Compression testing technique

[freybaby]

While looking at a boat tonight I attempted to do my first compression test.I know I made a couple of mistakes, one being motor was tested cold and the other was I was not able to move throttle linkage to WOT.The article I read said nothing about warming motor up (figured I would run motor after CT) and I could not move throttle to WOT at the engine. I tell you all this because on 4 cyl I got around 90psi but on other 2 only 35psi. I understand you are looking for consistency of 10-15 % and not so much actual reading. I was wondering if mistakes I made had a big enough effect on test that I need to go back and retest with engine warm--the boat was nice but I walked away do to test results . Also how do you have WOT with neutral lockout -- try as I might I could not move linkage at engine

Thanks just a rookie trying to learn!!!

 

[5150abf]

Re: Compression testing technique

Don't know where you heard to mput the throttle at WO but you don't need to do that, I can't think of any reason you would.

Numbers warm are alot more acurrate as everything is tight and sealed but cold is good also.

I really think you would get the same numbers either way adn you were smart to walk, 35 indicates some major issues.

When I run a compression check I pull and ground all the plugs so the engine can spin properly and just check each cylinder watching the gauge, spin the engine till the gauge stops going up, no need to touch the throttle, you really don't want a bunch of gas blowing around.

 

[rodbolt]

Re: Compression testing technique

depends on the engine being tested really, however what your looking for, hot,cold throttles open throttles closed, is consistancy.
you did not get it.
that motor has serious issues.
RUN AWAY

 

[freybaby]

Re: Compression testing technique

Thanks guys appreciate your reply--almost every article I read said to make sure both the choke and throttle were wide open to allow as much air as possible --figured it didn't much matter given condition of 2 of the cyl--

I will keep looking!!!

 

[seahorse5]

Re: Compression testing technique

--almost every article I read said to make sure both the choke and throttle were wide open to allow as much air as possible --

With a 2-stroke motor you can block off the carburetor or intake with your hand and still get an accurate compression reading. A 2-stroke doesnt' start compression until the piston is past the exhaust ports which are open to the air while the piston is down. Bottom line is a 2-stroke does not have to "suck" air into the combustion chamber like a 4-stroke does.