If the main jet is adjustable, try turning it in a quarter turn at a time, then look at the flame and test the throttle response after each adjustment. If it is adjustable there is no need to change the jet.
As said, the only real test is to actually put the engine under load long enough to get a good feel for adjusting the main jet. You'll hear it when it's right. And just because you get it right once doesn't mean it will be right next time, that's why they had adjustable main jets!
As for the black plugs, they will turn black if idled around and never put under load. If this is a hard working tractor, they will clean up if the mixture is right. If it gets minimal load, short runs, lots of idling, try going up on the plug heat range.
MS carbs are good, simple carbs. Simple is better on something like a tractor that sees a lot of dirt, or does a lot of sitting unused. That's why they are designed as they are, simple and few moving parts. Ford tried using Holley carbs for a while, complex with lots of internal moving parts, but would get the mixture closer to right across the power range. Good theory, but they were near impossible to tune and keep working properly. Don't even consider going there!
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Today's Featured Article - Earthmaster Project Progress Just a little update on my Earthmaster......it's back from the dead! I pulled the head, and soaked the stuck valves with mystery oil overnight, re-installed the head, and bingo, the compression returned. But alas, my carb foiled me again, it would fire a second then flood out. After numerous dead ends for a replacement carb, I went to work fixing mine.I soldered new floats on the float arm, they came from an old motorcycle carb, replaced the packing on the throttle shaft with o-rings, cut new ga
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