73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: thefarmboy21 on September 09, 2014, 07:29:44 am
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So I have a fair understanding of setting the base timing and such, but I need help to REALLY Dial it all in. I have a fairly mild 350 with Comp Extreme 4x4 cam (.447x.462 lift I think) Edelbrock performer intake, early 90's center bolt 191 "HD" truck casting heads, 650cfm Edelbrock carb, probably about 9-9.5:1 compression and an HEI distributor. Everything I've read on the heads says they're made for low end torque and fuel economy. Everyone who runs them says they run BEST with the total timing set around 25 degrees as apposed to the normal 32-36 range. Truck is an 86 K20, 4-speed, 4.10 gears.....mainly stays under 55mph and will probably rarely see over 4500RPM.....My questions are these:
1. To get 25 total, what base timing would I want it set at?
2. What RPM would I want to reach total timing?
3. What spring combo from the curve kit would get me X-degrees base, 25 degrees total at X-RPM???
I realize I'll have to tweak and adjust, but I need a starting point and I'm not sure what RPM I need total to be at, or how to hold the total timing back to 25 degrees without setting initial at 0-5 degrees. Thanks
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Why do you think 25° is your total timing target #?
You need to experiment to find what your ideal initial timing is and what your total timing should be. Determine what your maximum total timing is and start a baseline for making modifications
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Because I've read tons of articles and forums on these heads and all say they run best and make best power at 25 degrees total. Sounds weird but whatever works. I'll tweet it however I need to, but I'd like to start out close to where I need to be rather than try everything under the sun til it's good. I've tried that and it just takes way too long to not have a ballpark starting point.
I set my initial at 10 degrees this evening and it sounded really good. Crisp throttle and everything.....then I tried to tighten my dist and it's maxed out and still spins >:( I think it's probably because the cheapo chrome hold down is basically flat and has no grip. I'd much rather it be that than need shimmed. I really don't want to have to pull the dist back out and start over, BUT it is an aftermarket intake :/
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There is enough bad information regurgitated throughout the internet to steer you in the wrong direction. You need to experiment and determine what's right for your engine or you will not be optimizing your performance.
Make sure both tangs are in contact with the distributor base as they can move out of position.
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I used a factory distributor clamp and it fixed my problem, now I just have to get the timing and carb dialed in. Set the base timing at 10deg and then hooked vacuum advance back up and took it for a spin....seems to run really good, but has a pretty bad flat spot/hesitation if you mash the gas. I don't know what my total timing is because I have a plain jane timing light and my tach wasn't hooked up, but I'll tune on it more this weekend after I hang my exhaust. Right now it's got a curve kit with medium springs, vac advance hooked up, 10deg base, 93 octane, AC Delco R45TS plugs.....I may need to adjust the secondaries on my carb too.
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I just advance it a little and take a wide open test run
Advance it a little more on the next test run
Etc. until It pings under hard acceleration
Then retard it a little
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Where is the vac advance hooked up, ported vacuum or manifold vacuum? Ported vacuum will produce the hesitation you are seeing. Unless you are running a full emissions kit, you need to be on manifold vacuum.
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Where is the vac advance hooked up, ported vacuum or manifold vacuum? Ported vacuum will produce the hesitation you are seeing. Unless you are running a full emissions kit, you need to be on manifold vacuum.
this got rid of the hesitation problem with both my trucks.
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It's hooked up on ported vaccum right now, BUT I bumped it up to 16 degrees and it's a powerhouse now. I'll probably swap it to full vac and see how it acts. Right now I'm trying to figure out why my oil smells like gas.....hoping I can re torque the intake and solve the problem.....guess I'll have to change the oil to be sure.
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Chevy V8s like 36 degrees of total timing (base + centrifugal advance), with another 15 degrees of vacuum advance (usually specced as 7.5 camshaft degrees -- that is, 15 crankshaft degrees). That gives 51 degrees of advance at cruise. So I was surprised when you said 25 degrees base + centrifugal for your engine.
And they like the vacuum advance engaged at idle. That is, manifold vacuum, not ported vacuum. The idle rpms will probably go up 100-200 rpms when you do the switch, and most off-idle hesitations will go away.
