the mixture.
While you tune to the highest vacuum reading, go back and check your idle RPM. If it has gotten higher, turn the idle screw back down to 750. The idea is to keep the RPM the same after each turning of the mixture screws so that you are actually tuning the mixture. As the idle goes up it naturally creates more vacuum so you much check this after each turn of the mixture screws.
When you finally get the highest possible vacuum reading, it should hold steady. Most people will say you should have a reading of 18-20 on the vacuum gauge. There are many factors that could change that number such as elevation, what kind of cam you have etc, etc. A hotter cam will lower your number and give you a slight flicking of the needle. Higher elevation means a lower number as well.
After all this, put some new spark plugs in and drive it a while. Try to not idle before shutting the engine off when you go to check them. They should be a slight brown color. If you are still getting dry soot, it's still running rich.
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Also, that popcorn sound could be pinging.
With timing at 10* BTDC, that's unlikely UNLESS it's a bad load of gas or it's old gas.
You could pump the gas out of the tank and replace with fresh, and use the old gas in your lawn mower.
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Also, that popcorn sound could be pinging.
With timing at 10* BTDC, that's unlikely UNLESS it's a bad load of gas or it's old gas.
Depends on the curve in the distributor??, the total determines if it detonates. But old/bad gas will definitely cause havoc..
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Fairlane: It's mechanical, I guess I was assuming that turning the key helped as when I don't the truck dies, but when I do it starts up nicely. I've checked and double checked the plug wires, I'm pretty sure they're correct. I can't find any holes in the exhaust.
Rich: I will definitely try that with the timing light, as well as advancing the timing a little more. How old is old gas? I haven't driven the truck that much as I don't want anything to get worse, so the gas is probably about 4 moths old.
79GMC: The sound is definitely coming from near the mufflers, could that still be pinging?
Boone: I'm at sea-level, I will attempt the rest this weekend, thank you.
Thank you all for your replies. I've been lurking this site for awhile now, I'm glad everyone is so willing to share their knowledge.
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Gas today is crap, after 30 days it starts to deteriorate if you let it sit you need to put Sta-Bil in it. Also Edelbrock carbs are notorious for gas going away when they sit for a few days, you either crank the heck out of it or you squirt it to get it going, once the carb fills with gas it's ok, that could be your problem if it's doing this after sitting for a while, I've got one on my truck but I have an electric pump(since my motor don't have provision for a mechanical)so I turn the key on for a few seconds before I start it and it fires right up and runs.
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I would challenge that gas is bad after 30 days...
to the OP check this out, it will teach you more about your engine vacuum readings than most can tell you about.
http://www.secondchancegarage.com/public/186.cfm (http://www.secondchancegarage.com/public/186.cfm)
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I would challenge that gas is bad after 30 days...
Didn't say it was bad after 30 days, it starts to deteriorate after 30 days. ;)
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May not have anything todo with your problem but.........do you know if your truck has an oil pressure switch on the drivers side block about midway down near the back?
This switch provides a 12v current to the choke (if its electric) and basically allows the choke to operate when it senses good oil pressure. Dont know if turning the key ON has anything to do with this precess........just a wild ass guess.
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On the '74, the choke runs off the ignition.
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mess with timing to see if your idle smooths out might be a little retarded. just turn it clock wise a inch or two. then if it smooths out you know that's where to start if it doesn't and your sure your getting fire to all cylinders i.e.( touch the exhaust manifold quickly on each cylinder WHEN THE ENGINE IS COLD and see which one is colder than the rest that will be your dead cylinder).
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Clockwise and inch or two will really retard the timing......counterclockwise will advance. Also an inch or two is a lot of adjustment, I would use smaller increments. Using a timing light would also be advised.
Does it run hot? That can also be an indication of retarded timing.
As someone mentioned earlier, using the timing light on each spark plug wire will indicate if its getting juice.
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fair your right about counter-clockwise im not going to give any excuses why i thought clock wise lol. but if his timing is so off that its popping or running rough than a inch or two should put it right back where it should be my main thing was dont be afraid to turn the distributor
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An inch or two and it won't run, LOL, put a timing light on it... ;D
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it will work if the timing is off. if your timming is dead on an 1" will mess it up and 2" will prob kill it.
