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 - Restoring a John Deere 2010 Diesel Tractor - by Jim Nielsen. Following seven years working in California's Silicon Valley, my wife, baby son and I moved back to Australia to retire. We bought a small 'farm' of about 50 acres near Bendigo, in the state of Victoria. I soon found that it would be very useful to have a tractor around the place for things such as grading our long drive and brush-hogging the fields. I was also embarking on planting 1000 eucalyptus trees, and hence I would need a ripper, small disk plow, sprayer etc. to get these things accompli
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