Thread: Quadrajet tunning help needed

Tweet

Hello , first post.
I have a 84 c-10 that I just bought a newly rebuilt quadrajet carb to replace my old one. I miss diagnosed my problem though since I am still having the same issue as with the old carb. The main thing is a rough idle. I have been searching on the net and came across this info that give a little tuning info on the carb.Tech Tip #3
How can you tell if an off-idle stumble is caused by a lean or a rich condition?
A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. To narrow it down, tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. I actually use a small, broken drill bit that’s just the right size. Using a hammer, gently tap the roll pin in towards the choke air horn wall. Don’t jam the pin right up against the wall: Leave just a little bit of a gap so you can get a screwdriver blade in between the wall and the pin to pry it back again. With the pin tapped out, remove the accelerator pump lever. I like to do this with the engine running so I won’t have any trouble starting the engine without the accelerator pump. Now, rev the engine a little with the throttle. Notice if the engine seems quicker and more responsive, or if the hesitation & stumble is worse. If the engine actually feels more responsive with the accelerator pump disconnected, you have a rich condition. If the hesitation is worse than before, you have a lean condition. If there is no change whatsoever, you have a non-functional accelerator pump.

To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition. You need to select a jet/rod combination that will give you a little more Cruise Metering Area. Make these changes in less than 10% increments using the Figures provided in this paper. After checking I found that it seems I am running a lean condition by the tips givin in the article. I also installed a regulator for the fuel and found that I was around 7-8 psi. I lowered this to the recommended psi of 5-5.5 psi. All though while playing with the regulator I raised it to 10psi and the idle got better but I put it back at 5psi.

Ok, Psi is 5lbs. Newer rebuilt carbs comes with a APT adjustment , adjusting this is sort of like installing bigger/smaller main jets. I turned this to its maximum which was 1 and a quarter turns CCW richening the mixture and it helped a lot but still not idleing perfect as it should. What else should I look at to get a better idle? I think I might need to go to bigger main jets. Thanks

As this is a '84,truck, does it have the accel pump soleniod in the right frt corner of the carb ?? If it does, try to disconnect it and see if it's any better. Sounds like the idle mixture screws may be too lean. Have you atempted to adjust them. Preset years ago was about 2 1/2 to 3 1/2 turns on a Q-jet. Possibly a low float level (most Q-jets set at 1/4 "). D

Don't be so quick to blame your carburetor. More often than not, when I have chased down a suspected fuel delivery problem, the issue was somewhere else. Have you verified your timing, ignition? (plugs, wires, rotor, etc..) Check for vacuum leaks...they will create the symtoms you describe.

Good luck

IIRC the 84 has the ESC system and that was often problematic. Personally, I would never buy another "over the counter" rebuilt carb. I've handled way too many of them that were cobbled up junk.
Good basic troubleshooting info you posted though.