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Marvel Carburetor...Help!!

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I 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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I have shared a number of Marvel horror stories on different posts before you go crazy with the carb check your points.

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I have shared a number of Marvel horror stories on different posts before you go crazy with the carb check your points.

I did. None of them matched my problem.

Not a very helpful response.

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Check the heat riser tube in the manifold. They will cause a big vacuum leak.

JB

22-6-55 Sport Touring

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Check the heat riser tube in the manifold. They will cause a big vacuum leak.

JB

22-6-55 Sport Touring

Well, the heat riser tube was replaced by a machine shop who specializes in working on manifolds. There shouldn't be any vacuum leak there or really anywhere else I can think of......... The windshield wiper motor is diasbled and there is a tight fitting plug on the wiper vacuum tube. No leaks there.

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Rig a vacuum gauge into the wiper vacuum line and check what kind of vacuum you have at idle and running speeds. If vacuum is too low, intake manifold gasket leaks, tubes bad in the heat riser, or vacuum wiper line leaks are the most common causes. Burned valves can cause low vacuum also. A compression check will find this problem.

If vacuum is good and ignition timing is correct, then it is most likely a fuel problem. You could have plugged jets in the carb causing the engine to run lean. a set of 60 to 80 drills can be used to open the jets. The opening in the jets can be increased in size with these drill also. Be very carefull with opening the jets. It's a pain to solder the jets shut and redrill them.

Bob

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I agree with Bob.

To check for vacuum leaks around the intake manifold, spray WD-40 around the gaskets while the engine is idling. If the engine speeds up, you found a leak....

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I'll try looking for a vacuum leak at the intake manifold. It's the only likely place there could be one. I'm thinking that the jets may need to be opened up as Bob suggests. Since the car has ALWAYS run like this, I think its more of a problem like that rather than a transient problem like a leaky manifold gasket. I'll try the WD40 thing tomorrow and report back.

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Michael,

My '32 has a similar problem but not nearly as severe and I think my problem is the vacuum line to the wiper which I need to address. I will also review the other hints, so thanks to all for posting. I know you cover a lot more ground then my car.

John

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Michael,

My '32 has a similar problem but not nearly as severe and I think my problem is the vacuum line to the wiper which I need to address. I will also review the other hints, so thanks to all for posting. I know you cover a lot more ground then my car.

John

Oooo, I'd love to own a 32! My vacuum wiper port is currently disabled with a plug. It doesn't leak at all. But a vacuum leak somewhere could cause problems. Unfortuantely I don't think my problem is a vacuum leak.

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Well, the heat riser tube was replaced by a machine shop who specializes in working on manifolds. There shouldn't be any vacuum leak there or really anywhere else I can think of......... The windshield wiper motor is diasbled and there is a tight fitting plug on the wiper vacuum tube. No leaks there.

Did they replace the heat tube with a N.O.S. item or make up a new tube from stock materials?

The internal diameter of the tube is critical to the carb settings, if the walls of the tube are thicker than the original ( which are quiet thin and why they rust/burn out ) it will make setting the carb difficult.

The air valve spring is also critical and shouldn't be altered - i.e. don't try and stretch it. New ones are available and should always be used when rebuilding a Marvel carb.

The other thing you can check is to make sure the heat riser casting hasn't been cracked when the replacement heat tube was pressed in by the machine shop. I had this occur with my 1928 Roadster and it exhibited the symptons you describe.

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I did. None of them matched my problem.

Not a very helpful response.

Michael - I think the post from above meant 'check your ignition points' - not check your 'posts' which is a good call too. If points are too far open or weak, sometimes you need to 'choke it up' to get it to run OK.

I have had similar problems with the marvel on and off - but it is almost always muck in the carb, points or timing. If you suggest that yours has ALWAYS run this way, and requires excessive choke, than it sounds like a jet or vacuum related problem, as others have said. You reallt can't go to wrong with a marvel if you know the basics (and these too have been well described here by others).

Vacuum leaks must be tested with a guage to be sure. Jets can be tested with a drill. I would suggest that you go to the effort of checking these before running in circles on the twenty or so things that it could be from here.

Best of luck. :)

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If you go into the carburetor, you should look at the tubes that hold the jets. I have seen a number of these with cracks in the tubes. This is caused by embrittlement of the brass over the years. They can be soldered by a patient person. If these are cracked, fuel can leake out below the venturie and you get a poor fuel air misture. Choking the carb will increase the manifold vacuum and improve the condition a little bit.

One other item, If you do have an intake manifold gasket leak, do NOT just tighten the manifold bolts. You will only succeed in cracking your exhaust manifold. Follow the proper procedure for installing a new gasket. If you remove the manifold, check that there is a guide ring in each runner to the cylinder head. If any of these are missing, it will increase the chance of an air leak.

Bob

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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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Mike,

You should also check the float level which should be just below the lowest jet.

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Marvels.

I recently had my first experiences with these dreaded beasts. I friend recently picked up a neat 1930 Essex roadster which had been stored un-used for the last 25+ years. When cleaned up and first run it barely run and then only on near full choke. It took three sessions for us to figure out the basics. It has the die cast body and air valve. Problems, the air valve would not move freely on the hinge pin. Careful filing of the seal ridge on the air valve flapper was require to restore free movement. Ran but still required choke.

Next, the passages looked good, but we found build up in the low speed jet. Carb cleaner and fine wire cleaned out the barely visible varnish. Started up and pushed the choke in and it stay running. We took it around the block and he said this was the first time it made it without stalling and quiting.

Next round is to better tune the mixture etc to reduce some smoking.

It was a great help that he had a spare carburetor that came with the car. We took it apart and put it together twice for practice to see how to work on the parts before attempting to fix the one on the car.

Key learning, the air valve flapper distorts with age and can bind inside the body. It should be check on occasion by removing the carb from the car and checking for free movement of the valve and piston.

Looking forward to learning more as we get his Essex to run properly.

Drive Safe

Jeff

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Since I cannot seem to find a Marvel Carb. does anyone have a recommendation on a replacement Carb for the 1930 Buick Series 40 ?

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Jeff -

 

Thanks for your findings in regards to the Marvel carburetor.  I think answers like this will help all of us down the road.

 

Jay

1932 8-86 Victoria Travelers Coupe

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