Compression test yamaha 25 4 stroke

[maineiac5586]

I bought a 2004 YAMAHA 25 HP 4 STROKE MANUAL START TILLER with damaged lower unit it seems to start easily and REVS UP strongly. I am about to buy a complete 25 hp with a blown powerhead that I want to swap this running powerhead to. Before I buy the one that needs the powerhead I tried to do a COMPRESSION TEST by taking both spark plugs out holding the throttle wide open with tester in one spark plug hole but coming up with 90 PSI. Other cylinder is about the same. I expected a higher number but I read there is a decompression device which might need to be disabled to do this test. Anyone know how to disable the device? Also keep in mind that I can't spin the engine over very fast because it's only a pull start. And I know a leak down test would be more accurate but I don't have that tester right now

 

[scout-j-m]

Most decompression valves are only activated by the starting mechanism, like the pull rope. When you first begin to pull the rope it activates the compression release briefly to release compression at the beginning but then disengages once the motor turns over. So as long as you are doing crisp pulls and getting a good number of rotations its likely working fine. Compression gauge should hold reading so I would imagine if you could pull the motor 5-6 good times you would be seeing as accurate of readings as your tester can give. You can also spin the motor with an impact wrench if you have one.

I'm not much of a 4 stroke outboard mechanic but typically they have higher compression than 2 strokes. 90 psi would be about the least you would want to see on a 2 stroke and well below what I would think you would want to see on a 4 stroke.

 

[boscoe99]

A leak down test won't be any more accurate when it comes to compression pressure. That test will only tell you how well the valves, rings and gasket seal at one particular point.

90 psi on a model with compression release does not seem to be bad. You can do a running compression test by running the motor on one cylinder with the pressure gauge in the other cylinder. You are only going to be running the motor for several seconds.

If the motor you are considering buying has a blown power head (whatever that means) why are you wanting to do a compression check on it? I am missing something. What is the seller saying as to what makes the power head "blown"?

 

[QBhoy]

Hi
id maybe be wanting to see over 100 psi and holding the throttle open on a 2 stroke doesn’t do much to be honest...even if I do it myself.
Anyway. Worth pointing out that I have 3 compression testers. 2 are cheap ones and both read between 10-25 psi less than a calibrated snap on tester.

 

[maineiac5586]

Update. I bought the complete OUTBOARD with the non running engine (found broken pieces of piston in the oil pan). I swapped my running powerhead onto the complete OUTBOARD and transferred the ELECTRIC START setup from the damaged engine to my good engine. I rechecked COMPRESSION when I was done and with the ELECTRIC starter spinning the engine faster than the previous manual start did I got 152 psi and 150 PSI. I am pretty happy with those numbers unless you all think otherwise. This was on a cold engine if it matters with throttle held all the way open. Thanks and post if you guys would be happy with those compression numbers.