Massey Ferguson 135 won't handle any load

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The intake manifold gasket was a culprit of the same symptoms on a 135 gas tractor that my dad had. To check this, while it is running and running rough spray wd-40, brake cleaner or carb cleaner all around the intake manifold and gasket and see if it clears it up for a few seconds. You could have a bad gasket or a cracked manifold that only show sup when it warms up.

You can use a propane torch, too. Unlit. Don't light it, just turn it on partly and run it around, near the manifold, hoses etc.

Tractor that age, possibly a lot of cracked vacuum hoses, gaskets, etc.