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If it's off that much now it won't run to begin with, LOL.. ;D
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wow if your 1/2" off adding 1" will only put you 1/2 over it will run. just like if your 1" off 1" will put you = 2" will put you 1" over
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Timing is in degrees not inches, LOL.... :D :D
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theres more than one way to skin a cat, you just have to think outside the box. i know i have a timing light (maybe he doesnt) and sometimes its not with me when i work or try to diagnose a problem sometimes you have to do things by ear to get it close. and when you dont have a timing light on hand its kind of hard to adjust by degrees. just like when you have a miss you can A put a timing light on each plug and see where its at B use a laser thermometer to see what exhaust manifold pipe is cooler or when the engine is cold and you dont have none of the above you can touch each exhaust manifold tube and see which one is cooler.
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When the engine is cold..............wouldn't all the exhaust pipes be the same temperature? Cold?
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Well, I threw a new HEI (old had corroded terminals inside cap), plugs and wires on and it started and running better, not great, but nothing a little time spent fine tuning won't fix hopefully.
All of the wires sent signal to the timing light, and I felt the exhaust manifold right after starting the truck from cold, and they all seemed to get hot at the same rate, after about 30 seconds they were too hot to touch, does that seem right?
My only real beef is that I'm still getting a lot of popping at idle, in gear. Popcorning stops in Neutral and park, but will continue as long as my foot is off the gas. I tried leaning out the carb as well as richening, but neither seem to have any effect. I guess my next step if replacing the exhaust manifold and flange gaskets?
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If the cylinders are firing you have heat 30seconds is about right. Where is it popping through the carb or the exhaust??, does it pop under load?, do you have an exhaust leak??..
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Where is it popping through the carb or the exhaust??, does it pop under load?, do you have an exhaust leak??..
Popping comes from exhaust, it is barely audible from in front of the truck. The truck runs great under load, no popping, only at a near/dead stop while in gear. I cannot see any holes in the exhaust, how else could i find a leak?
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You might not see it but you can hear it if you listen close, does it pop at any temp??.
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You might not see it but you can hear it if you listen close, does it pop at any temp??.
Not for the first few minutes of a cold engine, but it pops from then on.
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You might not see it but you can hear it if you listen close, does it pop at any temp??.
Not for the first few minutes of a cold engine, but it pops from then on.
If it is an exhaust leak check all your joints and the manifold to block you should be able to hear it.
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When the engine is cold..............wouldn't all the exhaust pipes be the same temperature? Cold?
lol nope they warm up pretty fast i was saying do it when its cold cause if you wait they will be too hot to touch
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I felt the exhaust manifold right after starting the truck from cold, and they all seemed to get hot at the same rate, after about 30 seconds they were too hot to touch, does that seem right?
see and he understood me but
since its running good when cold hows the choke operating?
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Choke closes correctly, and opens as truck warms up.
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what kind of exhaust are you running?
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Just the OEM manifold and what appears to be run of the mill true dual exhaust that exit behind the rear tires, no markings are obvious, is there anywhere to look to tell?
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just trying to get an ideal. my truck will pop every couple seconds out the exhaust but i have open heads. my buddys blazer will also not as much as mine but his does it he has true duals coming out in front if the rear tires with no mufflers or cats. and his 72 jeep with 5.0 also does it it also has true duals all these are mud trucks except for the jeep we just got that thing running after sitting for 20 years lol
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my truck pops on decel, not as much or often since I bumped timing to 12*. My '92 s10 blazer had a dynomax muffler and it popped no matter what I did, I actually started liking it. on that exhaust the pipe was 2.5" with a 3" tip, so on decel cold airwould rush into that 3" tip and hit the 2.5" pipe it'd pop. The colder the weather the more it popped and louder it popped.
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My cousin has an old Bel Air that pops on decel too, but his sounds nice, like an old engine that has more to give than he's asking out of it. Mine just does it at standstill and it sounds like I have a bunch of dwarves with hammers stuck in the bed of the truck trying mine their way through the metal. Not that cool.
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Full manifold vacuum is better anyways. The motor should run cooler as well.
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mine doesn't pop under 50 or so mph, just when decel from highway
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travisr: Check the location of that vacuum line from the distributor. On decel is when I would expect the location difference to matter.
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travisr: Check the location of that vacuum line from the distributor. On decel is when I would expect the location difference to matter.
