68 gto idle problems

[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.

[blazer74]

Unless your using taper length # drills your not reaching the restriction in the idle tube which is approx 1 1/2 inches down.

[Chevelle300]

I pulled one of the tubes out of the carb to check it.

[77cruiser]

Try opening the down channel to about .055. They can be easily replaced if you mess up the sizing.

[Chevelle300]

Yea that is what I was thinking, going to give it a try in the next couple days. I will report back how it goes.

Thanks everyone for your replies.

[Chevelle300]

Well I opened up the idle channel restrictor to .052 from the .043 and all seems well. Idles like I think it should. Will cold start it tonight and run it through it paces again to confirm it is all good.

[77cruiser]

Good deal. What happens when you adjust the mixture screws now?

[Chevelle300]

I only have mixture screws out about 2.5-3 turns now compared to the 4 or more before opening up the passage.

[Cliff Ruggles]

.040" idle tubes are HUGE in one of those carbs.

I would remove the triangular cover over the upper idle airbleed adjustment and gently seat it.  It may be open too far and leaning out the idle system.

Even for HUGE cams the biggest idle tube I'd put in one of those carbs would be .038", most get .034-.036" tubes.....Cliff

[Chevelle300]

Hey Cliff I had to seat that bleed screw completely. If I tried to open it it ran worse. It has been driven a bunch since drilling out the idle channel restrictor and it has been working great.

Hard to know what had been done to this carb before he bought the car. It was kind of a mess and we had to straighten out a bunch of things that were all wrong.