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1990 merc 15 hard starting and bogging

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Futile Tinkering

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Long time lurker first time poster,
Hard starting and bogging of my 1990 15hp Merc (SN OC278778) have been getting progressively worse. So far I've inspected and cleaned the carb, adjusted the float height which was a bit off, tested the coils stator and trigger (all good). I've got 120psi in both cylinders and excellent spark. I took the flywheel off today, and noticed fuel sitting up there on top of the crankcase. The upper crankshaft seal is damaged. So I think I've found my problem. Does the crankcase need to be disassembled to replace the upper seal? Should I be looking to inspect the upper crankshaft bearing?

Thanks
 
Thanks Jeff,
I got a new seal, managed to get the old one out and new one in. The good news is that it starts and idles much better. Still bogs down and dies when the throttle is opened. I took out the plugs after it died and they are wet with fuel. So now I'm thinking its the stator. There are two stators on the assembly, one for low RPM, one for high. I think the high RPM stator is dead or shorting out despite getting resistance measurements that are in range according to the service manual. Anyone know if this is a common problem with these motors? Any other ideas? The stator is $180, so I'm willing to entertain other possibilities. I've just about lost my cool on this thing.
 
Wet plugs in an idle situation is normal--it they read normal, they would melt at full throttle!

Before spending all that $$ try opening the idle mixture screws 1/2 turn OUT (CCW).

Jeff

PS: From my experience, the motor starts very hard (if at all) and won't idle below 2,000 with a bad low speed coil.
 
I own three models similar to yours, including a 1991 15 horse (the other two are 9.9's).

The stators are pretty solid on these for the most part, as is the switchbox. All of mine are original, I use the motors a lot and they live in Canada - so go from high 90's in the summer to -20 in the winter with everything in between - not exactly ideal conditions.

Anyhow. The common issues I have found with mine and the dozens I have worked on are (in no particular order):

1) the upper crank seal which you have just replaced - common issue

2) the intake gasket - not behind the carb, but the one on the starboard side of the power head - it "blows" and then you end up sucking air instead of gas/oil/air into the cylinder - runs like crap, stalls when trying to throttle up etc - look for gassy/oily mess in the lower cowl, particularly on the right hand side "back near the heads" (which aren't really heads), from the perspective of standing behind the motor - so looking at the prop

3) coils - develop spider cracks in the casings - really tough to see unless you clean them up really good - or, with the hood off, at night and the motor running, mist some water with a spray bottle back near the coils and watch for the lightening show.

4) bad base gasket (powerhead) - usually accompanied by a cough or a sneeze

5) carb/fuel pump - parts wear out - fuel pump bladder goes and there is no "outward" indication (no pool of gas etc) - worn out needle valve on the carb, bad carb gasket or seal etc.

That is some of the more "typical" issues with a motor that has been otherwise maintained and ran at least occasionally every year.
 
Thanks Graham and Jeff,
I know thetemperature swings about which you speak. I live a stone'sthrow from the Ottawa River in Petawawa, ON.
I rebuilt the carband fuel pump this past spring and went though both again last week,everything looks good there. Also changed the reedblock gasket andhad a look at the reeds.


The intake gasket you mention soundslike a good possibility, you mean the gasket behind the plate thatthe switchblock is fastened to? Procedure fairly straight forward? Unbolt, clean, new gasket, reassemble? I just had a look and no onehas had that plate off since original assembly, there's not even ascuff on any of the bolt heads.


I guess I'll pull the powerhead andalso check the base gasket and lower CS seal as well. Once I got itrunning last, I pulled wires while it was running and it'll run (atidle) on #1 alone, but not on #2 alone. That leads me to believethat either that lower CS seal is at least a bit leaky or that basegasket is.


Thanks again I'll post my results.
 
The intake gasket I'm talking about is also called a transfer cover gasket by some.

It's in the diagram below as parts 33/34 or 31/32 in the inset. It covers the transfer of oil/fuel/air from the crankcase into the intake port on the cylinder. You can check around that cover for any signs of an oily mess which could indicate that the gasket(s) are shot.

Some models (all three of mine) have a single paper style gasket. Others had a pair of rubber o-rings (well, rectangular rings I guess).

transfer cover.jpg

I had one go on me after driving in to a lake about 100 km north of Chapleau with the closest dealer back down in the Soo.

I had throttled back and it let go - sputtering and stalling - could get it to run if you gave it lots of throttle while trying to start it.

Fortunately had some of that rubberized cork gasket material back in the truck so "hacked out" a rough gasket with a filleting knife (after finally figuring out what the issue was). That was four or five years back - I'm still getting around to getting an OEM gasket to replace my "bandaid" :)

PS - is the free boat launch still there at Petawawa point? I lived out near the Airport back 10 years or so ago - launched a lot from there and caught many "messes" of Walleye in the river. Last time I was up that way they had done a ton of work on the path/waterfront there - figured it wouldn't be long before the town started charging to use the launch there.
 
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Yeah, I figured that out after I postedyesterday and looked at the exploded drawing. It seems that gasketis only available in the powerhead gasket kit which they want like$100 for. I'm in Ottawa for the weekend and I'm going to shop aroundfor just that gasket.
I had a similar problem 3 weeks ago ona particularly windy day. I could get it started with lots ofthrottle, and once started it would idle. Then I'd run up and pullthe anchor, run back, put it in gear and throttle up, at which pointthe motor would die and I'd drift closer to the rocks. So I'd throwthe anchor out and repeat. Did this for about half an hour until itfinally decided it would rev up in gear and get me out of there. That's the thing too, once I get it throttled up in gear, it'll staythere and run at what seems like normal WOT power.
Yes, the free launch is still there atPetawawa Point. They'd have a war on their hands if they startedcharging. The cottage owners on the little islands around there useit to access their properties. I use it every weekend to go afterthose same walleye. I'm starting to get the shakes from lack ofjigging, gotta get this little motor working before I lose my mind.


Thanks again,


Kyle


P.S. Unrelated, but I'm having a hardtime with this forum, the text box causes by browser to behavestrangely, can't type. The cursor is constantly strobing, like it'srefreshing multiple times per second. I have to type replies in aword doc and paste it into the browser.
 
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