Sudden drop in rpm’s

[fleetwin]

Hmmm, OK, well I am assuming no amount of fiddling with carb adjustments makes any difference…Can you estimate how much the WOT has dropped? I like the idea about an exhaust blockage, maybe this engine was sitting before you got it and has a nest up in there. Yeah, I probably wouldn’t mess with that exhaust cover, especially if the engine has seen salt water use. You mentioned the compression numbers haven’t changed. How does the engine run otherwise? Starts and idles OK, runs smooth at higher speeds?
You might want to try cleaning the plugs up, then running it hard at WOT for several minutes, then shut it right down and pull the plugs to have a look…Maybe there is a slight water leak into the combustion chamber(s), or perhaps there is an ignition fault weakening the ignition causing the RPM drop. This is where those neon testers would come in handy, although a little tricky to use and monitor on the water at WOT.

[chris-p]

I admit, I don’t work on salty’s like some of you guys! I feel for you with your corroded bolts.

I usually heat them up with torch, hit with PB Blaster or Kroil, then hit with impact gun if needed and the screwdriver isn’t cutting it. Or the hand impact driver to get them started.

I have seen chunks of carbon blocking the exhaust route, limiting power. Also dauber, or even mouse nests, as stated.

Just my 2 cents. Would hate to have you break bolts for nothing, but seems feasible here.

[billw]

Why not just pull the power head and look with a dentist’s mirror?

Here is a way-out-there idea…..I once had a Mark 5 whose cylinder liner turned in the block casing, cutting the effective port size in half. They are not supposed to turn, as they are cast in. Don’t know if that’s possible on an 18 OMC or not. Never seen it….

Long live American manufacturing!

[fleetwin]

quote BillW:

Why not just pull the power head and look with a dentist’s mirror?

Here is a way-out-there idea…..I once had a Mark 5 whose cylinder liner turned in the block casing, cutting the effective port size in half. They are not supposed to turn, as they are cast in. Don’t know if that’s possible on an 18 OMC or not. Never seen it….

Hmmm-
Very interesting idea, this exact problem is not uncommon on some of the later model OMC looper engines, but I don’t think the sleeves can rotate on the older OMCs, but surely worth consideration.

[dougs-outboards]

Thanks for all the comments guys.

Maybe I will try the bolts on the exhaust and see if they will come out and stop just shy of breaking them f I run into that situation. I am kind of wondering about a chunk or two in there myself now.

[fleetwin]

I’m assuming you had the gearcase off when you were working on the engine, if not, you may want to pull it off first, might be easier than wrestling with the exhaust cover. True, you can’t see the exhaust ports unless you pull the cover, but I would do the simplest stuff first…

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