I recently purchased a cheap 15' jon boat with a 1969 Johnson 25 hp. When I went to check out the boat, I bought my brand new compression checker with me, purchased at Anchor tools, the best one they sold. I got there and checked both cylinders, I was getting a reading of about 90 on each cylinder. A little low but atleast they were even. I noticed both plugs sparking when I was pulling for my compression check. I decided to go ahead and buy the boat since it was so cheap. Boat had set a couple years so I knew a carb, and water pump rebuild would be in order. After I got my parts and did the work, I went to start the motor. Wouldn't start but did caugh, I reset my link and sinc, adjustment and it fired right up. Idled great in the tank and shifted in gear easy. Took out to the river and it stumbled and bogged at full throttle. More work. Pulled the flywheel and looked at the points and there was a problem with one set of the points. New set of points and everything seems to be ok. I just didn't get the warm fuzzy feeling about my low compression readings though. I went to Auto Zone and rented (free) their Compression guage. On their guage I had 125-128 psi on the cylinders. Had my neighbor loan me his compression reader, a real good one. Exact readings on his 125-129. I've had a lot of people tell me compression guages are all different and a lot are junk. Turns out they are right. So just wanted to pass this info onto the BOC. If you want accurate compression readings, make darn sure you don't buy the cheap guage.