Thread: I've got a 1986 20 HP Mercury longshaft motor that has some problems....

If you got low compression on a out board motor 90% of the time it's going to be a galled cylinder or piston or both from running hot or to lean on one cylinder. If it was me and everything else seems ok on that motor I would at least get estimate to rebuild power head. That way it gives you a number to start with in your search for a used motor. Hopefully Archerguy will see this and respond on whether it's worth it or not to rebuild.

Should be a cover plate on one side you can remove and see pistons when they come down on exhaust should be able to see gauling on piston with low compression.

ditto on what he said. Mercury is well known for not having "heads" what you see is the water jacket so you won't be changing any head gaskets. sometimes, if you're lucky, it's just a stuck ring that was carboned up which was common in the really old Mercs. I have some oooooold Mercs ranging from 1941 to 1972 and have seen scored pistons, stuck rings, extreme carbon you name it. first thing, get you some sea foam, or find a dealer and get the Power Tune (Mercury brand) or Ring Free. spray it in the spark plug hole and turn it over a few times then let it sit. keep fingers crossed. if the comp comes up that's a good sign. if not, well looks like someone let it run too hot or forgot to add enough oil to the gas. happens all the time.

I'm no expert, just seen a few things dingling around with these motors. wish ya luck!

Mikey is right, no head gasket. Since it is a single carb going to the read block I would doubt the lack of oil in fuel, or both cylinders would be bad. My guess would be carbon build up causing the issue.

But you likely have a fuel problem. If the ignition is good and the compression is 100 and 40 then the motor "should" start if fuel delivery is good.

You really need to get the motor running and "shock" treat with a 1 gallon can mixed with a bottle of Sea Foam. I think your issue will be solved.

It's not a hard motor to work on compared to others.

Actually the compression is 100 and 60-65, and thanks so much for the tips and insight. I certainly will go and buy some Sea Foam and try to do a "shock" treatment to the motor.

The average between cylinders should only be +or- 10% so the one cylinder with 100 psi the other should be 90-110 min or max. I would guess that some where an air leak devloped allowing the cylinder with low compression to run lean, then get hot internally (not a cooling problem) and score the cylinder walls, the other cause could be a stuck ring from carbon build up and doing the same damage. One way to check,remove the spark plugs and get a very powerful pen light, shine it into the cylinders looking for scatches, scoring on the sleeve, and for dents and chunks in the top of the piston. These things most always involve a major rebuild but the cost may far out weigh the value of the motor. No head gaskets, so the compression problem is like I said, inside the motor. Don't get into costs of boats and motors but you may be able to find a left over model motor at a big discount, not a big call for 20 hp powerheads so most or the companies that deal in them don't mess with smaller motors, usually 40-50 hp and up. You may want to check with Hildebrand Marine Salvage in Coldwater Michigan, they may have a good power head assembly off a motor with a bad lower unit

Those xd 25's can develop lower crank seal leaks which can lean out the bottom cylinder and cause the problem you've got. You'll need someone that can bore blind holes and new pistons/rings most likely. You're looking at probably around 5-800$ to rebuild it yourself, somewhere in the $1500 range to get a pro to do it. I don't recommend trusting a shade tree mechanic since they may not be experts at diagnosing the root of the problem and it may pull an encore on you. The rebuild itself is no biggie as long as you torque properly, insert pistons the correct direction, have the proper rings on the proper grooves if different, etc. Finding a good used powerhead might be possible, and be the cheapest route. Check ebay---you might get lucky. Those are good motors and worth fixing if you can do it reasonably cheap. Problem is, 500-$1500 is a hefty sum, and you'd still have a lower unit that was an "unknown". I'd recommend buying a used outboard in good condition, and parting out the 25. You could probably part it out and make a few hundred to put toward a 40hp. The lower units are worth $300 if in good condition,by themselves.
JMO,
JBJ

I've got a 40 hp Yamaha for sale that would fix all your problems.

Originally Posted by Slipcork

I've got a 40 hp Yamaha for sale that would fix all your problems.

That sounds great, I'm sure I couldn't afford it though. What year and ball park figure? You can inbox me.