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15 hp stopped runnimg

2

2001rotax800

Member
Hello everyone, first off great board!!!! Everyone here seems to have the desire to help each other out, and I am very greatfull.

Now I have been a long time lurker and seem to need some assistance.

I bought a 1993 Mercury 15hp shortshaft this past spring. Never had time to test run it until last weekend, first pull started right up, I was extemly pleased. Took the boat out for variuous rides over the course of 3 hours, then I let a friend takee it for a ride, he only made it about 500 yds down teh river and the motor just quit, like it ran out of gas and would not restart.

In my trouble shooting I have verified both plugs have spark, both have compression, but after changing the plugs, only the top one will come out wet, I have done this 3 or 4 times. Sometimes it wants to start but peters out. My thoughts are either the wet plugs is not getting spark, which I confirmed it is, or the bottom cylinder is not getting fuel. With only one carb, what could cause this?

Thank you in Advance....
 
Clogged reeds could cause only one cylinder to get gas.

Since the motor is "new to you" you have to assume that it has "lacked" maintenance.

These are awesome little motor and almost never break on their own. I personally own three from the model family (a 15 and two 9.9's).

If I were you I would -

if you are even slightly mechanically inclined, spend the 35 bucks on a Seloc's manual - then

remove the carb, disassemble it and give it a thorough cleaning - replace the gaskets and any needle valves that look distorted

while you have the carb off, remove the reed cage (directly behind the plate the carb bolts on to - held on by 4 or 5 bolts) and get a good look at the reeds - there is two sides to the reed block with three petals on each side that feed their respective cylinders - one side could be gummed shut

at minimum, replace the water pump impeller and if any of the seals/gaskets looks worn, throw in a whole kit

change the lower unit gear oil

replace the sparkplugs (NGK BP8HS-15's gapped at .060" - nothing else works worth a damn in the 15's)

pull the thermostat cover (just above and slightly left of the top cylinder) - this model doesn't have a "head" but I'm talking about where the head would be. If there is no thermostat in there (which would be normal - these did not come equipped from factory), spend the 25 bucks for one (plus a cover gasket) and install it. These motors idle considerably better and you will extend it's life with a thermostat

while you are picking up your supplies from the Merc dealer, spend 5 extra bucks and pick up a can of "PowerTune" (spray can) and just follow the instructions - it will clean all the carbon and other cooties out of the crankcase and cylinders

For somewhat under 200 bucks and a few hours of your time you can restore this motor to "like new" - it's life expectancy from that point can easily stretch 20 years with minimal annual maintenance.
 
Thanks, everyone I had talked to said blown lower seal, so I am releived to hear you say it might be the reeds. I am pretty mech inlined so I will be looking at the reeds first. Now the water flow is intermitent and does not to hot where you cannot hold you hand in it but it does get warm so I must assume there is a thermostat in it already. Thanks for your help, I will be sure post what I find, Jesse
 
It could be a lower crank seal, but that would be a very rare problem on this model.

If it was the seals I would suspect that the lower main bearing would be toast as well.

So while you can't entirely rule it out, start with the easy to get to stuff first...
 
Thanks, I bought the online version of the service mannual and I am going to inspect the reed cages first, I really hope that is the cause. I will post what i find along with some pictures, If anyone needs a diagram, or some info out of the book email me. Thanks
 
Well, I removed the carb and reeds and see no apprrent damage so I guess I have determined by process of elimination that my problem is the lower crank seal. I will pull the power head and begin the replacment. btw I did check the compression, 112 on each.
 
Thanks does not look bad. I am still alittle shocked on how it just quit..... oh well, better now than in october on opening day for duck hunting .
 
Is there someway to pressure check the crankcase before I rear it down?
Couldn't I make up a plate fo the carb hole and block off the exhaust and see what the case will hold, or even better put a vacume on it and see what it holds. Any idea what kind of negitive pressure a crankcase sees during operation?
 