It runs to a port on the bottom of the carb, my dad put it there and said it should be there to keep the vacuum advance from advancing at idle.
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OK, here is a pic of the Qjet showing the ported and non-ported vacuum connections. The distributor (for best performance) should connect to the non-ported vacuum. Ported vacuum was a trick to meet emissions specs at idle, but was not the best choice for best operation.
http://www.gtoforum.com/attachments/f50/9784d1293232179-who-has-best-book-help-me-convert-quadrajet-img_20101224_163810.jpg
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OK, here is a pic of the Qjet showing the ported and non-ported vacuum connections. The distributor (for best performance) should connect to the non-ported vacuum. Ported vacuum was a trick to meet emissions specs at idle, but was not the best choice for best operation.
http://www.gtoforum.com/attachments/f50/9784d1293232179-who-has-best-book-help-me-convert-quadrajet-img_20101224_163810.jpg
Is that the same for the side entrance fuel line style? if so I believe mine is on ported, I'll switch it over I guess, which means (if it goes back to the way it was before) my timing at idle will be something like 18* will that be ok?
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Changing the vacuum port shouldn't change the timing at idle because you are supposed to set the timing at idle with the vacuum line to the distributor disconnected! If you do not disconnect the vacuum line to the distributor vacuum advance when setting your ignition timing, the ignition timing will be WAY off. I would set the timing to 16* BTDC with the vacuum line disconnected, and then make sure you don't hear any detonation when, say, flooring the engine from a slow speed while going up a hill. If you do hear detonation, back it off to 14* BTDC and test again. If you need to back off again, 12* BTDC should be safe for just about any engine setup.
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Seems like my comment went away. But I switched the vac advance to full vacuum and the popping went away. Thanks for everyone's help.
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Changing the vacuum port shouldn't change the timing at idle because you are supposed to set the timing at idle with the vacuum line to the distributor disconnected! If you do not disconnect the vacuum line to the distributor vacuum advance when setting your ignition timing, the ignition timing will be WAY off. I would set the timing to 16* BTDC with the vacuum line disconnected, and then make sure you don't hear any detonation when, say, flooring the engine from a slow speed while going up a hill. If you do hear detonation, back it off to 14* BTDC and test again. If you need to back off again, 12* BTDC should be safe for just about any engine setup.
I did set the timing with the vacuum hose disconnected and plugged, when I put it back on non-ported the timing advances, on ported it doesn't advance at idle.
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Yup, because with the vacuum advance line above the throttle plate (ported vacuum), when the throttle plate is closed you get zero advance. With "non-ported" vacuum from below the throttle plate, the high vacuum at idle will pull in the vacuum advance. The lean mixture at idle burns slower, and so needs more advance, but the pollution control weenies moved the vacuum advance above the throttle plate to pass idle emissions tests. Which didn't really help real emissions, but there you go, they were playing the game by its own rules. That's fine, but it makes the engine run crappy, as you found out. Prior to the emission controls era, the vacuum advance was ALWAYS taken from below the throttle plate.
On ported vacuum, the idle will be rougher, throttle applications from idle will catch the distributor in the wrong position (fully retarded) causing a transition stumble until the advance catches up, and the advance will be all wrong when decelerating with your foot completely off the throttle, causing incomplete burning and ejection of unburnt mixture into the manifold, hence the popping.
Or something like that. That's my story and I'm sticking to it. Anyway, pulling vacuum advance from below the throttle plate, as all cars did before they started gaming the emissions regime, is the way to go.
Thanks for letting us know that solved the problem!
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Here's an interesting read about timing and vacuum advance...
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
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Uh, yeah. Like I said. :-) 34-36 degrees of total timing (set to 14-16 degrees at idle with a 20*-centrifugal-advance stock distributor) and vacuum advance connected to manifold vacuum. Yoda says: "Run like a top it will."
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Thank you 74 C-10 Shorty, that was very helpful and a great read, I'll switch to a non-ported for my advance now :)
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Thank you 74 C-10 Shorty, that was very helpful and a great read, I'll switch to a non-ported for my advance now :)
You're quite welcome, glad to be of help.
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seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?
When I lowered idle back down to 850 it ran a little rough, so I tried to smooth it out with the air/fuel mix, it's pretty smooth now, but the timing at idle is still up around 50 degrees and the popping is back, not nearly as frequently as before, but in back nonetheless.
