Hesitant LT-1

[Jim Gagliardi]

I recently had my carb rebuilt. Ever since I installed it I am getting a flat
spot upon acceleration. It also pops through the exhaust between shifts. Any
ideas on how to fix this?

[Duke W.]

Re: Hesitant LT-1

The first thing I would recommend is to put this question to whoever rebuilt your carburetor and see what they have to say.

Afterfiring (popping in the exhaust when you lift to shift) can result from a faulty diverter valve on the AIR system, so you should check it if the AIR system is still installed. I should close tight at about 18-20" Hg. A leaky valve can also cause afterfiring, so a compression or leakdown check is in order.

Assuming the AIR system is not malfunctioning or not installed and you don't have a leaky valve and the rebuilder did not install the wrong main jets or alter the off-idle system (a lot of "ifs", here), afterfiring on trailing throttle indicates a lean idle mixture, and hesitation on light acceleration could be caused by not enough pump shot to make up for the leaness. Since the LT-1 was produced during the emission era, its carburetor calibration was driven by emissions requirements.

The idle mixture setting procedure in your applicable shop manual probably recommends a "lean drop" method, which is emissions driven. You could try setting the idle mixture using the pre-emission technique of setting the mixture screws to obtain maximum RPM/manifold vacuum. This should eliminate the afterfiring assuming it's not something else mentioned above.

If this doesn't eliminate the hesistation on acceleration you are back to what happened during the rebuild. Since you say the problem didn't exist until the car was rebuilt, logic dictates that something for the worse happened during the rebuild.

Duke

[Wayne K.]

Re: Hesitant LT-1

Jim,

Also check that you don't have a vacuum leak between the intake manifold and carb base or elsewhere.

[John H.]

Re: Hesitant LT-1

If your A.I.R. system is still installed, chances are that the vacuum diaphragm in the diverter valve has failed (they all do - nobody ever expected them to last for 30 years); when they go, they fail with the spool in the open position, so the pump output is full-time into the exhaust manifolds, which causes after-firing ("popping") in the exhaust system. The diaphragm failure also causes a manifold vacuum leak through the signal hose from the carb to the diverter valve.

[Jim Gagliardi]

Re: Hesitant LT-1

Thanks guys. My A.I.R. is installed and recently restored by Bill Hodel. After
Johns response I think I know what that extra piece of vaccum line is for.