Hal,
Since 2- of the cylinders have a compression reading of 175, the majority rule would indicate that this is the nominal compression of the engine.
A good rule of thumb is a + or - 5 psi variation is acceptable off the nominal value. Or a reading of 170-180 would be an acceptable value.
The 155 is out of the expected value.
The next thing to do, is to perform a leak down test.
This is where the engine is brought up to top dead center on that cylinder, held there - and then have air injected into the cylinder in question.
Most good mechanics will have the ability to do a leak down test on an engine.
Then, with the air being injected into the cylinder, and if air is leaking down - the crank case is checked for the hiss of leaking air - to determine a bad piston, bad piston ring or scored cylinder.
The in system is checked for a hissing sound to determine the presence of a leaking intake valve seal.
Then, the exhaust system is checked for a hissing sound to determine the presence of a leaking exhaust valve seal.
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Remember, since this is a 4-stroke engine - it is possible that the only issue with the engine might be a mis adjusted valve.
I would suggest that before much more checking is done on the engine, that the valve lash is checked on all cylinders. If the valve lash is too tight - i.e not enough space between the rocker arm and valve, the valve could be held open during the time that it is supposed to be closed.
One thing to then be worried about a bit, is that if you find that the valve lash is too tight and the engine has been running that way for a while, it is possible that the valve and or valve seat might have some damage due to being held partially open. Of course, a resetting of the valve - if it was incorrect and retesting will verify if any damage has been done.
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p.s.
If the dealer and mechanic say that this variation is all right, and if another similar engine is available - have a compression check run on that engine - whether it is new or used. I am guessing that another engine will not have the same reading differences.
Take care
REW
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