Testing the health of a motor

What is the best way to test the health of a motor without dissasmbly? Is a compression check valid with hot cams?

 

99YZ400F

Conventional compression tests are pretty useless even with no auto decompression.  If a YZ400 is really healthy, you can't really kick it through compression in the first place, and there are no standards published for what it should have.  Even in car engines, the best use of compression testing is to locate one or more cylinders that are lower than the others, not what pressure is ultimately run up.

 

The best test is a Leak Down Test.  Most shops can do one inexpensively if you show up with the tank off already, and the cool thing is that you can not only see if it leaks too much, you can tell from where.

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