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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: 86gmc23 on May 11, 2014, 07:58:48 pm
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I just bought a rebuilt 350 and rebuilt th350 with a b&m shift kit already installed. Just finalized install and took it for test drive. Engine runs great but th350 didn't shift into 2nd until around 45mph under a hard acceleration, not WOT. Got up to 65mph and didn't shift into 3rd. Was on residential Street so I didn't want to keep going to find 3rd. Slowing down then giving gas again it had no power. Any suggestions on what to look for or check? Any help would be much appreciated.
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Did you hook up the transmission modulator to vacuum source? If not this will happen.
If it is plugged into a vacuum source, make sure line isnt blocked/pinched. Could be a bad modulator.
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Right now it's tee'd off of the distributor vacuum line, does it need a separate source? I have another modulator I can swap with it as well.
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Where do you have your distributor plumbed into?
I am not 100% certain about the T, I would plumb it in to its own source.
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Usually it is plumbed to its own source on the back of the intake manifold between the carb and the disti. It may be taking too long to suck the diaphragm down on that connection, or you could have a vacuum leak in the rubber line. The trans connection to the intake manifold on mine is a hard line all the way.
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Usually it is plumbed to its own source on the back of the intake manifold between the carb and the disti. It may be taking too long to suck the diaphragm down on that connection, or you could have a vacuum leak in the rubber line. The trans connection to the intake manifold on mine is a hard line all the way.
I have the distributor ran off the carb's vacuum, brake booster on the rear of the carb and everything else ran off a tree on the back manifold port.
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I'll fix the vacuum routing and try again before swapping modulators. Thanks for all the help.
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Most of the time dist vacuum is a timed vacuum. like they said hook it to its own full time vacuum source and try it.
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Ran new lines, just need to pick up a fitting for the vacuum port on intake. Both will have own separate lines.
My oil pressure gauge was faulty and just checked it with hand held gauge and it's down around 10-15psi, can anything other than oil pump cause this?
Looking at oil pumps, should I replace pump only or all components with it as well? Also is high flow pump needed or no, looking at Melting pumps from advanced.
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10-15 psi warm? but run the distributor off a ported vacuum. rich should be able to tell you why
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Start up hit about 25psi then fell close to 10psi, never let it warm up during test.
Distributor is currently run off port on right side of carb, if it needs to be ran from ported I certainly will change it. If that's the case can the trans be ran off carb?
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sorry meant that dont run the distributor off ported run both on manifold vacuum. i wont say anything about the oil pressure but my 350 at start up hits 50 once warm 15 if i run her too hard it will drop to 10. but this is with a motor thats been in a couple of my trucks and neither one was babied
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Here's why to run the distributor off manifold vacuum (the lower port) and not "timed" or "ported" vacuum.
http://www.corvetteactioncenter.com/forums/c3-technical-performance/60830-ported-vs-manifold-source-vacuum-advance.html
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How are those different vacuum sources identified? I think I have an idea but want to be sure.
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Ported, or timed, vacuum will run to an orifice in the carb body above the throttle plate. Manifold vacuum is either off the manifold itself or via an orifice in the carb body below the throttle plate. So Ported vacuum will be higher on the carb body than manifold vacuum. You don't say what kind of carb you have, but on the Carter and Edelbrock, manifold vacuum is on the driver's side of center, and ported vacuum is on the passenger's side of center, on the front of the carb.
(http://i39.photobucket.com/albums/e163/HydroCutlass86/carb.jpg)
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It is a Holley carb (#80508), thanks for your help. I really appreciate it, I found a diagram for carb and where the distributor vacuum advance is plugged in, the diagram says its Timed Vacuum.
Can anyone tell me if I should replace my oil pump? Like I said before, start up is around 20-25psi, then drops down to 10-15psi. Engine was cold during this test.
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...Can anyone tell me if I should replace my oil pump? Like I said before, start up is around 20-25psi, then drops down to 10-15psi. Engine was cold during this test.
Approximately how many miles on the engine? Check the rod and main bearing clearances. Installing a new oil pump on a worn out engine may not gain much.
