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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: 78C20Daws on December 01, 2021, 12:08:23 pm
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I have a stock 78 C20 with a 350 and a QuadraJet and factory HEI, and I have been having some issues with drivability after going 3/4 throttle or more. Truck starts, runs, and drives fine until I get on it. The first time getting on it goes well, but after that it bogs/chugs bad on light acceleration, especially from a stop. Changed the advance weights/springs/centerplate with no luck in solving the issue. Also if I leave it parked for a day then the problem seems to go away until I get on it again. Any ideas?
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As a fundamental step, validate the fuel pressure (~7 PSI), draw (>20" Hg), and volume (>1 Qt in 30 seconds of cranking) while the engine is cold. Leave the pressure gauge connected and then monitor the fuel pressure when the symptoms arise. Also, using an HEI spark tester...
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=37117.0;attach=51885;image)
...check the spark color and fullness when cold and then recheck it when the symptoms occur.
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As a fundamental step, validate the fuel pressure (~7 PSI), draw (>20" Hg), and volume (>1 Qt in 30 seconds of cranking) while the engine is cold. Leave the pressure gauge connected and then monitor the fuel pressure when the symptoms arise. Also, using an HEI spark tester...
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=37117.0;attach=51885;image)
...check the spark color and fullness when cold and then recheck it when the symptoms occur.
I will try that out tomorrow hopefully and see what I get, thanks for the reply!
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Another thing to try is to remove or loosen the fuel tank cap the next time the symptoms occur to see what happens. This might reveal whether the tank is properly vented.
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Sounds like it might be heat related.
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Sounds like it might be heat related.
I thought it might be too, but the other day I tested that theory by driving it cold, then idling it up to temp, and then drove it around for 10-15 minutes at operating temp with no issues regular city driving/accelerating
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So it's at temp, you trounce it, and she starts choking?
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At any temp cold or hot, the first time I get on it is normal, pulls fine and sounds good, any acceleration after that it bogs badly to the point it’s hard to get it going at all.
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UPDATE: I didn’t have the right size fitting for my pressure/vacuum gauge so I didn’t get fuel pressure, but I did get pump vacuum. It was 17” Hg cold, and when I got back from driving it after the issue arose, it slowly climbed to 14.5” Hg and peaked there. Thoughts? Also, [mention]JohnnyPopper [/mention], I stand corrected, I guess I just wasn’t driving it long enough around town to notice. The bogging starts slightly after a while of regular driving, but gets exponentially worse after the first hard acceleration. So, maybe it is temperature dependent. Does that change potential diagnosis at all?
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Replace the fuel pump! And while you are under there, check the condition of all the rubber fuel lines between the tank(s) and the fuel pump. Make sure none are kinked, split, cut, pinched, spongy, etc; replace as needed. Don't neglect the other suggested checks in case there are overlapping issues.
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Will do ASAP! Thanks for the help, I’ll be back here if problems persist after other suggestions have been checked!
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Some of us would like to hear either way... ;)
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Over last weekend I got the fuel pump installed (with some struggles), but I haven’t been able to drive it around yet because I’ve been so busy and on top of that the weather has been next to horrible about every day. Hoping I can drive it around a bit this weekend and let you guys know how things turned out.
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Hi 78C20:
If you have gone through all the previously discussed solutions and you still have the problem, it might be your carb accelerator pump going bad. I had similar problems with my 76 C-20 with OEM Qjet and OEM ignition system. Most often it was fine, but after extended driving in warm weather it would not want to accelerate and sometimes would stall....it would not start so I would play around with it and after cooling down it would start and off I would go again. I replaced all ignition parts one by one and no fix. Then I replaced fuel pump and no fix. Then I made sure there was not vapor lock issues with carb and fuel tanks and no fix. Then I replaced the acc pump seal and the problem was fixed for a few months...but problem came back and I noticed that the acc pump plunger area was very wet. So I replaced the carb with a rebuilt Qjet and the problem has been solved for 5+ years now. So I think the problem was the acc pump bore was opened up to the point the carb was not rebuildable anymore...so replacing the plunger seal was futile when the acc pump bore is too large from age. The carb was the original one (!) and I had rebuilt it 2+ times after it was over 20 years old so it was probably so old it was not rebuildable anymore even though most features of it worked fine.
Regards,
Henry