Also, the truck revs really high before violently kicking into the next gear, this started after I switched to full manifold vac.
Without the vacuum advance my timing is 14 degrees at 850 rpm and all-in at 3000rpm with 38 degrees.
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Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24. That said, 14* BTDC should be OK. You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.
A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance. Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean. They gum up within about a year of normal operation.
While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft. Should be something like "6852".
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Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24. That said, 14* BTDC should be OK. You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.
A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance. Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean. They gum up within about a year of normal operation.
While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft. Should be something like "6852".
The distributor is just this one: http://www.amazon.com/gp/product/B005GRGOGG/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 . It's brand new, the only thing I changed was one of the springs for one lighter one as the all-in timing wasn't coming in until too late. Would this cause the extra degrees?
The metering rods are stamped 7547 and everything seems relatively clean in there.
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seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?
That's normal, ported vacuum you have no vacuum at idle, full manifold you have vacuum at idle and it's pulling the vacuum advance at idle, vacuum advance units come in different amount of degrees they pull and they are marked how many degrees on the arm, I like the adjustable ones myself. Go back and read the article I posted it explains how this system works.
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seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?
That's normal, ported vacuum you have no vacuum at idle, full manifold you have vacuum at idle and it's pulling the vacuum advance at idle, vacuum advance units come in different amount of degrees they pull and they are marked how many degrees on the arm, I like the adjustable ones myself. Go back and read the article I posted it explains how this system works.
Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24. That said, 14* BTDC should be OK. You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.
A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance. Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean. They gum up within about a year of normal operation.
While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft. Should be something like "6852".
The distributor is just this one: http://www.amazon.com/gp/product/B005GRGOGG/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 . It's brand new, the only thing I changed was one of the springs for one lighter one as the all-in timing wasn't coming in until too late. Would this cause the extra degrees?
The metering rods are stamped 7547 and everything seems relatively clean in there.
I can't find the spec on the centrifugal advance on that distributor, but it has an adjustable vacuum advance canister. Not sure about the vacuum advance arm.
7547 (.075" on cruise and .047" on power) sounds lean on cruise and rich on power to me. Do you have the stock intake and exhaust manifolds, or is it an aftermarket intake manifold, or headers, or both?
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All "GM" units have a number on the arm(only way to tell how many degrees it pulls), adjustable units don't have a number because, well they're adjustable,lol. The rod is hooked to the diaphragm, by turning the little allen screw puts more or less tension on the diaphragm which extends or reduces travel of the rod and controls the amount of degrees it pulls, I like the adjustables because you change them a degree at a time if you want where the non adjustable I believe are 5*.
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All "GM" units have a number on the arm(only way to tell how many degrees it pulls), adjustable units don't have a number because, well they're adjustable,lol. The rod is hooked to the diaphragm, by turning the little allen screw puts more or less tension on the diaphragm which extends or reduces travel of the rod and controls the amount of degrees it pulls, I like the adjustables because you change them a degree at a time if you want where the non adjustable I believe are 5*.
GM HEI distributors had a bunch of different vacuum advance curves depending on the year and vehicle. 77-78 light trucks (i.e. mine) had the least at 5 degrees @ 12-14 in Hg. The most I know of in a stock application is 15 degrees.
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Very interesting read, thanks guys!! I bet the majority of poor mpg's and performance issues stems from mix-matching parts over the years. I guess I have never really thought about it and was lucky the combo's I've had or have had the right "stuff" to make them run fine, pass emissions and get acceptable mpg's. Folks can't understand that I have averaged 15 mpg on my 85 C20 with a 454. All I can say is that it is untouched, original and that it just does...
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All "GM" units have a number on the arm(only way to tell how many degrees it pulls), adjustable units don't have a number because, well they're adjustable,lol. The rod is hooked to the diaphragm, by turning the little allen screw puts more or less tension on the diaphragm which extends or reduces travel of the rod and controls the amount of degrees it pulls, I like the adjustables because you change them a degree at a time if you want where the non adjustable I believe are 5*.
GM HEI distributors had a bunch of different vacuum advance curves depending on the year and vehicle. 77-78 light trucks (i.e. mine) had the least at 5 degrees @ 12-14 in Hg. The most I know of in a stock application is 15 degrees.
