Re: Yamaha 40hp compression
Hey the question is not so much what the number should be, but how close the mumbers are to each other. The general rule is that compression between cylinders should be no more than 10% from the others. For example if you read 100psi on one, you can be from 90 to 110 on the other two and it is stil fine. The farther apart they are from that point the more you need to worry about possible problem with the rings or pistons. Ideally you want them as close to the same numbers as possible. For example I have a 1984 johnson 115, and mine reads 125,125,122,and122 its a v4. Here is something to good to know, compression various greatly, and so does compression testers. Good luck. Hope this helped.
