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90's 25hp 2-stroke - bent spark plug electrode!

R

renns

New member
Hi folks, I joined here hoping to get a bit of advice on how to proceed with my Mercury 25hp 2-stroke, likely early 90's vintage.

This past weekend it spend a few hours flat out hauling gear and people up and down the river. There seemed to an increase in engine noise (clatter), but I thought maybe it was all in my head. It also started to have a bit of trouble idling. Then on Monday, when at full power, suddenly power dropped to about half, then another minute or so and it stalled out completely. The top plug was gray in colour, and had electrode bend over touching center pin. The bottom plug was brown like usual. Compression test showed about 140 on bottom, 40 on top.

I'm assuming the bent plug is a very bad sign - piston hit plug? These are the same plugs that have been in it the whole season. Would this be caused by crank or con rod failure? Any other likely causes? One guy mentioned maybe reed valve popped loose and went through engine?

I have a spare 20hp short shaft motor (25hp in question is long) of similar vintage, and could swap the powerhead over, but it is pull start only, with no gear on the flywheel. Any thoughts on swapping power head and adapting charging coil, flywheel, starter, etc from 25hp to the 20hp?

Many thanks for any input.
 
That's what I figured. Any thoughts on compatibility with 20hp of similar era? I got the better half taking the boat out herself now, but she'll need electric start. The 20hp is pull only, and likely doesn't have charging coil either.
 
Can you get me some serial numbers? These vintage Mercs were pretty interchangeable. Some flywheel only need the ring gear screwed on in order to electric start. I have converted these if the owner doesn't need an alternator. A great alternative is a solar charger in some cases. I have one on a pontoon with 25 Merc, electric start with no alternator.....works fine.
 
Jerryjerry05 said:
Pull the flywheels and see if they match underneath?
Click to expand...
That's the plan, but I've got one motor in the back of my car, and the other is at the cottage at the moment. Hoping to come up with some sort of plan that can be executed next weekend when I'm back on the water.

timguy, the serial number looks like: OC263922.

Thanks for the help, everyone. I'm going to get the dead 25hp out of my car and onto the stand tonight. Should I pop the head off first to see how bad it looks inside?
 
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