In small displacement motors, the compression gauge needs to be one with a small dead volume between the gauge and the fitting for the spark plug. If the gauge is at the end of a hose with significant volume, it will show a lower than actual reading. It’s likely that your actual compression is a lot higher than your reading.
For anyone that cares, here’s one way to imagine the problem: Consider a pressure gauge hooked directly to the end of a 50 cc syringe. Depress the plunger to the 25 cc mark and the reading should read somewhere around 15 psi. Then install a foot and a half of 1/4″ copper tubing between the syringe and the gauge. The tubing has an internal volume of about 20 cc. Now when you press the plunger to the 25 cc mark, you’re just compressing a total of 70 cc down to 45 cc. The gauge should read less than 10 psi.
Tom
