Technical Model a carb problem

I have a completely stock 31 A. It has a Marvel carb on it that will not idle or rev up without the choke partially on. I have had this carb apart, checked all the passages and everything looks clean. Spark timing is good, no air leaks, I’m at a loss on this.

 

Have you adjusted the GAV

 

Yes, no mater where that is set same result. The mounting flange was severely warped, I did flatten that out.

 

Sometimes the throttle shaft in the carb is worn and getting air thru it....If the choke is on looks to be running lean....

 

These are the most simple and best, in my opinion, carb ever made for the Model A. Is the float level correct? You have a partially clogged passage somewhere. Usually the main jet is the problem. Blow all the passages out with compressed air. If you see any indication of white aluminum oxide on any interior parts you must get rid of it. Remove all jets (don't ruin them, they can be tight) and submerge the carb body in straight vinegar overnight. Do not use any kind of alkali to clean aluminum. It will dissolve it. Rinse thoroughly and blow it out again. All passages must be clean.
Moisture or liquid water will make the oxide grow. You can't see the inside of the cast in passages. Even a little bit can screw up the carburetor. Check the gas adjusting and idle needles for wear such as a groove where it seats. You can't get a good adjustment with a worn out needle. Parts for these are hard to come by but Bill Stipe has some parts.

 

Is this a Marvel or a later Marvel/Schebler?

Can you identify the exact model and identification?

Jon

 

It’s a marvel schebler. This carb is remarkably clean inside I have soaked and blown out all the passages. Does anyone know what size the idle jet orfice should be? Throttle shaft is tight. Float level is also correct.

 

Look for an identifying tag.

Marvel/Schebler made thousands of different carburetors, virtually all for tractors.

It may be that the internal venturi in the one you have is too large. Too large a venturi will result in venturi air velocity being too low (you increase the venturi air velocity with the choke). Too low a venturi air velocity will result in a lean condition.

Tractor carburetors are rarely the best choice for a car, as tractor engines basically idle, or run at P.T.O. RPM (RPM rarely changes). Because of the application, carburetors built for tractors have no tertiary circuit for changing RPM. But one that is the correct air flow size for the engine should idle well, and run well at high RPM. Typically, these are used on cars that live in relatively flat terrains (with no power circuit in the carb, the engine doesn't like hills).

And to answer your question, the idle jet size will depend on the size of the carburetor being used; one cannot compensate (much) for the incorrect venturi size (check the second line in my signature block).

Jon

 

There is no tag on the carb. I measured the Venturi it’s .950 at the smallest point.

 

I have a pretty good inventory of original parts for them if you find you need something.

 

Problem solved float bowl air vent was plugged runs like a million bucks. It’s been sitting since 1965

 

Great! Thanks for the followup. My cousin had a problem with his '31 'A', similar symptoms. LAST place he looked was in the fuel line, somehow a 2" X 3" piece of broadcloth (bed sheet) was clogging the line.
Cousin Bob demonstrated his vast street vocabulary on that one!

 

Glad you found the issue. And thank you for posting that you did find the issue.

For future reference, the pictured carburetor is a Marvel/Schebler model H, and was sold as a replacement for the Ford models A and B.

Jon

 

I have one of those carbs on my '31 roadster pickup. Came with it years ago and has always worked perfect. My '29 Sedan had a Tilitson and it also worked fine. My '31 Coupe has a 32/36 Weber but I have run Tilitson, Zenith, Model B Zenith and all work well.

Dave