It's possible you have one or more cracked cylinders in your tractor's engine.
H Ferguson used Continental engines in the 20 and 30 series tractors. The engine used in the 20 has a reputation for developing cracks around the cylinder bases near the galley.
If you purchase a rebuilt block, it's a safe bet that the reman. unit you receive will have weld patches around one or more cylinders. It is quite a common problem in this block.