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Johnson 15 compression test issue

Jupiter Dinghy

Jupiter Dinghy

Regular Contributor
I decided today to pull the head off a parts motor, (trying to make one out of two) and took the compression reading after putting the head in the vise by spinning the flywheel with a drill and an adapter for the socket.
Readings go to 125 on both cylinders, but as soon as I stop the drill, it slowly goes down towards zero.....
How do I find out if I have a problem with the tester or with the power head?

Thanks.
 
Get another tester.----Most testers will hold the pressure for a long time.----Pressure in the cylinder leaks away via ring gaps !
 
racerone said:
Get another tester.----Most testers will hold the pressure for a long time.----Pressure in the cylinder leaks away via ring gaps !
Click to expand...
Bingo!.... t'is the tetster
After thinking a bit about it, I thought "let me try testing another motor and see what happens". Same thing.
Need to replace this POS tester.
 
Last edited:
In fact, it was not the tester.... (I know you guys were dying to know LMAO)
It was the flexible with the adapter that screws into the spark plug hole.
Found out using another quick connect adapter (the kind that has a rubber cone at the end and that you press against the spark plug hole), and the reading stayed on.
Case closed.
Thank you for your attention.
 
FWIW, whenever I get unusual testing results, the VERY FIRST thing I suspect is my equipment, and then my method.

So many times I find problems.
 
A lot of times replacing the Schrader valve in the compression tester is all that's needed. Just like if you had a leaky valve stem in a tire.
 
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