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From what I was told engine was recently rebuilt so not real sure on miles. I just installed it on my truck, just got it able to drive down the road last weekend. That's when I found the bad shifting, fixed that earlier today and now it shifts great. When engine tested between 140-150psi on compression I took the guys word for the recent rebuild. What would you recommend I do, definitely don't want to burn anything up
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Ten PSI at idle won't damage your engine, but it is unusually low for a stock configuration; 25 - 30 PSI is more common. The guideline for high performance engines is 10 PSI/1000 RPM. Still, 10 PSI at 700 RPM doesn't leave much margin.
If you're confident the hand-held test gauge you used is accurate and the oil pressure is indeed ~10 PSI warm, remove the oil pan and one-at-a-time remove the rod caps to inspect the bearing surfaces and measure bearing clearances using green Plastigage (http://www.summitracing.com/parts/sum-pg1/overview/). Do the same with the main bearings. Procedure and specifications should be in the 1986 Service Manual (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Service/X8632_1986_GMC_Light_Duty_Truck_CK_G_P_10_to_30_Service_Manual.pdf), small block Section 6A5. Smear some engine assembly lube (http://www.summitracing.com/parts/cca-104) on the bearings as you reassemble. Clean then lubricate rod and main cap bolt threads with 30 wt motor oil and torque all fasteners to factory specs. Diligence and cleanliness is important during this process, so take your time and be careful. Also, crankshaft bearing journals are very soft, so take precautions to avoid 'dinging' them. You're trying to determine whether bearings were replaced during the 'rebuild' and whether the correct size bearings are installed. But, be prepared - depending on what you encounter, this job can escalate.
Plan on replacing the oil pump with a Melling standard pressure/volume pump and screen, but upgrade to a hardened drive shaft (http://www.summitracing.com/parts/sum-121160/overview/make/chevrolet). Melling also markets a screen installer (http://www.summitracing.com/parts/mel-mcat-62) that makes screen installation a snap, but the tool is pricey if only used once.
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The oil pressure indicates engine wear. The more worn the engine, the lower the pressure will be because the bearings are passing more oil. The aviation people monitor at-operating-temp idle oil pressure on their engines over the engine lifetime to monitor engine wear. Sounds like some of the engine may have been rebuilt, but perhaps the main or rod or cam bearings, or some combination of them, were not replaced.
Remove the vacuum advance off the "Timed Spark Vacuum Source" and tee it off the PCV line. Timed vacuum is emissions nonsense, to clean up idle emissions because that is one of the things tested. Back in the day, vacuum advance always ran off manifold vacuum; timed vacuum came in in the 1968 model year when they added AIR pumps and all the other nonsense. Running off timed vacuum idles the engine way retarded, and will cause off-idle hesitation, until the advance catches up with manifold vacuum, after the throttle plate is opened and the two vacuum sources become the same. Running off manifold vacuum means the engine is always 'ready to go' as far as ignition timing.
Why do I call the emissions stuff nonsense? Because it makes engines run like garbage, and 90% of the emissions benefit is obtained from simply running a PCV valve. The rest is a lot of hassle and performance hit for not much emissions reduction gains.
Here's where your ports are, I think. Just move the vac advance line around the corner and tee it off the PCV line.
(http://gordonfrench.com/camaro/vacuum.png)
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Thanks a lot for everyone's help, will get into it this weekend
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Well the oil dipstick that was in engine when I got it is not the right one so I was reading full engine oil when there was only about a quart and a half in engine. Found that out when I did an oil change, so know my oil psi is good, glad I didn't run engine to much with that little oil in it.
Rerouted my vacuum lines and next thing today ill be tuning the carb. Hopefully ill be driving it to work on Monday, cant wait.
Thanks again for your help
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Well the oil dipstick that was in engine when I got it is not the right one so I was reading full engine oil when there was only about a quart and a half in engine. Found that out when I did an oil change, so now my oil psi is good, glad I didn't run engine to much with that little oil in it.
The simple things will getcha every time. lol
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Yes they will lol. Having another issue I'm hoping someone can shed some light on for me.
I have my Holley 750 idle air mixture tuned and idle screw set. Runs nice and smooth at idle pulling around d 16in of vacuum, only problem is when I give it WOT it instantly dies. I'm somewhat familiar with my carb but don't have about of experience, can anyone point me to an area to check out. Power valve?
