LT1 Carburetor Problems

[Erich A. Meyer]

My 72 LT1 dies during a coast down or braking into a highway toll. This only happens after highway driving for at least one hour in hot weather (air temperature above 75 deg F). After it dies it will restart but idles poorly and if the car is driven without allowing cool down, it starts to buck on partial throttle acceleration. The problem clears up after the car sits for about 20 minutes but re-developes once things heat up. During cold weather trips in the winter the problem does not exist.

The carburetor was rebuilt about 7000 miles ago and ran fine, even in 90 degree temperatures. The problem started when a backfire occurred throught the carb on our trip the the national in St. Louis. Since then I have changed the inlet filters and adjusted the bowl levels with no solution.

The only thing left is to rebuild the carb. Does anyone know what specific part in the carb can be causing this problem or am I looking in the wrong place for the solution? If this is vapor lock then it must be due to a carbureter malfunction since it ran fine after the rebuild. Can a carb rebuild fix this?

[Joe L.]

Re: LT1 Carburetor Problems

Erich-----

Although your symptoms don't exactly fit the typical scenario, your comment about the problem initializing after a backfire makes me suspect the Holley power valve which mounts on a threaded fitting on the primary metering block. I'd try replacing yours and seeing what happens. You will need to remove the primary fuel bowl to do this, but that's not too difficult. The power valve is no longer available from GM for your carburetor, but readily available from Holley.

Many "idiosynchratic" problems with Holley carburetors can be traced to power valve deficiencies. And, after a backfire condition, the power valve is very suspect.

Of course, the problem could also be due to fuel vapor lock conditions. I doubt that a carburetor rebuild will cure this. Usually, such problems are "externally" generated and can be quite frustrating to resolve, particularly if it's one's intention to maintain totally original configuration.

[Jim T.]

Re: LT1 Carburetor Problems

I think Joe is on the right track with the ruptured power valve. Easy repair, I don't go out of town without a spare power valve. Holley's today have a anti-back fire valve to prevent this and your ole Holley can be modified with this change to help prevent the ruptured power valve in the future. Power valves by Holley are available from Auto Zone as well as the new fuel bowl and metering block gaskets you will probably need. Ask for them at the counter and know which power valve vacuum rating you want and match you old gaskets with the new. About your car dying coming off the highway or interstate. I had this problem with my 70 years ago. It was really wierd problem in that it happened after being on the highway for a long time. I couldn't find anything wrong. In messing around with the distributor I thought the end play could use adjustment. I pulled it and added shims to put the end play to specs. Next time I drove the car on a long trip it didn't die on the off ramp. This may not be your problem but I was sure glad mine went away

[G B.]

Try a different brand of gas

Joe is right about Holley power valves failing from backfires. That does happen. If you're going to take your carb apart anyway, change the power valve for peace of mind.

But, I don't believe this will solve your problem. I can't think of any way temperature would affect the operation of a power valve. Your specific problem seems to be triggered by heat. Your symptoms sound like gas is boiling in the carb bowls during low flow conditions.

Oil companies do change the formulation of their gasolines between the summer and winter. They also seem to constantly alter their gasolines for economic and environmental reasons. I would try a different brand of gas before I took anything apart.

I'm surprised that no one has mentioned warping of the carburetor body as a possible cause. This can cause internal gas or vacuum leaks when high heat further distorts gasket sealing surfaces. The gasket between the main body and the primary metering block is the one that usually fails. Sometimes you can cure this temporarily by tightening the bowl screws, but more often you have to take the carb apart and file or machine the main body gasket surface flat.

[Mark]

Re: LT1 Carburetor Problems

I agree with Joe too. I had this same problem after a backfire on my 72 LT-1. I sent the carb out to Jerry Luck and it came back brand new and the problem was solved. Jerry said the power valve was shot. Mark

[bruce]

re: float levels& intake gasket

HI Raced a stock LT-1 for years..... check to see that float levels are JUST below running out the window, check both power valves and also make sure you have the thick insulating gasket between the carb and manifold. Also install the Heat shield, which is still available from GM, it's called an insulator

[motorman]

Re: LT1 Carburetor Problems

as i have posted before that most all hot weather holley carb/alum intake manifold corvette problems can be solved by blocking the heat riser crossover in the intake manifold. if you have a manifold mounted choke spring you will have to block the choke open, but in warm/hot weather you do not need a choke to start the car. you can block the crossover by just draining down the rad fluid, looseing all the manifold bolts( do not forget the bolt under the ignition coil)and slipping a thin .003/.005 thick piece of stainless steel between the manifold and the head where the crossover is located on both sides. do not use a alum pop can as it will burn through in no time flat. i put a 90 degree bend at the top 1/4 inch of the plate so if you want to remove it you just do the above and pull them out with a pair of pliers.