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Merc 15hp compression figures

T

tomf

Regular Contributor
"Hi all,

apologies if this

"Hi all,

apologies if this is one of those repeat questions that gets asked all the time... but what kind of compression readings should I expect from a 15hp 1993 Merc 2-stroke? My first tests give readings of about 77 psi for the #1 cyl and 71 for #2. Seems awfully low, or is this normal for an engine of this power range?

The caveat is that I'm not 100% sure about my readings since my meter is a cheapo model that seemed to have some trouble getting a tight fit. But knowing what I should be seeing would make troubleshooting easier, whether it's the engine or the meter.
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The engine is recently acquired with an unknown service history, so I haven't had any major diagnostics or shop work done yet. Seems to run fairly well, though, on the first quick test drives."
 
"Tom the factory spec for thes

"Tom the factory spec for these is not even published by Merc. My experience has been somewhere in the 90-100 psi is "normal" - but as you say, you are not sure of your gauge.

The 6 psi difference between them is good. If the motor runs ok, I would simply suspect that your gauge is not calibrated and that your compression is fine.

Your motor belongs to a model run that produced the best 15 horse motors of all time, by any mfg in my opinion. The split block design means no "head" - so no leaky gaskets etc.

Unless you have chewed a ring you are unlikely to have a compression issue and aside from a couple of motors I've seen where someone tried running unmixed gas, I have never seen one of these eat a piston or rings - just a super durable little motor...."
 
"OK, that's good to know.

"OK, that's good to know. I've heard positive things about the 15hp, so I'm prepared to put some effort and a few $ into getting this baby to run well.

So far it seems to work pretty nicely, with only some "coughing" (sneezing?) in the low throttle range. Idling and mid to high RPM seem fine as far as I can tell, and it does move the boat at a pleasant speed, GPS'ed at about 20 mph with two people and a slight headwind.

Before I got it, the motor had been sitting (outdoors, ugh!) for at least a year, so next on the agenda are a thorough carb cleaning and adjustments according to the shop manual. I'll see what that does and might then post some further questions in a separate thread.

Thanks again for the advice, it's already been very helpful in flattening the learning curve!"
 
"Tom, the cough or sneeze is m

"Tom, the cough or sneeze is most likely only a minor adjustment on the carb.

I have a 91, 15 horse - over 1500 hours on it (don't use it as much as my pair of 9.9's, basically the same motor as the 15, that have over 2000 hours each), but have had "zero" trouble with any of them (the 9.9's are 86 and 87 models respectively).

A new pair of plugs each spring, change the gear oil and a new impeller every second year and this motor could well last you forever.

20 mph sounds about right - I run mine on a 14 foot deep v aluminum, console model, with me and the wife (plus junk for the day etc) and get that speed - so sounds fine there.

I do have the factory service manual for this model (and have been working on them since the line came out in 86), so if you get stuck on something etc, just drop me an email and I can send you over the relavent instructions etc..."
 
"Cool, thanks.

Having spen

"Cool, thanks.

Having spent the last week and a half poring over Clymer's shop manual and 3 or 4 general outboard repair books, everything is starting to look more comprehensible and a lot less intimidating.
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The Seloc edition for 1990-94 Merc/Mariners is also in the mail, so between the two I imagine I'll do ok. But I'm sure there will be a lot more questions arising along the way."
 
"[i] >Tom, the cough or sn

" >Tom, the cough or sneeze is most likely only a minor adjustment on the carb.

Have to follow up on this discussion since your guess was so spot on, Graham.

Long story short: Cleaned & blew out the carb, adjusted float level, put boat in water, still coughing.

Half a turn counterclockwise on the idle mixture screw - end of problem. Motor purring like a Cadillac at all revs.

Sometimes things are beautifully simple and simply beautiful.
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"
 
My 1999 mercury 2-stroke outboard has similar issues. It hits on the first pull and then won’t start. Starter fluid eventually gets it running. Then starts like a charm rest of day. The next day, it needs starter fluid to run. I rebuilt the carburetor with no change. New fuel hose, fresh gas too.
What should I try next.
 
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