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sndtubes

1930 Buick Carb Troubles?

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have a 1930 Series 40 Buick with the dreaded Marvel Carburetor. A little background first:

The carb was rebuilt completely about 5 years ago by "The Old Carb Doc".

The manifolds have all been refurbed and the heat tube replaced. The heat system is totally disabled. The opening to the exhaust manifold has been completely sealed.

The fuel pump is working fine and was rebuilt when the carb was. I have added a 6 volt "helper" fuel pump for when the car wants to vapor lock.

The engine was rebuilt about 15,000 miles ago.

Ok, here's the problem. I cannot get the car to run right at high speeds without running the choke at least halfway out. When it is really hot (like yesterday), the choke has to be nearly all the way out for the car to run. If I push the choke in, the car starts popping and backfiring through the carb as if it's running too lean. I have tried and tried adjusting the air valve spring and I just can't seem to the the car to run rich enough at higher speeds. If the air temp is reasonable (mid 80s or so) the car runs fine with the choke all the way in, but still runs better on the highway with the choke partially out. It's easy to find the "sweet spot".

My question is this: Is there something wrong, or is this just normal for this kind of carburetor? Maybe it just isn't possible for this Marvel carb to deliver enough fuel for this engine at high speeds? Or could there be something else going on?

Would I be just better off by changing to a downdraft like a Rochester or Carter BB-1? I'd like to keep it as stock as possible, but it's a driver and that's most important to me.

Thanks!!

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Have you done an ENGINE CYLINDER CHECK? If you have a valve problem, it will tell you which cylinder has a problem and if the problem is the intake or exhaust valve.

If the manual valve adjustment is set too close, you can burn a valve over time. When the valve clearance is set too close, the valve will not completely close when the engine heats up.

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Have you done an ENGINE CYLINDER CHECK? If you have a valve problem, it will tell you which cylinder has a problem and if the problem is the intake or exhaust valve.

If the manual valve adjustment is set too close, you can burn a valve over time. When the valve clearance is set too close, the valve will not completely close when the engine heats up.

I haven't cheked the cylinders yet, but it is an unlikely cause of the problem since the engine has only about 15,000 miles on it since rebuild.

I just checked the valve clearances and they were all correct so I don't think that is the problem either.

I really think at this point the problem is likely that the jets are partially clogged in the carb. I'm gonna try cleaning them next......

They also might be too small and need opening up a bit since this has been an issue with this car from day one.

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Quick update:

I squirted WD40 all around the intake while the car was running. No change in engine speed, so I dont' think the problem is a vacuum leak at the intake gaskets

I put my hand over the heat tube inlet. I could feel no vacuum there, so I dont' think the heat tube is leaking

I can put my hand on carb air intake while running. The car immediately dies. So, I'm not so sure there is a vacuum leak. I'm going to try to find a vacuum gauge and check the vacuum at the wiper inlet port. At present that port is closed off with a small hose (not leaking)

I'm becoming suspect of the air valve. I'm wondering if it is working properly. When the carb was rebuilt a few years ago, the spring was replaced with the correct one.

More updates soon. Thanks for all of the great suggestions.

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