cub still overfueling

im still having problems with my 48 cub overfueling, it blows black smoke, and fouls the plugs frequently, now, what ive tried, the carb has been overhauled with 2 different kits, and it and its passages are clean as it can be, the adjustment is as lean as possible without stumbling, starting with 1 full turn from seated, is it possible somebody has changed the jet size in the past? where can i find the factory jet? i bought a new carb, its the zenith, and is a physically larger carb, at least the size of the air intake is, this one has to idle fast,the cubs linkage wont let it idle down without killing the engine, which is not really a problem,. but it too feeds more gas then the cub needs, not as bad as the factory carb but id still like to get it better, again its leaned out until it just runs smooth, the old cub is now semi retired and only pulls my feed trailer twice a day, which varies in weight between 500 and 2000 lbs depending on how full the water tank on it is , its on reasonably level ground, so the old cub handles it fine, i hate to have a old tractor just sit and do nothing, it bugs me, as well as keeps my other tractors available for use, including another cub, any help would be appreciated

All the cub carbs I have worked on did not have an adjustable main jet just an adjustable idle so if yours only has one adjustment it is the idle which does very little to stop the black smoke. When was the last time you checked the air cleaner and made sure it was not full of mud and water in the oil cup and also the cleaner it self to make sure it is not full of mud dubber nests etc.

hotter spark plug, may help

Float set right?

I agree with John, there isn't a main jet setting, but the float level has a great impact on the fuel delivered.

air cleaner has fresh oil, at the proper level, float should be set, ill check that again

Ah but is the air cleaner it self clean. Seen a dirt dubbers nest plug up the upper tube and cause all sorts of funny problem some including a no start due to flooding because of no air gettign to the carb

If someone drilled out, or installed a larger main jet to use Ethanol modified fuel, running straight gasoline in it might cause rich operation. Jim

ill check that out too, should be clean but i havent pulled the steel wool stuff out in awhile

my .02. I have a cub (53) which had a
zenith carb. The tractor wouldnever run right. had symptoms similar to yours. Problem solved with a rebuild IH carb.

Disconnecting the pipe at the carb will tell you immediately if thats the problem.

Pull the oil cup off and try to run it. If it runs like it should then you know the air cleaner has a problem if it does not help then not likely to be the problem but and easy way to test so you know which way you need to go to fix it

The guy I bought my Cub from obviously didn't understand the principle of how a carburetor works. He had bored out the carb venturi. It would only run at higher speeds as it would not idle like that. I got it rebuilt with replacement parts, installed it, and it ran like you describe. Turned out he had also drilled out the main jet. I bought a new main jet from Case IH, installed it and it ran perfect and has ever since. As others said, the main jet on the IH carb is a fixed size.