Q-Jet cold start idle remains high for 2+ minutes

[jvmagic]

Hi,

I had my Q-Jet rebuilt a few months back and the carb will idle (1.5k - 2k rpm) for a long period (2+ minutes) before it finally comes down to it's warm state idle rpm (750 rpm). When it's idling high upon a cold start and I wait one minutes and hit then hit the accelerator a few times, the rpm does not go down (remains at 1.5k - 2k).  If my mind serves me right, the rpm's should go down to a normal idle rpm after 30 seconds or so (upon hitting the accelerator).  Other than this the car runs well.

79' Trans Am
403 old motor (all original)
original Q-Jet
Bay area, California (been sunny and warm)

Thanks in advance.

[Cliff Ruggles]

If you are still using the original "hot air" choke verify that there is adequate vacuum at the line coming off the choke housing. 

Verify that the line leading down to the intake manifold's exhaust crossover isn't leaking.

If you have good vacuum at the choke housing port and the line is sound it should heat up very quickly.

If it doesn't heat up quickly (usually too hot to touch it in about 30 seconds or so) then you have one or more problems.  Either there is a vacuum leak before the heat tube, in the heat tube or the exhaust crossover passages in the intake are plugged or blocked.

The actual choke dial on the side of the carburetor is fully adjustable for how long it stays on but it will not work correctly unless there is an adequate heat source from the hot-air system.

I'll add here that the hot air system is a "closed system", bringing in filtered air from the rear of the carbs airhorn, pulling it thru the metal tubing in the intake, then supplying heated air to the choke on the side of the carb.

I'd start out by verifying that the choke is set correctly, most are marked with a line on them and there will be a "range" on the choke housing from rich to lean.  Rich to lean simply means how long it will stay on after start-up.

We don't see too many factory hot air chokes fail, but we do see a lot of them "crack" and develop vacuum leaks where they seal on the housing which pulls in cold air instead of heated air, so something else to check while you are troubleshooting the problem.......Cliff

[jvmagic]

If you are still using the original "hot air" choke verify that there is adequate vacuum at the line coming off the choke housing. 

Verify that the line leading down to the intake manifold's exhaust crossover isn't leaking.

If you have good vacuum at the choke housing port and the line is sound it should heat up very quickly.

If it doesn't heat up quickly (usually too hot to touch it in about 30 seconds or so) then you have one or more problems.  Either there is a vacuum leak before the heat tube, in the heat tube or the exhaust crossover passages in the intake are plugged or blocked.

The actual choke dial on the side of the carburetor is fully adjustable for how long it stays on but it will not work correctly unless there is an adequate heat source from the hot-air system.

I'll add here that the hot air system is a "closed system", bringing in filtered air from the rear of the carbs airhorn, pulling it thru the metal tubing in the intake, then supplying heated air to the choke on the side of the carb.

I'd start out by verifying that the choke is set correctly, most are marked with a line on them and there will be a "range" on the choke housing from rich to lean.  Rich to lean simply means how long it will stay on after start-up.

We don't see too many factory hot air chokes fail, but we do see a lot of them "crack" and develop vacuum leaks where they seal on the housing which pulls in cold air instead of heated air, so something else to check while you are troubleshooting the problem.......Cliff

Hi Cliff,
Thanks for the quick response.  I think you hit it right on the button as I now recall that after the carb rebuild I also had to replace the choke heater tubes  going to the intake. The old one had broken off inside the intake so I had to do some careful drilling to remove and be able to insert the new tube in.  I assume I may have drilled a bit more than needed or I did not fully insert the new tube which is causing some sort of leak.  If it's leaking due to the hole being a bit too large, what are my options (I would like to keep the oem look as much as possible). 

[Cliff Ruggles]

You need to have a tight seal on that system so it pulls heated air instead of cool air.  I usually convert to electric choke when we find or have issues with the hot air tubes and such.  Very easy to do and they work flawlessly.  Rochester started using electric chokes as early as 1975 and by 1980 most will be electric choke as it's just a much better set-up.

Don't get me wrong, nothing at all wrong with the hot-air set-ups, until the tubes rot out, crossovers plug up, blocked off, etc....Cliff