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Issues with 1998 Mercury 25Hp (20 Jet)

A

AUbuck

New member
A buddy and I have a 1998 20 Jet (25 Hp) and have a couple issues. The first issue was that no matter how much I cleaned the water pump and tube running to the motor it seemed there was some sort of obstruction keeping water from pumping strong through the tell tale nozzle. When I put compressed air on the tell tale nozzle it seemed to blow through the system unobstructed but when we would take it out to run it still seemed clogged (little water and steam coming out of nozzle). It seemed to pump better in reverse for some reason. Any thoughts on where the obstruction may be and how to clean?Seems like these jets could pull up big trash into some neat spot....

So, I'm not sure but the motor may have run hot due to above issues. My friend had the motor out and said it felt like lost power (would only do 11mph instead of 18 or so as normal) but still ran. Then next time he took it out it fired up one time and died at the boat ramp and would not start thereafter. We assumed the head was blown but now not sure. I put compression gauge on it and had around 115 or so on each cylinder which from my research does not seem too bad for a motor this age. Compression was pretty equal between cylinders. I then checked for spark and had spark to both cylinders. My question is, could the motor still be blown given the compression results? If not where would you look next to fix? Thanks.
 
I would check your thermostat first. Compression should be ok. There is no head gasket on that motor. Did you line the waterpump tube correctly? Did you put the washers on the impeller? Did you put the key in the impeller? As a last resort take the transfer port covers off and look at the pistons. Are you running 50-1?
 
Thermostat....duh...that makes sense. I can check diagram but roughly where is it located? As far as the water pump, I'm pretty sure I've got it all back together correctly. Done quite a few in the past and I have taken this one apart a bunch of times. The other thing is that sometimes it pump runs ok, then seems to clog again. Like piece of trash hung somewhere. I did not know if it was possible for trash to be somewhere in lines that circulate through motor, and, if so, how could check it out. And yes, running 50-1 fuel mix.

If I look at pistons will I have to replace any gaskets? Just want to know what I'm getting into there. That will be last resort for sure. It also appears that there is some rubbery melted looking plastic under one of the coil packs (or whatever those things are that the spark plug wires go to). Could I have an issue there? Know I'm throwing alot out but glad to hear probably not blown power head. Folks tell me these can't be rebuilt well due to mercosil lining, used ones are hard to come by, and mercury don't make power head for this motor anymore.
 
the thermostat is where the pee-hole line comes out of the block and it directly controls the pee stream. You can actually watch the thermostat open and close on that motor. The coil melting means it got hot may need to be replaced. People who tell you it can't be rebuilt don't know what they are talking about. That motor can be bored out .040, but you need to find a good machine shop that knows how to do a blind bore motor and has the special bits for the mercosil. You can also use the older chrome bore block on that motor. They were made from the mid 80s to early 90s.
 
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