Rough start question

[TMA62]

I have a 1971 Avanti II with a 350 V-8 and a Rochester Quadrajet carb. The engine/carb was rebuilt last year. Up until recently, the engine started fine and idled great. Over the last few weeks however, it will have more and more rough cold starts. It will idle very low and rough, stalling if I don't give it gas. I will need to apply the throttle to about 2-3K rpms for a few minutes before the engine will idle around 700-900. After that, it runs and idles fine. Warm starts will be routine and with no difficulty. This rough starting only happens during cold starts when the car has not started for at least a day. It is a garage car and I use Marathon 93 Premium gas along with STP carb treatment as a feul additive.

What do you think it is? Thanks in advance!

[kboyd]

Sounds like you have an automatic choke problem. Take the air cleaner off when the engine is cold and make sure the choke plate closes when you press the accelerator. If it doesn't, it might just need to be adjusted or you may have a mechanical problem with the choke mechanism.

[mfg]

I go along with a choke problem. The 'low idle' after a cold start would indeed indicate a choke plate that is not being activated to close...and a fast idle cam staying in the low idle (hot engine) position. The real question is why isn't the choke closing on a cold engine?....Something blocking the linkage? Thermostatic cover slipped to choke off position? As kboyd said, observe the choke for smooth operation and full shut when engine is cold (not running). ON THE OTHER SIDE OF THE COIN.....The choke may be operating just fine, but the choke 'vacuum pull off' may have failed (very common) causing the choke plate to remain closed too much...and your engine is actually running 'over rich' and 'choking' until it runs a few minutes and warms up thereby opening the choke plate.

[TMA62]

While starting and running, the exhaust is not clear (faint blue/gray in color) which may indicate that it is running rich during this time. Agree?

[mfg]

While starting and running, the exhaust is not clear (faint blue/gray in color) which may indicate that it is running rich during this time. Agree?

Agree.

[TMA62]

Mfg, it was the vacuum pull hose between the choke and the carburator. The hose was just long enough to begin to kink thereby shutting off all air flow between the to units. I cut the hose to shorten it so it will no longer kink and the cold start was routine as it was in the past.

Now, I am running into a another but slight problem. The engine slightly hesitates under full acceleration. At regular acceleration and constant speed, there is no problem. But when I apply the gas hard, the engine will act like its not getting all the gas/air mix that it needs. Its just slight but it wasn't there a couple of days ago. Don't know if it is related to the vacuum pull off.

[mfg]

The hesitation on hard acceleration is probably related to the choke pull off problem, because the hose you fixed probably never did allow the choke pull off to open the choke completley, which it is doing now. That would have given you a slightly richer mixture.....which Quadrajets are known to like. Hesitation, or stumble on acceleration, is a common Quadrajet problem... fixable if you bring the carb back into proper adjustment. Suggest you look on the internet under 'Quadrajet carb problems...hesitation'. You'll find plenty of info and tips there.