Force Outboard 120HP - Bottom cylinder not operating

[Chipd13]

This is in regard to my jet boat with a 120HP Chrysler Force OB 2-cycle 4 cylinder engine.

I've cleaned both carbs several times and the spark is strong. Plug looks like it's washed clean all the time. Starting is problematic due to excessive compression in the bottom cylinder. Runs on three cylinders pretty well, considering. Is this a mechanical problem??

Or is it excessive oil injection in the bottom cylinder?

Could water be entering the bottom cylinder from the cooling jacket?

Help!
Chip

 

[pnwboat]

Re: Force Outboard 120HP - Bottom cylinder not operating

What year is your motor. There were a couple of different ignition systems used.

Too much compression? Have you checked the compression of all 4 cylinders? If so what are your readings?

When you say "plug looks like it's washed washed clean". Which plug? Usually a clean plug means the cylinder is not igniting the fuel/air mixture. If you have water intrusion into a cylinder preventing it from firing, you can usually see evidence of very small water droplets on the plug itself

Your motor has carburetors so nothing is "injected" into the cylinders. The oil is injected into the fuel as it flows to the carbs.

 

[Chipd13]

Re: Force Outboard 120HP - Bottom cylinder not operating

Dear pnwboat:

Motor's seal says 1997.

The flywheel-mounted relucter magnets were replaced recently, and a new wiring assembly was installed. I really don't think it's an electrical problem.

Compression of all 4 cylinders is excellent, 145-150.
The bottom cylinder is the culprit. The next one up might be suffering from the same condition, but not nearly as bad. The third one up perhpas still to a lesser degree, etc. It's possible water droplets did appear on the plug, but it's been a while since I last took a look.

I know the oil is injected into the fuel as it flows to the carbs. I just didn't say it correctly. But, your response made me start to realize the head gasket may need replacement. Oil passing through the fuel system would not know which cylinder it travels to, so it could not single out a specific cylinder. A head gasket leak, on the other hand, would potentially leak "locally" and cause such an affect.

Any other thoughts? Looks like I'll be getting dirty again. Really tired of working on this piece of junk.
Thanks,
Chip

 

[pnwboat]

Re: Force Outboard 120HP - Bottom cylinder not operating

There is also a gasket on the exhaust manifold. See item 2 in diag. below. Water flows directly from the pump into the bottom of the block through a small passageway directly into the exhaust manifold which then feeds the water jacket in the block. Typically, water leaks on the exhaust manifold affect the lower cylinders. Probably because the water flows down and goes into the exhaust ports of the lower cylinders.

http://www.boats.net/parts/search/Merc2/Jet/120XR%20%28JET%20DRIVE%29%201997/0E151580%20THRU%200E288445/MANIFOLD%20ASSEMBLY/parts.html

Like you mentioned, the head gasket could also be a culprit. Its very easy to change. Your compression readings are excellent. I would think that if the head gasket was blown, it would probably affect the compression too. But you never know.

 

[Chipd13]

Re: Force Outboard 120HP - Bottom cylinder not operating

Senior Chief:

I'll double check compression. It's a pain 'cause I have to remove the engine cover to do this right. Best thing is probably to replace both gaskets, regardless.

Thanks for the help, and for your service. Your next beer is on me.
Chip

 

[jerryjerry05]

Re: Force Outboard 120HP - Bottom cylinder not operating

You can still have a bad head gasket and get good compression. If the plug is clean then you probably need a gasket.
You check the spark? J