Hey Seahorse

[donwb46]

1974 rude 50hp.... Pulled the head and there is some scratching in cyl walls... Can you explain how it would show 120 psi comp and not fire.... Does compression leak down so quick that the fuel/air mix doesn't burn completely ???? Both cyl were wet with fuel as if the fuel mix did not burn completely..... Top of #1 piston was washed almost clean..... Odd thing is #2 is scored worse than #1 but #2 is the cyl that would run..... What could have caused the walls to score ???? Many thanks for your advise and help.....

[SEAHORSE]

Oil in the gas helps seal the scratches at cranking speed so you can get a reasonable compression reading. A leakdown test is more accurate and its the same one used for 4-stroke outboard motors, aircraft piston engines, etc.

Those old twins are very touchy on idle when they get some scoring in them, when the lambrynth seal between the crankcases wear, and if they don't come up to proper temperature.