8n idles rough unless the choke is 1/2 way out

1952 8n idles perfectly but only with the choke 1/2 way out. It's been tuned up and starts easily and continues to
run but a little rough. Carb was overhauled last year. I thought something must be plugged up. Took carb off and
soaked it in paint thinner. Sprayed carb cleaner through all opening I could find. All seems clear. Put it back on
and it acts exactly the same way. I started trying to adjust with the front adjustment screw all the way in then
backed out 1 1/2 turn. The side adjustment screw- all the way in then 1 turn out. Then I adjusted the idle down to
a slow idle. Then adjusted on the side screw. It makes almost no difference. Seems to run a little better with
that side screw all the way in. It is a Marvel Scheibler carb. What am I missing?

Chuck,Something is still plugging up the passage ways in your carb.Clean them all out good with a long torch tip cleaner or a guitar string of the correct sizeand carb cleaner spray with a straw to clean and check passage ways.Use new gaskets when cleaning a carb as old leaking bad gaskets will have leaks and cause problems.

Pulling out the choke gives you a richer mixture. So, you could have either a fuel or spark problem.

A carb out of adjustment will also cause this problem; set the main jet at 1-1/2 turns out & leave it alone.

A dirty carb can cause the problem as well; start by cleaning the fuel screens. (see tips 45 & 56)

Dirty/old gas, or water in the gas can cause this problem.

Restricted fuel flow can cause this problem; remove the bolt at the bottom of the carb & check for flow. The flow should fill a pint jar in under 2 minutes.

On my 1950 frontmount, needing choke to run is the first sign that the points need adjusting/replacing.

A vacuum leak can do the same thing. Get a hand propane torch and carefully spray it (unlit of course) around the intake manifold at the carb to manifold interface & the manifold to block gasket of the tractor while it is running. Or, you can use carb cleaner or WD40. If it speeds up, you found the leak. Sometimes (rarely) manifolds crack or get rust holes.

Lastly, when all else fails, you could have an air leak around the carb metering section. The likely suspects are leaky throttle shaft seals and/or a worn throttle shaft. Or, just a real dirty carb w/ clogged passages.

Bottom line: Probably 90% of "needs choke to run" problems are fuel related, but do not rule out ignition problems, contrary to what others may tell you. I?ve had that very same problem before w/ a rebuilt carb, fresh fuel & good flow & a tight manifold.......and it was a spark problem. But, that was unusual. Just check the likely fuel problems first.

Please be sure to post back & let us know what the fix was. We all learn something if you tell us what worked!
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