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Chevy V8s like 36 degrees of total timing (base + centrifugal advance), with another 15 degrees of vacuum advance (usually specced as 7.5 camshaft degrees -- that is, 15 crankshaft degrees). That gives 51 degrees of advance at cruise. So I was surprised when you said 25 degrees base + centrifugal for your engine.
And they like the vacuum advance engaged at idle. That is, manifold vacuum, not ported vacuum. The idle rpms will probably go up 100-200 rpms when you do the switch, and most off-idle hesitations will go away.
This
...and if you are dabbling in this you really should get an advance timing light.
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I'm about 90% satisfied I how it runs right now, but I am going to switch back to full vacuum and see how it runs. Right now I just have a Actron? basic timing light from Autozone. I plan to buy a better one but right now my extra cash is all tied up in the truck.....as it seems to always be lol. Thanks for all the help this far, I'll report back tomorrow evening after I tweak things all day.
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...Right now I just have a Actron? basic timing light from Autozone. I plan to buy a better one but right now my extra cash is all tied up in the truck.....as it seems to always be lol....
Think of it this way... a good advance timing light is an investment in your truck and helps squeeze every ounce of potential from the investment you've already made. It's just another power making component. ;)
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Well I drove it around all day and seems to run really strong. Swapped the vacuum advance around and it idles smoother, but perfromance seems the same. I didn't get chance to flog on it today though. HOWEVER it did leave me sitting for a while today after it flooded out. When I finally got it started again I continued runnin my errands and watching the gauges. All seems well other than the fuel issues I'm having now. I guess my carb could be to big but I really doubt a 650 edelbrock is too big for a 350 with a comp 4x4 cam. Guys I talked to said sound more like a sticking float?
Here's the current symptoms:
1. Oil smells strong like gas.
2. After it warms up and you shut it off, you have to hold the throttle open and crank on it for several revolutions (when cool it starts right up)
3. Tail pipes are both coated with black soot and you can see black smoke puff out when you mash the throttle.
4. Probably unrelated but seems to collect condensation on the exhaust very fast OR I got some water in the tank because water rolls out for a while in the mornings and also if you let it warm up then shut it off on a cool evening and start it back up after about 5 mins it has water running out again.
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Condensation in the exhaust is normal. That's why exhaust pipes rust. They rust from the back forward, because it takes longer for the back to heat up enough not to condense water from the exhaust gasses. Lots of short trips eats up the exhaust faster than long trips, in which the exhaust gets hot enough to get all the previous water out and not condense any more.
650 Edelbrock is fine for the 350. That's what I'm running, with Edelbrock manifold, Comp cam, headers, duals.
Black soot and black smoke is rich mixture. Could be stuck float. Could also be floats set too high. Did you adjust them to spec? They are usually wrong when new. Also, did you tune the carb? Usually set rich from the factory to cover all applications, because lean is bad. I went a size down on jets, and changed rods, to lean the mixture out to best settings using an air/fuel ratio meter.
Mine also starts right up cold but likes to go a couple revolutions warm.
Oil smelling like gas sounds like leaking fuel pump, leaking into the crankcase. New fuel pump may be in order. (Mechanical pump on side of engine, right?)
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I have not tuned on the carb for a while. My buddy ran it and messed with the mixture screws and adjusted the secondaries , he may have also messed with the floats. I changed the fuel pump already thinking that may be a possible cause just didn't have an oil change yet.....I'll change the oil this evening if the rain holds off and try to adjust some carb stuff.
Rich, how many turns out should the mixture screws be for a starting point? I've got a vac gauge I can use. Also where do the floats need set? I don't have a book on it.....maybe I can find a manual on the web.
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Here's the owner's manual:
http://www.edelbrock.com/automotive/misc/tech-center/install/1000/1404_manual.pdf
You need the gasket to adjust the floats because you have to take the top off the carb.
http://www.summitracing.com/parts/edl-1499/overview/
Here's a post of mine on my tune. Includes how I set my idle mixture screws initially, though I tweaked them with an A/FR meter.
http://www.fullsizechevy.com/forum/general-discussion/classics/514682-just-got-1978-k-20-where-do-i-start-5.html#post5758323
That whole thread should be pretty interesting to you.
