2002 Yamaha 15HP 2 stroke carb mystery?

[old boat NC]

I just bought a 2002 Yamaha 15HP 2-stroke that had been sitting for several years in a garage. Obviously, it wouldn't start... the carb was all gunked up.

No problem, I took the carb apart, carefully cleaned all ports, valves, float needles, etc. Re-assembled, put back on motor, used fresh oil/gas, and it started on the first pull and ran like a champ in my driveway with hose attached.

However, once I got it on the water, when I shifted it into forward, and the throttle is opened wide open, the engine runs like it is starved for gas. HOWEVER, if I pull the choke open about 1/2 way, it runs wide open perfectly! The engine idles and goes into forward and reverse just fine at idle speed. It is only when I open it up in forward that it stalls...

(The vent to the fuel tank is open & the fuel pump looked great when I cleaned it.)

Is there an adjustment I am missing? I only see one adjustment screw on the side of the carb and it doesn't seem to change anything. Or did I not adjust this screw out enough? I only opened it maybe three turns past there it was. (About 5 turns total.)

I think this is something really simple I am over looking. Can anyone help?

Thanks!!!

John

 

[JB]

Re: 2002 Yamaha 15HP 2 stroke carb mystery?

My first thought is that the high speed circuit is not completely clean, but I prefer to do some fault isolation before tearing into the carb again.

With the engine at idle, disable each spark plug in turn. Each should cause the idle speed to drop, by a similar amount. If that doesn't happen you may have a weak or bad cylinder. A compression check would be in order.

Keep in mind that many sick engines will "run like a champ" with no load on them, sometimes on one cylinder.

If the compression checks out on a weak or dead cylinder, check the spark for a hot, blue spark that can jump at least 5/16".

Less likely but possible causes are a bent or broken reed or a leaky gasket preventing scavenging pressure in the crankcase. Gaskets tend to dry out in long storage.

If none of the above turn up a culprit I would go into the carb again, with particular attention to the orifices into the carb throat.

Good luck, and let us know what you find.

 

[old boat NC]

Re: 2002 Yamaha 15HP 2 stroke carb mystery?

Thanks for the thoughtful suggestions...

I will check out the spark when I am at the boat next weekend.

What exactly is the "high speed circuit"? I've never heard that term before.

My therory was that I missed something in the carb when cleaning it.

Do you know why it runs so well when the choke is pulled out slightly?

Thanks again for your help.

Also, if this helps, the motor has only ever been run in fresh water. And the the guy I bought it from said he only ran it a few times before storing it in his garage ... he claims 10 hours on the engine, max. Something I believe after looking at the spark plugs, and general condition of the engine. It is spotless.

John

 

[JB]

Re: 2002 Yamaha 15HP 2 stroke carb mystery?

The "high speed circuit" is that system in the carb that supplies fuel to the airstream when the throttle plate is fully, or near fully, open.

It differs a bit from one type carb to another, but the orifice into the carb throat is usually located right in the narrowest part of the venturi.

 

[locallawoff]

Re: 2002 Yamaha 15HP 2 stroke carb mystery?

Gidday from down under,When you strip the carby and have access to a air compressor use it to blow all the holes you can find.Locallawoff