You might be able to jury rig something up to do a leak down test (I'm just kinda thinking about it - which I will do out loud below).

Since the fuel pump is on the carb, with it removed the only point you would have to seal is the carb plate - but you would probably have to remove the reeds if you attempt to observe pressure via the carb plate - otherwise any pressure in the crankcase would keep the reeds closed and if it all leaked out through a crankseal etc you would never detect it on the carb side of the reeds.

Likewise, the reeds would have to be closed or removed and the cage blocked off to keep the pressure in the crankcase.

So you would probably have to jury rig a seal on the reed cage (not the carb plate) with the reeds out (but you would also have to separate the cage into the two sides - to isolate the upper and lower crankcases otherwise the upper would just leak into the lower and vice versa) and then go through a flywheel revolution just enough that the piston was creating some crankcase pressure but the piston was still covering the the intake and exhaust ports in the cylinder.

Then if you did get that to work you would have to re-rig it for the other half as well (if you were checking the pressure on the upper half of the case the center main bearing and liner makes a good enough seal that you may not detect a lower seal leak).

Your vaccum test idea would accomplish basically the same thing if rigged up the same way.

Regardless of method, it really is a leak down style test that you need to do - a simple static measurement will do little to tell you if a seal is leaking (or if the two halves of the block have lost seal)..

I have gone through the service manual and can find zero reference to the vaccum pressure required for the fuel pump (even looked through a few other manuals for larger horsepower models) - so have no clue what would be "normal"....
 
Yeah I understand what you are saying and I have been wondereing the same. I just have a hard time beleiving by looking at the seal I see no apparent damage and that what ever damage is there is enough to prevent the motor from running.
 
Another update, in case anyone is following, I removed the power head today and inspected the lower crank seal, it looked as if the inner ring, the part of the seal that contacts the actual crank shaft had grease on it, but when i scrapped a little off with a knife point and put it on my fingers it was pretty stiff, smeared pretty inconsistantly, and smelled a bit like burnt rubber, which leads me further to beleive the seal is toast, the resisdue I am seeing is the seal breaking down and mixing with the burnt particles of the seal material. This motor was not used much, and with out use it seems to have dried out the seals, causing them to fail upon start up after returning to service after some duration(not sure how long). I will post pics as I break the motor down and replace the seal.

Thanks to all who have helped so far, Jesse
 
Excellent observation Jesse and that brings up a good point.

For the past few years now the mfg's have been suggesting that motors be stored "wet".

Instead of just pulling the gas line and letting them run out of gas before storage the new "way to do it" is as follows:

Mix up a concoction of gas, TCW3 oil (mixed at 20:1) and your stablizer then run it through until the motor "smokes like crazy" - then shut it down and store it without running any of the fuel out.

That's their "new way" for both 2 stroke AND 4 stroke models (including EFI models). They state that the mix will keep all the gaskets/seals "wet" drastically extending their life cycle.
 
Do I have to split the case to replace this seal? I appers that if i can remove the seal carefully, I should be able to press the new one in without any problems.
 
Jeff, maybe the lesser of two evils - easier to clean a carb than replace a main seal (???)

If you can get the seal in and out without splitting the block, go for it - you generally shouldn't tear down further than you need to - sometimes opens new cans of worms...
 
its not the carbs, single carb and one cylinder was getting fuel fine, with a leaking crank seal the crank case was not able to produce a vacume to pull the fuel air mixture into the case thus there was nothing to push into the cylinder as the piston travels downward opening the intake port. It is the only thing that makes sense. It looks as if I can just pull the seal and press teh new one in and see what happens
 
Having followed the postings about the 15 horse Mercury with bad crankshaft seal, and removing my power pack, I'm curious if you were able to replace your lower seal without splitting the case. Did the seal fix your motor? Help me Mr. Wizard! If anyone else has input, please respond. Thank you.
 
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