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When adjusting the idle fuel mix, for example "3 1/2 turns" means full revolutions correct, not in 1/2's? I know that sounds stupid but me carb literally idles the highest and pulls the most vacuum at ONE full revolution OUT per screw. 1 turn each from dead closed?!?! I'm thinking it has to be pouring an absurd amount of fuel if it only wants 1 turn per side. Also I adjusted my vacuum secondary flap back to the edelbrock spec (turn screw til it barely opens then back off 1.5 turns" and somewhere between the idle mix and secondaries I've developed an exhaust "flutter" It's like it puffs and has a lisp after I let off the fuel? I haven't checked the floats yet because I don't have a top gasket, but I'm thinking I may rebuild one of my 1406 600cfm carbs for it instead. Sigh........I just want a reliable daily driver lol.
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Yeah, it means full revolutions. One revolution out is amazing.
Which truck is this? The '77, right? You are running the mechanical fuel pump on the block, right?
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No the 86......I just dropped a basically fresh 350 in it. It's a 70-72ish bottom end from a truck that I rebuilt a couple years ago and had been running as my derby motor with some heavily worked 2.05 double hump heads......the heads need new bronze valve guides, so I put a spare set of centerbolt heads on the motor and dropped it in my 86. Has a manual fuel pump with the return port, Comp 4x4 cam (.447x.462) Eddy performer intake, HEI dist, AC Delco R45TS plugs.
But yea it kinda threw me for a loop when it just wanted leaner and leaner.......and the exhaust will still burn your eyes!
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You must be leaking fuel somewhere in the carb. Choke not opening all the way? Float valve stuck open or not closing properly? Piece of grit in a float valve? Floats set wrong? Someone mentioned on a forum that the Edelbrock floats are always wrong, right out of the box, so I checked mine and sure enough they were set low, overfilling the bowls.
The floats are easy to check, but you do need to replace the air horn gasket when you take the top off the carb. Edelbrock 1499 I think. Set of five in the package. Don't drop one of those little snap clips when you take off the linkage rods. They're a bugger to find.
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Well the choke is currently wired open, so it's not that. I'm thinking there's something going on with the needle valves or floats. I've got a gasket kit on the way, so hopefully I can get it straightened out.
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I bought the edelbrock trick kit for my edelbrock 1405 performer intake flat tops 4 valve reliefs 0 76cc 882s 2.02 1.60 0.30 over same comp extreme 4x4 extreme cam you are running it is absolutely perfect the truck runs amazing ill let you know what size metering rods i am running we set the carb up with an air fuel meter and 02 sensor
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Yea I'd like to know. I'm going to try and tear the carb down today. I have two 600's on the shelf I may try if I can't find the issue with my 650.
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This thread I posted earlier includes my rods/jets/etc. I also dialed it in with an AF/R meter.
http://www.fullsizechevy.com/forum/general-discussion/classics/514682-just-got-1978-k-20-where-do-i-start-5.html#post5758323
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Did you buy an Air/fuel gauge and sensor combo, or just cobble your own system together?
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I bought a complete kit. NGK AFX Powerdex. It has the wideband sensor. Not a cheap unit -- you can find them for about $300. But well worth it to get the mixture spot on the numbers for idle, cruise circuit, power circuit, and secondaries. Makes it a snap to tune a carb, or to re-tune it when you change something. And a perfectly tuned carb is a joy to drive.
There are cheaper units, but for accurate readings you want one with a wideband sensor that includes a heater circuit, a thermostat circuit (to control the heater with), and the sensor circuits. This is a seven-wire sensor.
I had the local muffler shop braze the (provided) bung in the drivers side exhaust and chase the threads. When I have the unit installed (only when tuning), I run the cable up the firewall, hook it over the wiper, and run it in through the vent window (which will still close and latch). I plug the unit into the cigar lighter for power. The unit itself fits in the little glove pocket in the dash (I don't have A/C, so instead of the A/C vent I have a little glove tray in the dash) with a piece of packing foam around it to keep it from rattling around.