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Not to harp on a point, but did you move the vacuum advance hose to manifold vacuum? Poor off-idle transition is one of the results of having it on timed vacuum. Also, what is your base timing?
When an engine dies due to carburetion, it is almost always too lean. A very rich mixture will still burn, but you don't have to go too far lean for the fire to go out. When you open the throttle plate, and all the air rushes in, the accelerator pump on the carb should inject a long squirt of gasoline to keep it from going lean until the gas flow can catch up. You can test the accelerator pump easily. With the air cleaner off, and the engine off, open the throttle all the way with your hand while looking down the primaries. You should be able to see and hear a several second stream of gasoline into both primaries. Poor operation of the accelerator pump will cause your symptom. It's usually the gasket/o-ring on the piston, but it can also be clogged accelerator pump metering jets.
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Base timing is set at 14°. Yes I did move distributor vacuum. I'll try that when I get off work. I remember seeing a lot of fuel squirting in when I hit throttle, but I'll get a good visual today.
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It could also be clogged secondary jets or a really bad jetting causing lean mixture, but I would not think the engine would die instantly. That sounds like a transition issue, hence accelerator pump.
14* base timing is good. You can go higher, to 17-20, and get more power, but that is enough that it shouldn't be the problem.
Does it do this warm as well as cold? A properly tuned and functioning cold engine will do that anyway. That's why some carbs have lockouts on the secondaries until the choke turns off.
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I figured I would keep timing on the lower end until I get kinks out, then maybe advance somewhere around 36 total timing.
Yes it does it hot or cold. When I test the accelerator pump later today i should be looking for a nice solid stream of fuel shoting into both primaries for several seconds right
Right now my electric choke is not hooked up and I have choke held open with zip tie. Not sure if that has any effect of this but thought I would put it out there
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Yes it does it hot or cold. When I test the accelerator pump later today i should be looking for a nice solid stream of fuel shooting into both primaries for several seconds right
Yep, exactly.
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Here's a little more to think about and do...
Holley 4160 pump shot should be no more than 1 - 1.5 seconds in duration with rapid throttle opening. Wiring the choke open will have no effect on a warm engine, but a cold engine will be effectively lean. Verify the float setting to the base of the float bowl sights and accelerator pump adjustment (0.015" lash at WOT). Make sure the secondaries are not opening prematurely. What happens if you hover the carburetor with a shop towel and gloved hand (protection from backfire) to enrichen fuel delivery when you rapidly open the throttle? Are the symptoms noticeably decreased or alleviated?
Does your engine have a fully mechanical distributor - is the centrifugal advance working correctly - does the timing still advance with increasing RPM when the vacuum can is unplugged?
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just curious but why a 750 cfm carb?
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WOT accelerator pump jet squirts nice solid stream for around 2sec. I sprayed some carb cleaner over carb while I snapped throttle and it revved up without stalling. Jet is a 31, do you think that means I should set up the jet size?
Bd I didn't get a chance to check that since I just saw it but will get into it more tomorrow. Float levels are just below sights. Distributor is vacuum advance.
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WOT accelerator pump jet squirts nice solid stream for around 2sec.
In both primaries?
I sprayed some carb cleaner over carb while I snapped throttle and it revved up without stalling.
So you are definitely going way lean for the fire to go out. I wouldn't think it is the primary or secondary jets, though, since fuel delivery from them hasn't begun yet at the time it "instantly dies". The accelerator pump settings should dominate the mixture for about the first second after WOT application.
Here's the best on-line reference I've found.
http://www.stockcarracing.com/techarticles/scrp_0601_holley_accelerator_pump_adjustment/
Sounds like you need to increase the pump volume adjustment and go to a bigger accelerator pump jet. That will put in more gas, solving the lean condition, while keeping the pump shot down in the 1.5-2 second range. What diameter jet is in there now, do you know? Remember that flow goes as the area of the jet, so a 10% increase in diameter is a 21% increase in flow rate. Adjustment in small steps is advised.
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31 jet I was referring to is the accelerator pump jet, I will try to get some larger ones tomorrow and try it out.
The 750 carb came on the motor, I have a 650 but it needs a rebuild.
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31 jet I was referring to is the accelerator pump jet, I will try to get some larger ones tomorrow and try it out.
