Mariner 25hp 1987 Compression

[ngentile]

Does anylone happen to know what the piston compression should be on a Mariner 25hp 1987? You know when you pop the spark plugs and attache a pressure meter and give it a few pulls?.. Just looking for the PSI that it SHOULD BE. Many Thanks

 

[verado7]

Re: Mariner 25hp 1987 Compression

without being exact you want to see 100 psi but whats more important is the variation between cylinders - more than 10% is bad

 

[lckstckn2smknbrls]

Re: Mariner 25hp 1987 Compression

I have a 1983 Mariner made by Yamaha. I had 124psi in both cylinders last year.

 

[ngentile]

Re: Mariner 25hp 1987 Compression

yeah when I tested it I had 95 and 95 from both pistons no variation. Would rebuilding the carb improve the compression? (I don't think it ever has been rebuilt).

The motor does run like a bat out of hell so power loss is not an issue at all. Idle is rough though and konks out if I'm not giving it a little gas.

 

[Bondo]

Re: Mariner 25hp 1987 Compression

I had 95 and 95 from both pistons no variation. Would rebuilding the carb improve the compression?

Nope,... rebuilding the motor restores the compression...

Rebuilding the carbs restores the fuel delivery...

 

[verado7]

Re: Mariner 25hp 1987 Compression

precisely ... thats why i mentioned the 100psi ( without being exact ) for newer motor - under 100psi is a fairly worn engine yet it it maight still provide a few years of use without any real noticeable power loss - the rough idle is a normal characteristic of a merc 2 stroke - but if its stalling an idle screw adjustment and increasing the idle slightly may help - we had the idle up to 900 on a buddies 60 hp and it ran for years in fact he bought another one before it even died so it probably is still working .mercs are the toughest motors out there i swear - anyone can kill any motor quick but taken care of i wont even look at another brand yet . you should be able to make a couple of adjustments to get it to idle better yet though - if you wanted to .

 

[lckstckn2smknbrls]

Re: Mariner 25hp 1987 Compression

You could try decarbing the motor. It might loosen the rings and up the psi's.

 

[ngentile]

Re: Mariner 25hp 1987 Compression

Thanks for the input guys. I will try all of that.

However I guess what I'm looking for is what is the ideal compression for my engin model. I just want to know how far off I am from the norm.

For example if I brought it to a shop and the mechanic with 40 years experience checks the compression, he has a number in his head of what a good PSI for my year/mk/model is. What is that magic number? I know all engines are different. It runs perfect and powerful.. maybe this model has a low PSI range. (remember both pistons have exactly 95PSI) Thanks