Carburetor on a Classic Trans Am

Hi everyone. I really need help before I take my 1978 Trans Am to a shop.

When I drive my 78 Trans Am and I am going up hill or pass 55mph, The original Carburetor chokes and shuts down my engine. I want to change the Carburetor and buy a new one. Which Carburetor/manifold should I buy or is recommended??? This is the only problem getting on the way of passing smog here in California.

My engine specs: 403 Oldsmobile, 6.6liter V8 Automatic.

How do you know it’s the carb? Have you checked the fuel pump and fuel filter?

I think maybe your floats are set too low-an easy fix. Or have you replaced that filter where the fuel line goes into the carb? Also I would check the fuel pump. Thats the old style mechanical pump correct, and may just not be putting out enough volume anymore.

Check your fuel pump, If there are 3 lines connected to it the smaller of the rubber hoses is to a pressure regulator at the fuel tank that can stick in the open/return position and drastically reduce fuel pressure and flow. If pinching the smaller rubber hose shut eliminates the problem the repair requires dropping the tank or replacing the fuel pump without that connection.

Quadrajet? Leaky float bowls? 3.08 gears, 3spd auto so maybe 2200 rpm at 55. Punch it and it stumbles? Could be vacuum advance, leaky intake gaskets? Ever had intake off? How is distributor? Ever change rotor, cap, or coil?

Fuel Filter. It’s in the carb fuel line inlet fitting. Don’t strip the threads…You will spend a fortune swapping the carburetor and manifold for one that won’t work as well…

@Caddyman, new carb and intake won’t work as well? Match them up correctly, and you’ll find all kinds of power the factory left on the floor. Late '70’s was the Dark Ages of muscle cars due to EPA and emissions. The intake manifold and carb are purposely undersized to help these big blocks meet tailpipe standards. A properly sized carn and intake will wake this beast up. Provided nothing else is amiss.

The cam, ignition advance curve and compression ratio are the (very) limiting factors, not the carb. Quadrajets have ample flow rate to feed anything up to a 455 inch mill…The OP just wants it to run normally. He didn’t mention rebuilding the engine with the goal of increasing performance…

Be sure to consult Calif’s Bureau of Auto Repair about their requirements when replacing parts that affect emissions. They probably have a list of acceptable carb replacements for your car. (I know they have a similar list for cat replacements.) Best to make sure to use one on the list .

Are you using ethanol gas in your vehicle?


As a matter of fact, @Tester, the ethanol blends could be causing all manner of problems from deteriorated rubber fuel lines that have become porous to power valves becoming gummed up.

The OP may need to find a “seasoned” mechanic to look into the problem. All the scenarios above are highly possible causes for the problem.

Did anyone mention a catalytic converter being plugged? I forget what year they became mandatory.

Catalytic converters became mandatory in the US in 1975.

These cars used the “pancake” style converters which had a removable plug so the ceramic pellets could be “serviced” . On most Trans-Am’s, those pellets were removed before the car was a week old…Back then, these cars could pass an emissions test even with an inactive converter, the test standards being very loose…

@RodKnox it was 1975 for almost all cars. I was working for a Ford dealer’s garage. We had customers who lived in sparsely populated areas who could NOT get unleaded gas within 30 miles of home. The shop actually removed cats in some cases. I think the statute of limitations has run out on that crime.

BTW, I think you have one of the best handles on the BBS.

Yeah but “test pipes” were perfectly proper and sold everywhere. How else could you tell if it was a cat problem if you didn’t put a test pipe in for a while? Some diagnosis just take longer than others.

I’m glad you can appreciate my “handle” @MGMcAnick. Gearheads recognize the term while the uninitiated think it is my real name.

RK’s is good, another handle I liked was “Malty Tasker”…

THANKS everybody for your comments/replies, I will take in considiration many of your answers when I take it to the mechanic. I really want this beast to run smooth and powerful within the limits of the California emmision.