Well, first you want to have even compression (within 10%) between all four cylinders. That is more important than having 125-140 psi. And, don't believe everything you read on the internet.
Second, you have not told us whether or not the tractor has flat-top pistons or stepped pistons, or power crater pistons. I have a rebuilt Super M with overbore (4-1/8") stepped pistons (around 1/4" step) and it registers 125+/- per cylinder with an 8060 head. If it had the stock flat-tops, I'd guess it would be around 90 to 95. It runs very well, by the way - and many tractors do with 85-95 psi while making a ample power.
You sure as heck don't need 93 octane with an 85-90 psi compression.
After 10 years of not running good, I'd look into worn cam, worn distributor, etc., before I worried about 90lb compression.
That is another thing..... did you swap distributors? Various models have different advance curves and different maximum advance degrees.
Let us know what cures your problem when you find out. We all learn from each other.
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Today's Featured Article - Talk of the Town: The Saga of Grandpa's Tractor - by The following saga is from the Tractor Talk Discussion Forum. Someone. The saga starts with the following message: Hey guys I have a decision to make. I know what you all will probably suggest and it will probably agree with me way down inside, but here it is. I have a picture blown up and framed in my "tractor room" of a Farmall M. It was my Grandpa's tractor, of which whom I never got to meet. He froze to death getting this tractor out of the barn to pull a truck out of the ditch before I was born. Anyway my dad and aunt had to sell it at the auction,
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