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Mariner 25hp compression issue

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lormanalex

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"hi there all, and thanks in a

"hi there all, and thanks in advance for the help.
i have a 1998 25hp 2s mariner OB.
i did the real quickie compression test of holding my thumb over the spark holes, the top cylender seems ok. the lower seems quite shot.
any suggestions to fix this?"
 
"yes, quite possibly. getting

"yes, quite possibly. getting my hands on a compression tester. could it be a broken ring? or the seals in the powerhead? are these good places to start?"
 
"Alex, if you confirm a compre

"Alex, if you confirm a compression problem, it's easy enough to pull the cylinder cover (simply remove the thermostat and then 5 more bolts) and take a good look...

This model is nice and straight forward to work on if it goes that far..."
 
"Graham;
pulled the head &#40

"Graham;
pulled the head (the metal plate with two holes in that the thermostat mounts in) last night however it doesnt seem to then let me access the top of the cylinder as it seems to be blind bored. any thoughts?
(thanking my stars that it's a two stroke, so much easier to work on, so much easier to get running well)"
 
"Sorry Alex, was thinking of s

"Sorry Alex, was thinking of something else when I commented above. This is a "headless" split block motor. What you removed was simply the waterjacket cover.

Bandit nailed it - you will be able to see into the cylinders by removing the exhaust cover.

I would stop by a Merc dealer though and pick up a couple of gaskets (for the water cover plate and the exhaust). Best to put on new ones anytime you have a plate/cover off.....

If the pistons or cylinders need work, the motor must be torn down completely to get at them..."
 
"Alex,
If you see that the

"Alex,
If you see that the bottom cylinder is scored, don't worry about those gaskets that Graham suggests that you pick up. I had a scored top cylinder on my 20/25 Merc and I would have come out a whole lot better off (financally) if'en I had bought a new motor.
2_cents.gif
"
 
well heres a major update.
an

well heres a major update.
and again thanks for all the help and good ideas from everyone.
i dismounted the powerhead from the lower unit and currently its sitting on my workbench totally taken apart. the crankshaft has been taken out of the block.
the lower piston rings were both jammed on the piston and couldnt expand properly to seal the cylinder. this seemed to be because of a small score on mainly the piston but a little bit on the clyinder too. it seemed to score along the ring. i had to break both rings to get them out and then used a knift blade and cleared the slightly gouged metal so that the rings will seat properly now (now to find new rings).
i have photos of all of this is people are interested.could it be that with new rings and maybe dismounting the wristpin and mounting the piston turned 180 so that the scores arent alighned might seal everything better and it all work?
thanks so much again
alex
 
"Alex, If'en you can get y

"Alex, If'en you can get your hands on a cylinder hone and clean the cylinder up as close to smooth as you can get it, install a new piston with .025" oversized rings you may be getting somewhere back to 105 psi compression on that cylinder. Did you do a proper compression test on the top before you broke everything down? If you didn't there is a chance that you will have an out of balanced compression situation so you might as well do both cylinders. When I discovered that my 20 hp had a scored cylinder I was able to purchase an entire used 25 hp power head and installed it on my motor. It works like a charm but I did have to reset the tandum tilt angle to compensate for the increased top-end speed."
 
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