Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Idle problem
dlphil10:
I can back out the mixture screws to 6 turns and it doesn't really bog. It may lower the vacuum a quarter to half an inch after 1.5turns out but it doesnt drop anymore. Could that be an issue? It really seems to have a better response at 1.5turns out though.
Cliff Ruggles:
The idle mixture screws are set for the best idle quality at the leanest settings, then move on to something else.
This is done by backing them out far enough so it idles great, then slowly turning them in until you hear a change in the engine speed or very slight drop on a vacuum gauge or hand held tach.
Move them back about 1/4 to 1/2 turn or so until the engine smooths back out. Repeat with the other side and balance them if you are running an open gasket under the carb or single plane intake.
So basically turning them in until you hear the engine slow is finding the "lean drop", then back out till it idles the best and you have the best A/F ratio at idle for your engine. Of course make sure you do this with the engine fully warmed up and fully heat soaked.
If you are unable to establish good idle characteristics in this manner and can continue to back out the mixture screws with zero effect (doesn't go rich), then the idle system is marginal for what you are doing and may need a tad more fuel made available to the mixture screws via larger idle tubes and/or opening up the DCR's slightly.
Keep in mind when doing this that you need to first check for "nozzle drip" to make sure ALL the fuel at idle is coming from the idle system. Also be aware that increasing fuel to the mixture screws for better idle control also puts more fuel to the transfer slots. If you are having issues or lean symptoms right off idle during transition, and/or minimal fuel at part throttle or limited control with the APT system, adding more idle fuel will help that scenario as well.......Cliff
dlphil10:
Thanks for that explanation, that helped.
I opened up my DCRS to 0.052". Then I had bad nozzle drip so I opened the idle bypass from 0.040" to 0.080". I also noticed the screws holding the baseplate and gasket had worked a little loose. It's better, but has nozzle drip at 750 rpm idle. At 700rpm the edge of the nozzle looks wet, but there no visible drip.
Sonce I just barely got the drip to stop at 700rpm should I add more idle bypass air and open up the idle circuit since it's lean? The idle tubes are 0.037, DCRs are 0.052, mixture screws are around .1. The idle vacuum is 14ing. It seems like that should be enough for it..
Cliff Ruggles:
Sounds like you are getting close.
A good way to decide whether to add bypass air vs idle fuel is to remove a small manifold vacuum hose and re-adjust the idle speed.
Then look for nozzle drip and tweak the mixture screws to make sure you have full control from rich to lean.
If you do not additional idle fuel is needed. After you add some idle fuel then do the same test to make sure no nozzle drip and full control with the mixture screws from rich to lean......Cliff
dlphil10:
I remember the original motor would bog when I turned out the mixture screws on the 2 barrel. I seem to only get this one to bog down with turning the screws in. Turning them out does make it flubby amd erratic at around 3 turns out so it makes a difference. Is that enough or should I expect a clear bog when riches the mixture?
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