Marvel carb Question

[Ron Lachniet]

I have read good things about Marvel carbs recently and decided to find one and try it out. I found one in really good condition with a tag on it that said "ready for installation". I put it on and it idles good and starts good but pulsates at running speed. What causes a carb to pulsate?

[H. L. Chauvin]

Hi Ron,

Not trying to be funny, but sometimes it matters "who" wrote: "Ready for Installation."

Sometimes we see similar phrases on Ebay .....

Not a bad idea to maybe first look to see what is going on inside.

A good Marvel is a good carburetor.

[duke36]

There are some great prior posts or threads on the Ford Barn site re: Marvel carbs.
As stated, check the inside... for intact gaskets, etc. float height parallel to the gasket flange, etc. and jets open and clear. I run Marvels, but without proper gaskets, non- scored float valve and the GAV needle washer, they can leak .

[Ron Lachniet]

The carb was listed as coming out of an estate and the tag was very old and faded but said it was ready for service. The carb looks to have been rebuilt with new gaskets and everything as clean as new. It didn't leak and ran strong but just seemed to run uneven like it was surging.

[H. L. Chauvin]

Hi Ron,

FWIW: You may want to try to see if the float level is set correctly.

One way is if one removes the carburetor and holds the top half of the carburetor upside down.

The horizontal soldered metal float seam seen on the exterior of the float should be horizontal and "parallel" to the joint between the two carburetor halves where the carburetor perimeter gasket is compressed.

Check if the float seam is parallel from both left to right .... and also from front to back.

If the float is not parallel and set too high while the engine is running at higher RPM's, it is possible that not enough fuel is entering the carburetor bowl through the float valve, thus allowing the engine to run unevenly.

Let us know what you find .... please do not give up.

[Tom Wesenberg]

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Does pulling the choke a little help? A lean mixture can cause pulsing.

[Ron Lachniet]

Thank you, I was hoping not to take it apart because the new gasket is stuck pretty tight and it will probably get ruined taking it apart but I am curious to see how that float works and inspect all the insides. I will update you when I do it.

[BILL WILLIAMSON]

Originally Posted by Ron Lachniet

Thank you, I was hoping not to take it apart because the new gasket is stuck pretty tight and it will probably get ruined taking it apart but I am curious to see how that float works and inspect all the insides. I will update you when I do it.

Have you tried adjusting the GAV?
Bill W.

[Ron Lachniet]

I didn't try choking it, but I did play with the gav and that didn't stop the problem. I have it back off now because I needed to put on a lot of miles in the next couple weeks and didn't want trouble so I will take it apart and make sure everything is in good order and try again when I am close to home. Thank you for your help and I will give you a report soon.

[1930artdeco]

Don't forget that Renners Corner sells parts for these carbs. I have one that I will work on in the near future to install on my car for testing.

Mike

[H. L. Chauvin]

Hi Ron,

Throughout live it is always interesting to investigate anything re-assembled, altered, or repaired by a human mind ...... for ages they have walked among us.

Removing a Model A oil pan, disassembling a Marvel carburetor is never much different from opening Pandora's Box in the ancient Greek World.

Truth is ........ like eager students in a classroom ....... everybody on this Forum is all ears waiting anxiously to hear what you find.

Hope this helps.

[Ron Lachniet]

I split the carb and everything inside is spotless. The float was a bit low so I adjusted it to the correct height. It has a spring loaded valve in the bottom of the bowl that lines up with another spring loaded pin that comes from the top section of the carb. I don't see this in any diagrams of this carb and don't know how it functions. Both springs look like brand new. Any info would be appreciated.

[duke36]

The Marvel's we seem to see for the "A" appear to have only a spring on the GAV,(choke. gas valve), the external idle air screw and the throttle /speed adjust screw. Could this be a Marvel for another car make ?

[Ron Lachniet]

I don't think so because everything lines up and looks correct externally. I think the two internal springs work on vacuum and as one spring is pulled up it allows the lower spring to expand and richen the gas mixture. I hope someone has seen this and can better explain it.

[Mitch//pa]

Pictures

[Ron Lachniet]

I will try to take pictures but I am having a hard time loading them into our computer. I will try tonight or tomorrow.

[Ron Lachniet]

These are the pictures of the springs.

[[email protected]]

I am guessing that those springs are not for adjustments but just for venting the top and quick draining of gas from the bottom. Don't like the angle of the float from your pix. Could it be a replacement float? More pics please.

[Ron Lachniet]

I think you are right about the purpose of springs. The float is actually set correctly but the camera plays tricks at such close range. From other pictures on this site the float looks to be correct but none of them show the spring arrangement. Thanks for your help.

[Tom Wesenberg]

That appears to be a vacuum operated power valve. When the vacuum drops more gas goes to the main jet.

[Ron Lachniet]

Thanks Tom, it sounds like you have seen this type of arrangement before.

[Tom Wesenberg]

Originally Posted by Ron Lachniet

Thanks Tom, it sounds like you have seen this type of arrangement before.

Yes, but only on later carbs.

The strangest thing I ever ran into with the vacuum operated power valve was during my trip home on leave from the Army. I was in western Kansas and a sandstorm suddenly came up. It was so bad I had to pull off the road and wait it out, so I slept in the front seat until morning. I was driving my Studebaker, and when I started driving again in the morning the car wouldn't go over about 35 MPH. I slowly made my way 150 miles to my uncle's house in Wichita, where I took the carb apart. I found the vacuum rod bent to the side, instead of pushing straight down on the power valve. I'll never be able to explain how that could have happened. I straightened it and the car was fine again.