DaySailer.org Forum

This post isn't really related to daysailers, but thought I'd see if anyone might be able to help.

A 1998 Yamaha 9.9 hp outboard has come into my hands. I wouldn't be surprised if it hasn't run in a year, but the engine oil is clean, and spark plugs look ok (not great, but not fouled or rounded).

I changed the fuel, and started it. It will run if I nurse the throttle, but it won't idle (or maintain any rpm). If I leave it at a particular rpm, it will die within 10 or 15 seconds. But then it will start again easily.

Any thoughts?

--john

Assuming it's a 2-stroke, are the exhaust passages in the lower cowl, midsection and lower unit clear? If they're partially clogged with oil residue, a buildup of exhaust back pressure would explain the stalling followed by easy restart.

good questions. can I just disassemble from the bottom up to check?

--john

Generally, yes, but it depends on the motor, and I don't have specific experience with this one.

Because it starts up easily, I would guess that it has good compression and the ignition system is good. That pretty much leaves the fuel system. I had not thought about plugged exhaust but that's a good thought. If the fuel lines from the tank to the motor are good and have good seals and the bulb in the fuel line stays firm I would next look at the diaphragm in the fuel pump, next I would go for a carburetor kit. Also, if it has been sitting around for a long time, other rubber parts may have cracked or hardened. In particular, make sure that it is pumping good water and if not put a new impeller in for the water pump.

Though,I don't have that motor and these are just general thoughts, I do think you got a good one. Those Yamaha's are really dependable.

Thanks, KC.

Yes, it is pumping water well. I put "earmuffs" on it when I run it and there is a strong stream coming out.

I'm going to replace the fuel filter tonight, and if that doesn't help then I'll take it to someone for a carb kit. I did a carb rebuild on an old MG, but the consequences of failure there are walking. On the water, it could be worse.

Thanks.

--jf

John,

I don't know if this holds true for 9.9 hp engines or not, but on my 3.5 Nissan, I found out after I had paid a mechanic to rebuild my carb that I could have bought a brand new, genuine Nissan carb from Boats dot Net for substantially less. It might be worth looking into.

Chris