OMC 15hp Running on one cylinder

[OptsyEagle]

Re: OMC 15hp Running on one cylinder

The reed operation would have no effect on cylinder compression. As the cylinder rises up the reed opens and fuel comes in. When the cylinder powers down the reed is supposed to shut tight so the fuel is pushed into the combusion chamber. This is where a falty reed usually has its problem. Instead of the fuel being pushed up into the combustion chamber it gets shot out the open reed, through the manifold and out through the carb. So of course there is no fuel to spark and therefore the cylinder is dead.

The compression happens at the other end of the engine from the reed, when the combustion chamber is closed tight. It would make no difference to the compression readings whether the reeds were working or not.

 

[2shallow2sail]

Re: OMC 15hp Running on one cylinder

Thanks for the clarification, as well as the brainstorming. If I get a chance to look at it this weekend i will post back with my findings. If anyone else has ideas I am open to suggestions.

Summary:
-Bottom cylinder not firing
-Has spark on both plugs
-has 110psi compression on both cylinders
-swapped plugs and coils and didn't make a difference
-Was running perfect last week
-Top cylinder is running pretty well still
-Some fuel can be seen pooling in the bottom of the airbox
-New fuel pump, carb kit, points and condensers, plugs, and coils
-Currently thinking it is either a fouled Reed, a flooding issue, or something with the timing.

 

[2shallow2sail]

Re: OMC 15hp Running on one cylinder

I finally got around to pulling this motor apart over the weekend. One of the reeds had a piece cracked off and missing. I ordered the new parts and gaskets and am going to do some investigation to see if the broken piece damaged anything. I didn't see any scores on the cylinders looking in from the side exhaust ports and compression is was still good before I pulled it apart. Pulling the power head really ins't bad at all. I took a bunch of pics and have the diagram so hopefully reconnecting the linkages and whatnot will be easy. If i had to do it again I think I could have it apart in about an hour. Thanks for all the help.

 

[OptsyEagle]

Re: OMC 15hp Running on one cylinder

Might want to change the bottom crankcase seal and the upper water grommet while you have the powerhead off. The spring around the crankcase seal usually goes by now and that grommet can wear out as well. Both are easy fixes when the PH is off and the parts are cheap.

 

[AlTn]

Re: OMC 15hp Running on one cylinder

+2 with Opsy

 

[2shallow2sail]

Re: OMC 15hp Running on one cylinder

Can you tell me the part# or exact name on the diagram for the bottom crankcase seal? Also, do you know the function of the small round "Check Valve" and Screen on the edge of the leaf plate? It looks like it fits right in one of the bolt holes. Trying to figure out if i need to replace it or if cleaning will be sufficient. Thanks again, looking forward to getting this motor running again.

 

[2shallow2sail]

Re: OMC 15hp Running on one cylinder

I believe the seal you are talking about is 0315288 SEAL. This part of the motor looks to be different in the 70's vs the 80's. I see an extra piece in the newer version, almost a cap of sorts. To replace this, do i just need to unbolt #11 CrankCase Head? I'd prefer to not tear it down any further at this time.

 

[OptsyEagle]

Re: OMC 15hp Running on one cylinder

I think that is the right seal, at least you have the correct number. I think that part might be superseded by 321831. By the way, here is what mine looked like when it came out. You can see how the spring inside it was broken.

 

[Rick.]

Re: OMC 15hp Running on one cylinder

Can you tell me the part# or exact name on the diagram for the bottom crankcase seal? Also, do you know the function of the small round "Check Valve" and Screen on the edge of the leaf plate? It looks like it fits right in one of the bolt holes. Trying to figure out if i need to replace it or if cleaning will be sufficient. Thanks again, looking forward to getting this motor running again.

Glad to hear your making some progress and you'll be enjoying your "sunny days" on the water. Reed problems are uncommon but they do happen. I would say just clean the Screen/valve you mentioned. I've no idea what function it performs. Rick.