That's mostly because of the emissions crap over the years, but the curve is in the mechanical not the vacuum advance it only pulls x amount of degrees after the mechanical is all in, when properly setup that is..
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Very interesting read, thanks guys!! I bet the majority of poor mpg's and performance issues stems from mix-matching parts over the years. I guess I have never really thought about it and was lucky the combo's I've had or have had the right "stuff" to make them run fine, pass emissions and get acceptable mpg's. Folks can't understand that I have averaged 15 mpg on my 85 C20 with a 454. All I can say is that it is untouched, original and that it just does...
I manged to squeeze 15 in my 86 dually running empty, got 12 with a camper and towing the race boat, now my 73 only got 13 but had a heck of a lot more power than the 86, couldn't complain nothing's free, lol..
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seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?
That's normal, ported vacuum you have no vacuum at idle, full manifold you have vacuum at idle and it's pulling the vacuum advance at idle, vacuum advance units come in different amount of degrees they pull and they are marked how many degrees on the arm, I like the adjustable ones myself. Go back and read the article I posted it explains how this system works.
Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24. That said, 14* BTDC should be OK. You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.
A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance. Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean. They gum up within about a year of normal operation.
While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft. Should be something like "6852".
The distributor is just this one: http://www.amazon.com/gp/product/B005GRGOGG/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 . It's brand new, the only thing I changed was one of the springs for one lighter one as the all-in timing wasn't coming in until too late. Would this cause the extra degrees?
The metering rods are stamped 7547 and everything seems relatively clean in there.
I can't find the spec on the centrifugal advance on that distributor, but it has an adjustable vacuum advance canister. Not sure about the vacuum advance arm.
7547 (.075" on cruise and .047" on power) sounds lean on cruise and rich on power to me. Do you have the stock intake and exhaust manifolds, or is it an aftermarket intake manifold, or headers, or both?
I've tried adjusting the vacuum advance, it seems to be the same amount of advance regardless of if it is all the way closed, or 12 turns open, I plan to get a non adjustable one, would you suggest a particular one?
The intake seems to be stock, if it isn't it has been a long time since it had one (I've only had the truck 6 months, and the guy I bought it from inherited it, but not the knowledge of what was done to it.) as it is painted the same as the engine and you can tell that was painted awhile ago. The exhaust manifold seems stock as well.
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What you are adjusting is the vacuum when it comes in, not the amount of advance.
I don't know the exact needles to use for stock manifolds, but 7547 seems off to me. I would probably get a set of 7052's (a bit richer on cruise, a bit leaner on power) and see if performance and drivability improve. http://www.jegs.com/i/Edelbrock/350/1452/10002/-1?parentProductId=
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What you are adjusting is the vacuum when it comes in, not the amount of advance.
I don't know the exact needles to use for stock manifolds, but 7547 seems off to me. I would probably get a set of 7052's (a bit richer on cruise, a bit leaner on power) and see if performance and drivability improve. http://www.jegs.com/i/Edelbrock/350/1452/10002/-1?parentProductId=
I see. So is 55 degrees BTDC at idle too much in your opinion? Or is it fine and I should adjust the Tranny Vacuum modulator to shift earlier, as right now I probably get up to 3500-4000 rpm before it shifts to third, and its a really rough shift
Sounds like a plan, although I was looking towards rebuilding a Q-Jet to replace the Edelbrock, unless this is just a stupid idea... I'm eventually going for a very stripped down stock-ish look to the engine, and the Edelbrock is anything but that.
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That sounds like a LOT of vacuum advance to me. Like 40* worth! Yowzah. The stock disti on the 77-78 350 trucks was 5 degrees of vacuum advance, on top of maybe 16 degrees of base.
Have you adjusted the TH350 cable properly? That will set your shift points. You can also set it a bit off the standard setting to move the shift points around. Takes about 10 seconds. http://www.novas.net/forums/showthread.php?t=9025
The Qjet has little passages and air bleeds that need to be set up properly for your induction, heads, exhaust, etc. Great carburetor -- probably the best -- when it's set up properly. This is not that easy to do, and I wouldn't attempt it without an air/fuel ratio meter. There are some good books on how to do it (one by Roe, one by Ruggles), but it is involved.
