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I've posted here before about my 1989 Yamaha FT9.9 4 stroke. Last year it had a confusing issue where it sputtered when hot. Anyhow I cleaned the carb very well (carb soak and ultrasonic cleaning) and that issue is gone. But it's been replaced with another.

To test the hot sputtering issue I made a test tank out of a 275 gallon tote. It seems to work great, and allowed me to prove that the sputtering is gone.

However I noticed after running at high throttle for a while that the engine started exhausting white smoke and dribbling out a nasty sticky black gunk from the upper exhaust hole. This is the first time experiencing any smoke since I figured out how the pilot screw worked. The motor had good (119/110) compression in the past but I don't have a tester and so haven't checked since running in the test tank.

Is it possible that running in a test tank with a normal prop (it's a high thrust motor) presumably at low RPMs could cause this issue?

My first instinct was to check float bowl height but it was fine.

Any ideas? Thanks!

I described in my previous thread an issue with my Yamaha 9.9, which was "making water" in the oil sump. We did two things over the winter 1) replaced the thermostat (which was not functioning), and replaced the electronic control module.

I recall that you have replaced your thermostat too. Is that right? The ECM was causing my timing to not advance, which might be a similar problem if you are having trouble at higher rpms.

--john

I've always taken white smoke to mean water in the cylinder or exhaust stream. Black means too much fuel, and blue means oil is being burned. But those are just the rules of thumb I've always used.

Someone had posted about wet goo from the exhaust port a long while back, but I doubt I could find it again. Sounds like 'wet-stacking' in a diesel, especially a 2-stroke diesel like the old Detroit Diesels, caused by running too cold. Happened a lot when an engine was running with light or no load, and they would make a slobbery mess. Not as bad with 4-strokes diesels, but can still happen. But I've never seen that happen with a small gasoline engine.

So basically, I'm no help at all here.

Interim wrote:I recall that you have replaced your thermostat too. Is that right? The ECM was causing my timing to not advance, which might be a similar problem if you are having trouble at higher rpms.

Hey John,

I'm not sure what you're referring to as the "thermostat". Mine has to bimetal valves and a diaphragm on the carbuerator that regulate starting based on temperature. I've not been able to figure out how they work or what they do. The diaphragm is a little stretched, but I think it still does the job.

The engine has spark on both cylinders at both idle and high throttle.

DRAIN OIL
POUR DIESEL IN OIL RES
WAIT A FEW DAYS
DISCONNECT SPARK AND ROTATE ENGINE
WAIT
DRAIN
IF SUPER NASTY REPEAT
DRAIN AND FILL WITH FRESH OIL

fritz3000g wrote:

Interim wrote:I recall that you have replaced your thermostat too. Is that right? The ECM was causing my timing to not advance, which might be a similar problem if you are having trouble at higher rpms.

Hey John,

I'm not sure what you're referring to as the "thermostat". Mine has to bimetal valves and a diaphragm on the carbuerator that regulate starting based on temperature. I've not been able to figure out how they work or what they do. The diaphragm is a little stretched, but I think it still does the job.

The engine has spark on both cylinders at both idle and high throttle.

Fritz--

I think there is a coolant water thermostat. A mechanic replaced mine, so I didn't see it in person. But it makes sense that it would stop raw water circulation until temp gets up, so I didn't question it.
--john

https://www.vansoutboardparts.com/yamah ... 6840,67607

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