need some carb help...I think

[67sss]

Have a 72 gmc, smog stuff taken off, 350, auto and am having issues with the carb. I've adjusted the idle mixture screws out to the starting point of 1 1/2 turns, set idle at 600rpm, and the hi idle at 1000rpms. When I start it each morning,this only happens when the engine is cold, I step on the throttle to set the choke, start the engine, it runs for 10-15 seconds then dies. I do this same procedure 4 times and usually on the 4th time it stays running. Also as I drive the engine seems to be starving for fuel, I need to "feather" the throttle to keep the truck going. Thanks in advance for your help..Oh yea, this engine originally came from a 75 4x4 GMC 3/4t auto truck, and the motor is a GM crate motor.

[al's71gmc]

My 71 has similar issues. Mechanic told me I have a vac/air leak in the base of the carb. He offered to install a rebuilt QJ for $650. Nice guy. huh? When he set the timing and idle correct, it wouldn't even run without keeping my right foot on the gas while braking with the left. I've got a rebuilt one to put on now, but in the meantime I had to set the timing waaaaay BTDC to get it to run decent. Only problem with that is it slashed my mpg down to about 7 or 8. But at least I can give it gas and it takes off instead of bogging down. I'm hoping the new carb will improve everything.

[pjmoreland]

I had the same problem on a '69 I used to own. Turned out that the choke pull-off diaphragm was opening the choke too far once the engine started. I straightened out the little rod between the diaphragm and the choke so that it wouldn't pull the choke open so far, and it fixed the problem.

[jhow66]

Idle mixture screws should be out at least 3 1/2 turns--you are running to lean a mixture.

[Ron 586]

Just trying to point you in right direction. Make sure that you don't have a problem with a vacuum leak. Check all ports that have been plugged off for a while with a plastic or rubber plug to make sure that they are not cracked or leaking. Rubber hoses will dry and crack over time. Make sure you have the distributor vacuum line hooked to a ported/part time source on carb. Late sixties early seventies when thermo vacuum switches and delay/part time vacuum smog control came in to play it was a year to year change as to what was the hot ticket. Our trucks and engines fall right in the middle of it all. With 30+ years to mix and match it is easy to end up with a combination of parts that are not compatible. So if you don't know for sure that it is all 100% original do some research and find out what you have and if it will work before spending a lot of money and never getting it quite right. This is not an answer, it is a guide to try to help keep you from buying a $300 carb. that you don't need.