The other option is to get a Qjet all adjusted to your setup, which is pretty common. Sean Murphy Induction offers Qjets set up to spec -- there is no standard product off the shelf with them, you tell them what you are running for manifold, heads, exhaust, and they build it up for you from their copious notes on proper tunes for different setups. Sean was previously with Jet, and has probably tuned the Qjet to more different setups than anyone else alive. For about $50 more than a new off-the-shelf one you still have to tune, you can have a properly tuned one from Sean. http://www.smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/32/productID/18
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That sounds like a LOT of vacuum advance to me. Like 40* worth! Yowzah. The stock disti on the 77-78 350 trucks was 5 degrees of vacuum advance, on top of maybe 16 degrees of base.
Have you adjusted the TH350 cable properly? That will set your shift points. You can also set it a bit off the standard setting to move the shift points around. Takes about 10 seconds. http://www.novas.net/forums/showthread.php?t=9025
The Qjet has little passages and air bleeds that need to be set up properly for your induction, heads, exhaust, etc. Great carburetor -- probably the best -- when it's set up properly. This is not that easy to do, and I wouldn't attempt it without an air/fuel ratio meter. There are some good books on how to do it (one by Roe, one by Ruggles), but it is involved.
The other option is to get a Qjet all adjusted to your setup, which is pretty common. Sean Murphy Induction offers Qjets set up to spec -- there is no standard product off the shelf with them, you tell them what you are running for manifold, heads, exhaust, and they build it up for you from their copious notes on proper tunes for different setups. Sean was previously with Jet, and has probably tuned the Qjet to more different setups than anyone else alive. For about $50 more than a new off-the-shelf one you still have to tune, you can have a properly tuned one from Sean. http://www.smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/32/productID/18
I'm guessing then, that the vacuum advance is broken or malfunctioning, guess I'm headed to NAPA. The reason I would lean towards the Vac Modulator would be that the truck shifted perfectly prior to me switching to full manifold vacuum advance, which leads me to believe that maybe it just isn't getting enough vac now.
I've got the book by Cliff, and trust me, I don't head into this rebuild lightly, I know it will take some serious time and learning, but that's why I got the truck, and if it doesn't work out, I know the Edelbrock will always be there to throw back on. When I have more fun-money, I will keep Sean in mind.
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I'm assuming you had the vacuum on full manifold when you adjusted the vacuum advance right, and at idle you still have the same degrees of advance no matter no matter how far you turned the adjuster?, if so the diaphragm is ruptured. Check the Modulator you may have an adjustable one like the vacuum advance, you adjust it the same way and it changes the shift points.
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I'm assuming you had the vacuum on full manifold when you adjusted the vacuum advance right, and at idle you still have the same degrees of advance no matter no matter how far you turned the adjuster?, if so the diaphragm is ruptured. Check the Modulator you may have an adjustable one like the vacuum advance, you adjust it the same way and it changes the shift points.
Yes, it was on full manifold when adjusted, and I adjusted mech timing with it plugged, and mech timing was 14 degrees at idle, when I add the vac adv it would jump to 55 degrees at idle and not change. I'm going to buy non-adjustable, at least until I'm better at fine tuning.
I don't really want to mess with the Modulator just yet (it is adjustable) as it shifted perfectly before the vac adv (read: I) messed things up, I'll replace that and then see where my shifts are at.
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I'm assuming you had the vacuum on full manifold when you adjusted the vacuum advance right, and at idle you still have the same degrees of advance no matter no matter how far you turned the adjuster?, if so the diaphragm is ruptured. Check the Modulator you may have an adjustable one like the vacuum advance, you adjust it the same way and it changes the shift points.
Yes, it was on full manifold when adjusted, and I adjusted mech timing with it plugged, and mech timing was 14 degrees at idle, when I add the vac adv it would jump to 55 degrees at idle and not change. I'm going to buy non-adjustable, at least until I'm better at fine tuning.
I don't really want to mess with the Modulator just yet (it is adjustable) as it shifted perfectly before the vac adv (read: I) messed things up, I'll replace that and then see where my shifts are at.
That's right, only one change at a time.
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And don't forget to adjust the transmission's throttle cable. It's quick and free.
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And don't forget to adjust the transmission's throttle cable. It's quick and free.
