Make sure the tractor is at operating temp; that usually takes 10 15 minutes at idle depending on ambient temp.
Both Ford and Marvel/Schebler (assuming you have a M/S carb ) say to set both the side-pointing idlemix and the down-pointing mainjet to 1-turn as a starting
point. I set the down-pointing mainjet to 1-1/2 turns and don't fool w/ it until the final step.
Then adjust the side-pointing idlemix for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed set-screw for very slow 400-rpms
idle. Do that idlemix adjust for maximum idle at least 3-times. Make sure that you turn the screws slowly, like 1/8 of a turn at a time & wait a second or two
for the engine to catch up. Take your time!
Do it like this:
1. Adjust idle mix jet until RPM increases
2. Adjust idle-mix set screw until the engine nearly stops (as slow as you can get it unless you have a tach that tells you 400 rpm)
Repeat steps 1 - 3 three times.
Remember the side-pointing idlemix is out for lean, in for rich.
If you do not have any problems inside the carb, it is easy to get the idle down to 350 - 400 rpms.
Your last step is to go back to the main jet. Remember, in for lean, out for rich. If you end up turning it OUT more than turn for max power (remember, you
already had it 1 turns out) then stop right there because you have a dirty carb or a fuel problem.
Lastly, each of us has his own definition of a carb rebuild. If your rebuild did not include soaking the carb 24 hrs in a bucket of caustic carb cleaner &
blowing out every orifice w/ a rubber tipped air gun, you didn't do a rebuild, IMHO. You just replaced parts.
_____
75 Tips