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bcripps 10-11-2014 10:54

Erratic RPM's - Yam15hp
Since new, this Yammie has never run as well as the one I bought back in 1993, but has been reliable.

Engine is a 2009 two stroke, (Yamaha 15MSH. Model #684KS1057789), that I use daily for the dingy. Still starts well, choke and a couple of pulls in the morning, but sounds like a bucket of bolts on the way to the dock, a quarter of a mile away. It idles ok but if I shut down the fuel right after starting, I'll get a couple of misfires.

RPM's drop and rise constantly, every second or two, enough for me to feel the dingy surging, but not so much anyone would notice who is watching. I'm guessing a couple of hundred rpm's, up and down.

The engine seems to rattle like something's loose and I thought it was the cowling because when I put my hand on it, things quieted down, but now I believe the sound is just reverberating through the cowling.

Changed plugs. Cleaned and changed fuel strainers. Ran 6 gallons of fuel through it with PEA fuel system cleaner. Stuck a piece of rag under the cowling catch.

I hardly ever run the engine above quarter-throttle; as indicated by the little bars on the grip. Never bring the dingy up on plane.

And things are much better when the engine warms up, but still erratic rev's.

Does this thing just need a tune-up? Or is a bearing going?

No decent mechanics where I'm located so will have to do my own work. (And hopefully learn something.)

Would appreciate any ideas as to how to make this little baby sing!


Cheechako 10-11-2014 11:01

Re: Erratic RPM's - Yam15hp
hmmm... sounds like an OB with the vent screw on the tank shut. Yours may not have that type of vent though if newer. Blocked vent or tube?
If you were under higher power it could be a prop hub problem... but likely not.
Is there a mini fuel filter in the motor housing on those? Some OB's have them inside where the fuel tube comes in....
I had one of those great mid 90's Yamahas... a main bearing went out and it vibrated and wouldn't reach optimal rpm.. and surged... but yours seems awful new for that....

bcripps 10-11-2014 11:53

Re: Erratic RPM's - Yam15hp
Thanks, Cheechako:

That's a good call, but the fuel vent is open and taking the cap of the tank makes no difference in performance.

There is a fuel strainer under the cowling, which I cleaned, and an in-line fuel strainer that I installed myself. Everything looks clean and proper. Fuel is fresh.

I had water in my gas a couple of months ago and the motor refused to start until I blew out the lines and drained the strainer. That's when I changed the plugs (with the proper replacements), and to tell you the truth, I can't remember if the erratic rev's started before or after that event.

The RedLine fuel system cleaner with PEA seems to have helped the engine in general, but not the problem I'm having.

Thanks for your input...


SVNeko 10-11-2014 14:38

Re: Erratic RPM's - Yam15hp
Sounds like you have one cylinder out. But try this. It helped me.
Won't Idle - Here's what I did to fix it! - Yamaha Outboard Parts Forum

bcripps 11-11-2014 07:09

Re: Erratic RPM's - Yam15hp
Thanks, SVNeko... I'll give that a try! Sounds promising!


KnotCrazy 11-11-2014 07:46

Re: Erratic RPM's - Yam15hp
Not a whole lot to them....Fire, air and fuel. Has to be jetting or air.


a64pilot 11-11-2014 07:54

Re: Erratic RPM's - Yam15hp
Erratic RPM or surging is usually lean mixture, rich tells on itself as excessive smoke like you left the choke on.
Gets better as it warms up generally points to being lean as engines need a richer mixture when cold, hence the choke.
If it's only at idle or just off idle, then it's likely the low speed circuit, but not much above idle, your off the low speed circuit and onto the main.
The misfire is indicative of a lean condition also, sounds either like carburator or a vacuum leak.

Neither explains the bucket of bolts sounds though, that sounds too much like a main bearing to me, and a bad bearing can cause the crankcase seal to leak, which can cause your running issues
Carry a paddle.

a64pilot 11-11-2014 07:56

Re: Erratic RPM's - Yam15hp
This motor wasn't ever sunk was it?

bcripps 11-11-2014 10:02

Re: Erratic RPM's - Yam15hp
SVNeko: Too easy I'm afraid. No such screw found on that side of the carburetor. The attached photo shows what I have.

a64pilot: Thanks for joining in. I can fiddle with mixture but could use a little instruction. Is it a matter of twisting a screw, one way or the other? I'm in the middle of nowhere without a mechanic in sight!

Thanks, guys...

bcripps 11-11-2014 10:07

Re: Erratic RPM's - Yam15hp
1 Attachment(s)
Oh, The only water this engine has seen is rain water!

azsailor12 11-11-2014 10:45

Re: Erratic RPM's - Yam15hp
What does the spark plug look like? Too lean causes the insulator of the plug to look white vs light brown for good jetting.

a64pilot 11-11-2014 11:07

Re: Erratic RPM's - Yam15hp

Originally Posted by azsailor12 (Post 1675349)
What does the spark plug look like? Too lean causes the insulator of the plug to look white vs light brown for good jetting.

This is exactly correct, a plug reading can tell you a lot, and oily / wet is too rich or possibly a bad ignition system. Look to see if both plugs are the same color. Plug reading was the old school way of jetting carburetors and is surprisingly accurate.
Idle mixture screw is exactly that, it adjust the mixture at and slightly above idle, not anywhere else.
An idle screw can either adjust an air bleed or most often it's fuel, just one way turning it in makes it richer while the other is opposite. Doesn't really matter, you adjust it so that it runs at it's highest RPM when fully warmed up.
Often now days , it's under a plug that has to be removed, that came about as a result of air quality standards.

I'd find someone who knows what they are looking at, your box of rocks sound, doesn't sound good to me.

bcripps 11-11-2014 11:17

Re: Erratic RPM's - Yam15hp
I wish I could provide you with a sound bite!

a64pilot 11-11-2014 11:29

Re: Erratic RPM's - Yam15hp
If I were a betting man, I'd bet your adjustment screw is under that brass plug that is at the upper left portion of your picture, but don't go drilling it out on my word, verify that before you do.

SVNeko 11-11-2014 11:44

Re: Erratic RPM's - Yam15hp
I can't see a pic but the screw should be there. It may be under a little brass cover, which you have to drill out. That was mandated by the EPA so you couldn't mess with the mixture.

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