I believe all he has is a TH350??, that cable is just a kickdown and don't do anything for shifts.
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If it's set wrong, it can screw up your shifts, by calling for a kickdown at part throttle.
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Only if you're in high gear, if you're in second it's already there and won't do anything in first, I've tried it before just to see what would happen.. :D
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If it's set wrong, it can screw up your shifts, by calling for a kickdown at part throttle.
actually, it MAY (I say may because I haven't driven it since, so I haven't really studied when it happens) be doing this, I'll check it just to be sure. I can't mess anything up doing this can I (if done correctly, obviously)?
and, yeah, its just a regular th350.
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At full throttle cable should be all the way out.
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OK, so I replaced the vac advance with a non-adjustable one and I get 36 degrees at idle. The detent cable is maxed out at WOT and the vacuum modulator is getting full manifold vacuum, but when I adjust it (got up to 4 turns counter-clockwise at half turn intervals) the gear shifts don't seem any shorter or softer, so I'm guessing that this went bad during all of the turning of the vacuum advance and whatnot. The truck shifted great before all of this, so hopefully I just blew the diaphragm, I get some white smoke upon cold-start, but I didnt see any ATF in the vac mod line.
Does timing advance affect gear shifting at all? ie the more advance the later the shift? This is the only other thing that I can think is messing up my shifts, and now the clutch is really starting to smell, even after just one short run, so I need to figure this out before I have a huge problem on my hands.
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36 degrees at idle is way to much. From your vac can all you need is 8-10 degrees this is my setup tried and true from Dave ray aka the ignition man. 375/ 41 center plates and weights from the distributor in an 84 suburban. They provide 21 degrees of mechanical plus 12 initial then 8 of vac. That is all you need. Nothing more or less.
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You want 36* of advance at 3000 rpm with the vacuum advance disconnected. For stock distributors, this should 16* at idle with the vacuum advance disconnected.
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rich why you have to be so confusing lol i was confused about all this stuff from the get go. but why not just say 16° at idle with the Vacuum advance disconnected
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Because it depends on the distributor advance setup. What you want is 36* when the advance comes in. The idle setting depends on the distributor. Stock distributor, sure, then you want 16* at idle. But if you have, say, 24* advance in the distributor, then you only want 12* at idle. You only need to check it at rpm once to see what kind of distributor you have, then you can just set the timing at idle once you know what the subtraction is.
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so there is no way to check what type of distributor you have unless you "zero" it out, then run to 3k then see what your advance timing is at then minus that from your "zero"
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That's the best way because who knows what's been changed over the years:-) better than taking a wild guess and detonating a good engine... Or just look up ignition man and follow his past threads...
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Find the lightest advance springs and put them in (only to find out how much advance you have) then take them out. You should only have to rev up to 2k max probably Less
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36 degrees at idle is way to much. From your vac can all you need is 8-10 degrees this is my setup tried and true from Dave ray aka the ignition man. 375/ 41 center plates and weights from the distributor in an 84 suburban. They provide 21 degrees of mechanical plus 12 initial then 8 of vac. That is all you need. Nothing more or less.
What size springs do you run on those weights, and is there a vac I can buy that only gives 8? Or do I have to modify one?
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My apologies, I still have my good initial timing 12 at idle and 36 all in before 3000rpm. I just can't find a vac advance that gives me less than a ton of advance. Any advice on finding one would be much appreciated.
My bigger concern now is the hard & late shifts, is it just a coincidence that these started now, or is there a connection between advance and shift times?
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Here's a chart with numbers for vacuum advance canisters, maybe this will help..
http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf (http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf)
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Here's a chart with numbers for vacuum advance canisters, maybe this will help..
http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf (http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf)
Awesome, thank you.
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36 degrees at idle is way to much. From your vac can all you need is 8-10 degrees this is my setup tried and true from Dave ray aka the ignition man. 375/ 41 center plates and weights from the distributor in an 84 suburban. They provide 21 degrees of mechanical plus 12 initial then 8 of vac. That is all you need. Nothing more or less.
What size springs do you run on those weights, and is there a vac I can buy that only gives 8? Or do I have to modify one?
I run the medium springs in the moroso kit. I bought the accel vac can because it pulls down to 5hg.
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Vac advance limit plate
http://m.summitracing.com/parts/crn-99619-1