Yamaha 15 hp two strokes dies at low rpm

[mikekohut]

I had my Yamaha 15MSHU two stroke overhauled last year. The first time I used it in December it started on the first pull and ran perfect. In February I went to use it and it would only start at a very high RPM setting. After it starts it runs fine until I reduce the RPM setting down below the starting position and then it dies. It almost sounds like the spark quits. I was thinking fuel pump, that doesn't make sense that it would run at high RPM and not at lower RPM. Anyone have any experience with this type of problem? Mike

[TownsendsFJR1300]

Sounds like the low speed jet in the carb is clogged(varnished up), especially if it won't idle once warmed up.

If it does idle when warmed up and you have an automatic choke solenoid(prime start), it may not be working when needed cold..

A few more details would help as well as a year of the engine..

Leaving un-treated/un-stabilized fuel in the engine will easily clog the jets as they are extremly small. Pulling the carb and float bowl will allow you to access them(low speed jet and main jet). Make sure you physically pull them out and can SEE thru them. JUST BLOWING/CARB CLEANER DOESN'T COUNT..

Bringing up the parts fisch for your particular engine, in red, above), will show you exactly where their located..

[mikekohut]

15 HP 2 Stroke

Thanks Scott. I have the diagram printed out and will check out the low speed jet. The engine is a '96 two stroke. After I used it in December I was using fuel with stabilizer in it and pulled the hose and ran it until the fuel ran out. I may have picked up a piece of crap and clogged the jet. I will try to get at it next week and let you know how I make out. Mike

[Yamahataos]

I have a 2011 2stroke 9.9 modified to 15 hp the 9.9 hp and 15 hp are the same engine besides for the reed valves that's the only part difference and I have a similar problem at low speed , like others have said it could be the low speed jet plugged or the low speed mixture screw circuit pluged mine had a plug over the screw I had to remouve to get the mixture screw out to clean it , also you should be using fuel that contains no ethanol as if used will cause varnish in the Jets of the carb and cause poor running conditions even if you run it out of fuel as I had a problem with another new small engine that I would shut the fuel valve off each time I used it and running it until it stopped running and even doing this cause the jet to plug up when I used regular fuel with 10 % ethanol so now I use premium fuel because in canada the premium fuel at most gas stations contain no ethanol .

[mikekohut]

Thanks. I have always used non-ethanol fuel in it with a fuel stabilizer.

[mikekohut]

Jet

Townsend,

You were spot on. The low speed jet was completely plugged with a white calcium looking substance. It was in so hard that I had to drill it out. Cleaned it and put it back together and it runs great. Thanks again. Mike

[99yam40]

If you had to use a drill , the jet is probably not the same size and shape it was originally.
Might want to order a new one and just replace it.

[TownsendsFJR1300]

Townsend,

You were spot on. The low speed jet was completely plugged with a white calcium looking substance. It was in so hard that I had to drill it out. Cleaned it and put it back together and it runs great. Thanks again. Mike

Glad to hear and your very welcome Mike.

As 99Yam40 noted, if you weren't super careful drilling that jet, I'd probably replace it. It doesn't take much to change the jetting of it (and if larger, will run richer on the bottum end).

Just as a side note, running Yamaha's "Ringfree" is a fuel system cleaner / top end cleaner (isn't cheap) but works extremly well.

Unless you drain the carbs with the drain plug (if it sits for awhile), it'll goo up again W/O that additive.. Well worth the investment (and time savings IMO).

[mikekohut]

Thanks

I used an undersized bit and drilled only enough to break loose the deposit. I do not think that I damaged the jet, but will buy one just in case I did. I will also buy the Yamalube Ring Free. I always run the fuel out when putting the engine away by removing the fuel line. But I don't drain the bowl on the carb.

Do you use the Ring Free in your 150? I just bought a new Robalo 206 Cayman with a Y 150 on it.

[rodbolt17]

you simply CANNOT empty the fuel bowl by running till it quits.
you can by using the drain.

if it idles well don't sweat the jet.
I use wire torch tip cleaners on them occasionally.

[TownsendsFJR1300]

.

Do you use the Ring Free in your 150? I just bought a new Robalo 206 Cayman with a Y 150 on it.

I use Ringfree 100% of the time in my F150, motorcycle and all my gas powered engines. I usually mix it stronger than required.

I've run it in engines that wouldn't idle down, had a slight misfire at 3600RPM (WOT) (Briggs 8 HP chipper / shredder) and it cleared out the misfire and it idled down on its own with NO other adjustments/fixes.

I also run K100 in the OB (strictly Ringfree and K100 ONLY), both work great in my experiance. The fuel tank is 1997 vintage, NEVER been out, never been flushed, original screen in the PU.

As Rodbolt noted, running a carb dry will leave a little fuel in the bowl, which, if un-treated, WILL varnish up.

I had to clean the main jet on a another 8hp Briggs generator when my brother in law ran it (no additive) and stored it (after running the fuel out). Pulled the main jet, sure enough, clogged (in a MONTH). Cleaned it, started first pull. Once running, we drained the fuel, I pulled the bowl, made sure it was dry and re-assembled, done.

For your engine, you should have a drain bolt on the float bowl(s). If the boats going to sit for any lenth of time, simply pull that bolt/plug, let dry and re-assemble..

I can't remember the last time I pulled one of MY carbs apart. Customers, yes...

[robert graham]

Yep, drain carb float chambers whenever motor won't be run for any extended period of several weeks or more. Put a rag under carb so when you drop the drain plug it don't fall down in some dark place. Draining carbs several times a year is just darn good preventive maintenance and a lot easier/cheaper than plugged jets/poor running/lean mixture conditions...

[mikekohut]

Scott, Robert and Rodbolt,

Thanks for all of your excellent advise. I am getting the Ring Free for the little engine and will also use it in my new F 150. I will also be pulling the plug on the carb bowl when ever it is going to sit for any period of time.

Thanks again, Mike

[TownsendsFJR1300]

Your very welcome. You'll like the new engine.. I wouldn't run ethonol in it....

Just an FYI (that'll work on any engine).

I rigged this up awhile ago (as I run in brackish/salt water) as I had the pitot tube clogging up on me occassionally. You don't necessarily have to use the air compressor but at least a shot of fresh water into the pitot hole will help keep it clean and open;

http://www.yamahaoutboardparts.com/f...g-th23159.html

Cheap and easy (if you have the plastic hose / water pressure speedometer). Works great. The speedometer, after 8 years, still reads down to 4 MPH (that's as low as it goes stock)..