2000 25 hp Mercury compression

[rzrbkrd]

Just put two new sleeves and new pistons in this motor and compression is 150 plus on each cylinder. Searching online this shows to be inline with what its supposed to be. My Merc manual did not have this info. My question is how do the small engines run with such high compression and Merc recommends 87 octane fuel? My 200 Merc would need at least 93 octane to run that high compression with out doing damage.

 

[QBhoy]

Hi there. Seems like strong numbers for a 2 stroke. Good going.
Not sure what your questions is really asking though, haha. You’ll obviously know that without it, they won’t run at all, haha.
I would also add that unless your gauge is calibrated every year or known to be good...they are typically known to be wildly not accurate generally. Any non calibrated gauge should only be really taken as a general guide or indication of health between or amongst cylinders. I’ve known gauges to read up to 30 psi out from a calibrated snap on unit...used seconds later on same engine.

 

[flyingscott]

Why was the motor re-sleeved and not bored? did you test that right after you put oil in the cylinder? Were the decompression holes plugged? Compression for those is usually around 120 psi.

 

[Faztbullet]

Should be a Mercosil block....

 

[rzrbkrd]

It is a Mercosil block but would not clean up at .040 so had to go to steel sleeve. I have bored the Mercosil blocks but never got this much compression. Usually 125 or so which to me is good.

 

[flyingscott]

My guess is there is an excess of oil in the cylinders or the sleeves do not have the decompression hole. Or your gauge is bad. I have had a lot of chrome bore blocks re-sleeved and never had compression that high.

 

[flyingscott]

Do you have the correct rings in the motor?

 

[rzrbkrd]

I used the rings that came with the WSM pistons. I will check it again after I run it some more. I don't spare the oil when I put an engine together and I was running double oil in the gas.

 

[racerone]

Don't worry about it -----Run it and go fishing.

 

[flyingscott]

Are those direct replacement pistons for that motor. Once you sleeved the motor you went to a cast iron bore which requires different pistons/rings.