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Is pulling the prop through a good indicator of compression? Should it be about equal difficulty for each cylinder? I recently purchased my M20C and flew it home to PA from Texas with no issues. About 3 weeks after having it cylinder 3 fouled the lower spark plug and I couldn't get it to check out during run-up. I cleaned all of the lower plugs, reinstalled them, passed run-up and flew it with no issues. The next time it wouldn't check out again... I am under the assumption that I was not aggressively leaning it and just needed to the hang of it. I decided to replace all lower sparks plugs because looking through the logs I noticed they haven't been replaced in many years (12 to be exact). I replaced them with Tempest UREM37BY's. After replacement I started it up and aggressively leaned, passed run-up, and took it for a flight with no issues. All compressions were in the 70's in March when I picked it up but it is a high time engine (1800 SMOH). The weather around here has sucked so I went to the airport just to put fuel in the tanks. I decided to pull the prop through for fun and noticed that two cylinders are fairly hard to pull though, one is slightly less and another that is not very hard. I just want to see if others think that the fouling of the cylinder 3 spark plug and having one cylinder with significantly less compression during a prop pull though could be related or if it should be checked further.