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1987 25HP Merc "Coughing"

R

rmcderm313

Regular Contributor
Hi Folks,

My 1987 25HP Merc is "coughing" periodically. That's the best way I can describe it. When it does a puff of white smoke comes out of the drive housing. Please see the video. This was using fresh gas and I've just rebuilt the carb.

Any thoughts as to why this is happening? I'm going to post a separate question about the water coming out of a hole in the engine block. You'll probably notice the water in this video. I mention it here as well in case it is related.

Thanks,
Rob

 
Classic symptoms of a motor running too lean on the low speed agjustment or carburetor open when it should be closed at idle.
 
Thanks. So should I be backing off the idle adjustment screw (over the carb opening) to see if it clears up? The carb butterfly is closed at idle.
 
You might have a bad lower seal that's allowing water to come in (and exit at that hole). That would also explain the stumble.

The lower seals on those are easy to change without slitting the cases.

Jeff
 
I was able to open that fuel mixture screw and eliminate the stumble ("cough") so thanks for that tip. When it finally stops I do feel like the motor is idling way to high to be good, it's probably 5-6 turns out when it finally stops. How can I tell if my perception of the motor idling high is correct? and if so, what would cause an engine to need to idle high to avoid those misses?

Thanks for the replies...
 
I believe he is talking abouit the seal at the bottom of the crankshaft.-------To get the motor to slow down at idle you will have to look at making adjustments to rotate the stator assembly that is under the flywheel.--------Once throttle plate is closed timing is retarded to control idle speed.----6 turns out seems a lot. there may be other issues with the motor.
 
What types of other issues with the motor could be causing it to need to run high? Compression is good and when I do have the fuel mix rich it runs pretty well. What is actually happening when the motor coughs/sputters like that? Is that sound the plug firing into a mix that doesn't ignite correctly? Are there other things that I can check out for issues that might be leading to the coughing at a leaner mix?
 
What happens is the mixture is too lean to fire.-----During the next revolution the lean mixture is expelled into the exhaust.----Then the next stroke a richer mixture is in the cylinder and it fires.----Then when that burning mixture is expelled into the exhaust there is fuel there and because the ports are open you hear the " cough / sneeze " from the engine.------A simple study of the 2 stroke pricipal or U-TUBE videos might help you understand this concept.
 
Very helpful. I have reviewed the 2-stroke principal so your explanation made perfect sense. I just couldn't figure out at which stage and what part of the motor the pop was occurring.

Thanks.,
 
I believe Jeff may be on the right track regarding the lower crank seal.

If I'm remembering correctly it's almost identical to the upper seal (Merc calls it an oil seal).

It's a thin metal (cup - kinda looks like the little cup on a tea light candle with a hole in the middle) the hole has a little rubber (sleeve) around it with a round coiled spring integral to the rubber (which is integral to the metal cup).

Over time, from the rotation of the crankshaft, the rubber gets worn out and tears leaving a big gap (the spring starts rotating as well around the shaft = no seal.

Now the motor can run apparently well even with a torn seal (my 15 horse had a torn upper for 3 years before I got around to finally replacing it - my motors always end up at the end of the work list :)).

One of the "signs/symptoms" of a busted seal is having to richen the idle screw or fiddling with the timing (neither of which should need adjustment after initial set-up).

If the seal leak gets bad enough, such that it's now sucking enough air that it drastically changes the air/fuel ratio, you need to add more fuel (many more turns on the idle needle than normal) to compensate for the extra air in the crankcase.

However, if it's only the lower (or upper) seal as the case may be, you end up with a "balanced" cylinder and a very rich running cylinder (one plug significantly darker than the other) - plus you are burning more fuel and loading up one cylinder with more carbon - all bad scenario's.

The seals can easily be "picked" out and replaced with simple tools (I use a socket just a hair smaller than the race that catches the edge of the seal and tap them back in with a hard mallet).

The seals themself run about 13 bucks each - it's a quick job and since you just replaced the base gasket (so know how to pull the powerhead as well), it shouldn't get torn up if you pull the powerhead to get at the lower seal (normally you would replace the base gasket along with the seal).

So for the minimal cost and an hour of your time, I would be inclined to replace the lower seal since it is indicative of your current issue (and would also have accounted for the cough).

At worst, you would never have to do it again for the life of the motor (or at least 20 years)...
 
Right. That motor has a hole going right through the crankshaft so 2 stroke oil can lube the splines. That lower seal is the only way to keep water out of the splines (and, hence, the motor).

Jeff
 
The guys got it correct here - 7 and 11 are the upper and lower seals.

They are fairly small (not even as big around as a quarter).

Merc calls for them to be (packed) with 2-4-C with teflon. I only ever use high speed lithium grease (use the same thing in wheel bearings on cars/trailers etc) - generic brand from discount auto places and it has worked fine for me for years - unless you want to pay OEM prices for a bit of grease.

I usually "gloop the snot out of them" and then tap them in place leaving whatever oozes out alone - figure it maybe gives the seal a few more years of life.

You can pick out the old ones with a screwdriver or an awl (a seal picker works awesome if you have one but wouldn't buy it special)..
 
Well that didn't go as smooth as I planned.. The seal just wouldn't budge. I tried being gentle but the rubber inside the seal just kept giving way every time I tried to get a grip on it with a screwdriver. I finally got it out, but I had to be rougher with it than I had planned and in the process I nicked the metal around the edges :(

Am I in trouble here? Will this affect the seal to the point that it will cause me problems? Is there anything I can do, such as some additional sealant that I can add when I replace the seal?

Here are some photos of the "damage". BTW, some of this damage was already present so I assume someone before me had trouble getting this seal out.
Any help is appreciated.

photo 3 (7).jpg
photo 2 (12).jpg
photo 1 (12).jpg
 
That crank is about 60 C on the Rockwell scale--it doesn't gouge! Just be sure to seal the outside of the seal with something and you'll be fine. I'd use some epoxy here--putting Lock Tite anywhere near a bearing would scare me!

Jeff
 
Thanks guys. Probably not as dire a situation as I thought last night. Yes I lost sleep over my lack of patience getting that damn seal out. I should have walked away last night and taken a fresh start this weekend. I don't even have the replacement yet. :mad:

Sorry for what will probably be a dumb question, but could you guys elaborate on "Lock tite" and "epoxy"? I'm familiar with both to an extent, but first on Lock Tite, which formulation? I assume you wouldn't want anything that would permanently affix the seal? Aren't there different formulas for different purposes?

And on "epoxy", I was always under the impression that it was a permanent bond. Am I wrong on that? Is there just one type of epoxy?

And finally, Jeff, what is your reason for recommending epoxy over lock tite? What characteristic does it have that makes it a better solution in this case in your opinion.

As always, many thanks to everyone who is helping me out with all of my questions. This started out as a half-hearted attempt to try and get a cheap motor running, but it has turned into an obsession (in a good way) to learn as much as a can about how this motor runs. I'm very thankful to all of the guys with deep knowledge that are willing to take the time to help out a newbie.

Rob
 
Epoxy is a two part, thick glue that bonds nearly anything. (I recommend J B Weld Quick--available in most hardware stores.)

Lock Tite is an a thin liquid adhesive that flows where it shouldn't go sometimes and can cause problems--like if it gets into the lower main bearing. Not good!

Jeff

PS: Should have posted this before: The easiest way to get a seal out is with sheet metal screws. Poke a hole in the metal shell with an awl, then tread in a small screw or two. Grab them with pliers and out the seal comes.
 
Jeff, I have some Permatex Aviation on hand, do you think that would be OK to seal the outside of the oil seal?

Thanks as always,
Rob
 
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