The 750 carb came on the motor, I have a 650 but it needs a rebuild.
Understood. I was just ruling out other variables.
What's in it now, a 28? Going from a 28 to a 31 is a 23% increase in cross-section, which is about a 23% increase in flow rate (actually, a little more, since the edges have to be discounted a bit -- the whole diameter does not flow). If you kick up the volume about the same, you will get 23% more fuel into the transition. The A/FR has to be above 17:1 to be lean enough for the fire to go out. Adding 23% more fuel to a 17:1 A/FR should take it to 13.8:1 A/FR, which would be just about perfect on acceleration. Of course, you might be higher than 17:1, but you can't be much higher or it wouldn't be so responsive to the carb cleaner spray. So 31 sounds like a good thing to try. You should see a lot of improvement. If that doesn't quite do it, there's still a little balking, you could try just a touch larger, or try polishing out the 31s a touch.
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31 is the one thats in it now.
Im going to have to order them, how big of increase is recommended. Think ill start with .035
*Just got couple small drill bits, .035 & .038 ill try and if works ill order one in correct size
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.035 is 27-1/2% bigger cross-section than .031. That ought to be plenty.
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If I drill out the .031 to .035 and it works, I should still buy a new .035 rather than using drilled one right?
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If I drill out the .031 to .035 and it works, I should still buy a new .035 rather than using drilled one right?
If it works, why bother? They are conical shaped for better flow, but drilling it out won't change that. All they do for the different diameter orifices when they make them is drill them out.
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Sounds good to me, should know how it works later today.
Thanks again for your help Rich
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De nada. Hope you get it sorted. I think we are on the right track, though. The carb cleaner proved the issue.
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Having same issues after opening it up to .035 and at .038. Not quite sure where to go from here. Should accelerator pump lever be a lot stiffer with fuel in it then one on the bench? Also from the primary vent tube I'm getting lots of fumes and some fuel spatter spraying out, not sure if that's from me snapping throttle or not.
Is it possible for a vacuum leak to cause this? I can hear a hiss but when I cover carb I can't hear it anymore and motor is steady pulling 20" of vacuum at idle.
*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean
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*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean
Yes.
Did you also increase the accelerator pump volume adjustment, or did you just go to the bigger orifice? Is your squirt still 1.5-2 seconds long? Did the behavior change, or stay exactly the same? Like, does it take a bit longer to stumble now, maybe a half-second, or does it still die instantly?
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Having same issues after opening it up to .035 and at .038....
*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean
Did you let the engine run to operating temp or is it cold?
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Having same issues after opening it up to .035 and at .038....
*closed choke blade 3/4 of the way and snapped throttle and it took off fine. Is this more proof that it's to lean
Did you let the engine run to operating temp or is it cold?
Excellent question. I missed that one.
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I let engine warm up. I did adjust accelerator lever screw, it seemed to help some but didn't fix. Only difference was one time I snapped throttle is bogged way down, then caught it self at last second and shot some flames out of carb. I'm thinking I just need to rebuild my old 650 carb, the 750 just needs to much fuel.
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WOT accelerator pump jet squirts nice solid stream for around 2sec. I sprayed some carb cleaner over carb while I snapped throttle and it revved up without stalling. Jet is a 31, do you think that means I should set up the jet size?
Bd I didn't get a chance to check that since I just saw it but will get into it more tomorrow. Float levels are just below sights. Distributor is vacuum advance.
I let engine warm up. I did adjust accelerator lever screw, it seemed to help some but didn't fix. Only difference was one time I snapped throttle is bogged way down, then caught it self at last second and shot some flames out of carb. I'm thinking I just need to rebuild my old 650 carb, the 750 just needs to much fuel.
Lift the distributor cap, grab the rotor and rotate it. Does it rotate freely ~10° then snap back when you let it go? Before making any more changes to the carburetor, make sure the centrifugal advance mechanism is functioning as it should. Weight pivots wear out and reluctors seize. You can also unplug the vacuum advance then watch the timing marks with a light to verify centrifugal advance as RPM changes. Verify that ignition timing control is totally functional before expending any more time on fuel tuning. If ignition isn't right you're chasing your tail and prolonging your agony!