Please help

[rfd223]

I bought a Force 15 hp I think it is a 98. I know its from when mercury took them over only has like 10 hours on it the guy said. The guy I bought it from said it stopped running one day he took it to the dealer and they put new coils it and it ran fine for a few more boat trips and then quit again he decided to sale it and buy a new one. So, I bought it hope it would be a easy fix, pull it over it wont run. I have plenty of spark from both cylinders. so I left the top plug out started right up ran great, put spark plug back in wont run. I pulled the bottom plug out same thing started right up ran great. I put the plug back in wont run. I took it to the local repair shop guy asked me what its doing so I told him. he never even looked at it and said the motor is junk and a waste of time and money. He said the cylinders were cross firing. basically one cylinder is blowning pressure into the other cylinder therefore fighting each other and the only way to fix it is the put a crank seal in it. he said for what it would cost to put a seal in it I could buy a new motor. I don't believe that's the problem. he didn't even look at it.

Does anyone have any ideas that can help?

 

[pnwboat]

I think the guy didn't want to work on your motor.

I don't know much about the 15HP motors, but I don't see a crankcase seal between the two cylinders on the crankshaft on the parts break-down diagram. The larger horsepower motors have this but I don't see one on your motor. The only crankshaft seals I see are located on each end of the crankshaft which would have no affect on blowing pressure into the other cylinder.

I don't know, but I would guess that you might have a bad reed valve? Maybe some of the other guys on the Forum with more experience on this motor might have some suggestions.

 

[jerryjerry05]

ON the crank center main bearing there is a "liner" or case that encloses the bearing.
I think??? this also acts like a seal keeping the pressure from going between the cylinders(again I think)

Possible it has gone bad and allows back pressure to mess the things up.

Start taking pics of the motor as you disassemble.
That way you can look back when you find you don't know how to put it back together(helps me)

Have you done a compression test.

Pull the intake and check the reeds.

See how I broke the message down into small paragraphs?
Yours is real hard to read and follow.

Oh yea Welcome.

 

[Jiggz]

You are in the right direction for troubleshooting. The next step is to install both spark plugs and then have the top plug wire disconnected. Did it run? Do the same for the bottom cylinder. Did it run? If it ran with only one plug wire connected, then the problem is with the CD module. The blocking diode in the CD module is bad.

If it still won't run with both plugs in and with only one plug wire connected, do a compression test before going any further. I'm a little puzzled with the mechanics diagnosis of cylinders fighting each other considering the pistons alternates from each other.