Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

68 gto idle problems

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Chevelle300:
Helping my dad try to get his gto to idle correctly. Motor is a 400 with 067 cam and 7028268 carb. I can turn the idle screws in and the engine wants to stall out. Backing them out about 4 turns seems to produce the best idle but I noticed if I take my fingers and block the main air bleeds the motor seems to smooth out and idle better? Also blocking air into the carb seems to help as well. My understanding is the main air bleeds should not have an effect on idle.  Here are all the measurements I got off the carb:

Idle tube  .040
Idle down channel .043
Upper air bleed .052
Lower air bleed  .070
Idle bypass .090
Idle mixture screw holes .090

Any suggestions would be great, thanks.

Chevelle300:
Ok so after thinking about this for a day I have to assume blocking the main air bleeds is making some fuel dribble out of the primairy fuel system richening the mixture. But where I am stumped based on reading cliffs book is my idle down tube size seems very big already. Do I try to open up the just the channel restrictor to see if I can get the extra idle fuel that I need to make this motor happy?

Thanks again for any help.

Frank400:
are you certain the idle tubes are .040 ?  That is huge for such a combo, IMO.  Also, are you sure you are blocking the main air bleeds and not the upper idle air bleeds ?  I'm asking because on such a carb, they are right by each other in the air horn.

Frank400:
Also, the timing is an important player here.  By the way, which intake manifold are you running ?

Chevelle300:
Yea I am sure the idle tubes are .040. I am using a numbered drill bit set to put through the holes to check sizes. The base timing is set at 12 degrees. Ignition system has been upgraded to an HEI and rewired for full 12 volt power. The intake is the stock cast iron unit. Yes I am sure I was blocking the main air bleeds I did it a couple times to be sure.

The specs on the factory cam say 273/289 advertised duration and 112 lsa. I could not find any .050 values? Motor is making about 14- 15 inches of vacuum when I checked.

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