Tohatsu M9.9 (2-stroke) very smokey but good compression

[bfriendly]

I picked up a 90's tohatsu 9.9 longshaft as-is with no history (same basic engine as M15, and also nissan 9.9hp, 15hp, 18hp) and did a tune up incl. new plugs, impeller, gear oil, and a full carb cleaning.

I am running 50:1 quicksilver semi-synthetic oil and new gas. The oil is measured carefully. I am sure that the lines are good with no air leaks and good fuel filter. I've adjusted the carb idle screw to spec. It fired right up and ran but smokes about 2-3 times more than any other normal 2 stroke I've seen, and doesn't clear up at all once warmed up. At idle there is maybe 2 times the normal expected amount of white-ish "cold start 2 stroke" smoke, but when revved up, even a little more, it smokes a lot

I checked the compression and both cylinders are right at 100psi (I believe my gauge reads a little low based on previous experience). Both plugs are wet and not steam cleaned, nor any signs of water droplets. With a flashlight both cylinders look good (and similar to each other) with normal amount (but not a lot) of black (not perfectly clean).

I ran it with and without the thermostat, and tried a full decarb procedure, and new plugs. The engine is pumping a good amount of water out the exhaust/tell tale area half way up the leg, and also from the opening below the lower swivel mount. After the decarb I ran it on the lake for a few hours, and it still smokes just the same, and still has good even compression.

The engine seems to run well, although a little rough, perhaps, at low speeds.
Despite the good compression, and lack of signs of water entering the cylinders, the only thing I am able to come up with is - possibly blown head gasket? Is there any way to know for sure, short of doing a leak down test?

Any other theories or suggestions as to what to check out would be great. Thank you!

 

[Scott Danforth]

100 psi isn't stellar

 

[bfriendly]

So you think it may need rings, and this is causing the smoke, or ?

After running it hard for about 15 min today, I tested compression and got 90psi on both cylinders hot, same compression gauge. After it had cooled down for the day I got 100psi again on both. Does this say anything? Plugs were both slightly wet from fuel, and slightly black as they normally would be. The bottom plug may have been ever so slightly cleaner (less black) than the top.

One thing I am not sure about is the choke flap in the carb. This is an electric choke/electric start model with a manual choke knob also.
With the choke pushed in (carb not choked) the flap in the carb is not opening the flap fully horizontal, it is only open part way. I am reading some places on line this may be how the 15hp is detuned to 9.9, but I want to be sure all the choke linkages etc are all right. That said if I prop the flap open fully horizontally with a screw driver, the smoke stayed exactly the same.

After running it hard the amount of smoke does seem to have gone down slightly, but still not anywhere close to normal levels..

 

[ondarvr]

Pull the head off and look at the condition of the gasket and cylinder walls.

 

[pvanv]

Assuming your model is M9.9D2. The choke butterfly should be fully open, however, a wee bit less than fully open will not hurt. No, that is not how the 15 is made into a 9.9. You need the thermostat, or the plugs will foul fast. They do idle a bit rough. As long as compression is even between cylinders, it sounds OK. If you have much too much smoke and have done a good decarbon, you may be running a bit rich.

 

[bfriendly]

The model is m9.9b2

So again last night i ran it hard for about 15min in gear (thermostat is in, that was only a temporary test with it out) and right after i got 100psi on top cyl and 95psi on bottom. I put a drop of oil in cyls, pulled it over a little, then tested again and got 120psi on both. After cooling down for a few hours i got 125psi on top and 120psi on bottom.

Compression test procedure is the same, 7 hard pulls back to back

I am reluctant to pull the head because i dont have a new gasket on hand, the motor runs now, and despite lots of penetrating oil and heating w/ a mapp gas torch.. I have had a bad string of luck breaking head bolts off (usually the lower ones that require pulling the powerhead to drill out). But those were yamaha and suzuki. So i would like to rule everything else out for certain before pulling the head.

Paul if it is running rich anything in particular to pay attention to if i tear down the carb again? As mentioned i have alread rebuilt it and carefully checked all orifices, and blown every passageway out with carb cleaner and compressed air. Carb was not gummed up to begin with... and i have also tried various settings with the low speed idle screw.

As i have tested with the butterfly valve propped fully open...
I am sure the butterfly valve is not the problem, but i have been looking at the manual and parts diagrams trying to get it right. This is the model w/ electric choke actuator with 1 wire going to it, and manual choke also.
Today i tested again and 115top, 110psi bottom
P.s. it is still smoking just as bad

Thanks for all the help! Any other thoughts appreciated!

 

[km1125]

When you're doing the compression test, are you setting the throttle wide open? (sorry if that sounds like a silly question, but I don't know your history).

 

[pvanv]

Whoa. The 9.9B series is from the 1980's. In salt water, that is really antique. In any event, it is super-obsolete, and very few parts are even available any more. I would definitely not attempt to pull the head, as you will not find a head gasket anywhere. No carb kits, either. If it's running, I would not touch it. When the day comes that it stops running, it then becomes a display item.

 

[bfriendly]

this one is 1994

The throttle has been all the way open when testing compression

the head gasket 351010051m or 35101-0050 is in stock several places online

 

[pvanv]

this one is 1994

The throttle has been all the way open when testing compression

the head gasket 351010051m or 35101-0050 is in stock several places online

Sorry, yes, that's the same gasket that fits the D2... But... Good luck getting any carb kits. Treat the motor as well as you can, and you may get a few more years out of it. To what end would you pull the head? It's running OK but smokey now. IIRC, those motors always were a bit smokey. If you find a bad ring (unlikely, since the compression is OK), would you rebuild a motor of that vintage, knowing that you can never get carb parts?

 

[Sea Rider]

Some issues...

1-Have you tried runnning normal non synthetic 2 stroke oil mixed 50:1, to see if smokes same, less ?

-If OB is easy to start say, second/third pull works ok.

2-Try piston rings, piston decarbinization with OMC Power Tuner or CRC Engine Tuner, aply as per instructions.
*Apply at least half can while OB runs on muffs and geared forward between 2-2.5 K revs, will smoke a lot. Go for a one minute wot spin after cleaning procedure to clear all product remain out.

3-Butterfly must close fully when key is pushed in, check that fully opens when key is released out.
*If jamming, could have a miss aligment on solenoid plunger, with OB OFF push plunger fully down with finger and release it, if jamms, there's your culprit, realign solenoid height with side screws till plunger works flawlessly. Apply thin oil drop to both sides of butterfly's shaft for smooth axial operation.

-Head gasket failure won't show at a compression test, only when it's completely blown up/kaput .It's useles to remove head gasket to have a look, can't be put back again. Head gaskets have a special glue coat at both sides that melts and seals both mating surfaces, that's right after head gasket has been properly torqued and OB is put into operation. New head gaskets must be retorqued to specs after 20 hours of operation.

Happy Boating