That's the thing, my father in law gave me this tester. It's kinda like a homemade pocket type of deal. He is a mercury mechanic. My better tester is a $30. one from an autoparts store that I can't use w/o taking off the tank. it seemed like it was only fire intermittently on the left cylinder after a few minutes warmup, and a couple of passes around the yard. I called my buddy, and he felt that I just fouled a plug due to some oil used during assembly. So, I checked compression was 140L & 155R. Changed plugs (left was fouled) ran for maybe 15 minutes on some trails behind my house and came back and rechecked compression and than it was 145L & 140R. Overall, it seems to run good, I was hoping to be a little higher with my compression, but was told I would not be able to run pump gas with smaller domes. They're close, so I'm happy I guess, Will try to get a better tester for a more accurate reading. Thx