Piston condition

[JimmyG]

I pulled the head off my '94 70hp last night to verify condition of pistons and cylinders. 2 of the 3 pistons have a flaky black 'soot' on the top of them, 70-90% coverage. The other piston seems ok. the motor has a history of stalling and rough running. Is this from too much oil in the mixture?

thanks

jim

[Ken]

Jim,
Probably from clogged carburetors and/or not running a detergent like RingFree or SeaFoam in the fuel.
I assume you have a pre-mix. Should be running a 50:1 fuel/oil ratio. That's 1 qt oil to 12.5 gallons of gas.
Motor stalling and running rough is probably from clogged carbs. May want to consider a carb teardown, cleaning, and kit install.
If it has been more than a year since new plugs,
put 'em in.
If that fuel pump is more than five years old,
get a new one.
Two strokes build up a lot of carbon from burning oil - you need to run a strong detergent like Yamaha's RingFree or SeaFoam in the fuel to keep carbon buildup at bay, and to keep the fuel system clean. Some owners run a detergent all the time. I like to run SeaFoam about every third tank - 1 oz to 1 gal of fuel, and then I do a "shock" treatment once a year.
Good luck [img]smile.gif[/img] ,
Ken K

[JimmyG]

Good advice, thanks. Its a precision blend, so I'm going to replace the oil pump as well ( I have rebuilt the carbs and replaced the fuel pump). I think the oil pump may be bad as every time I pull of the carb cover I can see what appears to be some oil pooling in the front section of the carbs...

thanks

jim

[Bazooka Joe]

Carbon deposits are rarely (probably never) the result of plugged carb jets.

Rather, it is typically the result of:
1- RICH jetting
2- Certain brands of oil
3- Malfunctioning float bowl components
4- Excessive low speed operation
5- Crappy modern gas

All of these causes will result in the formation of carbon on the piston tops, cyl heads, and exhaust chambers.

btw, Excessive oil usually doesn't cause carbon either, and sometimes (depending on oil brand) actually will wash the carbon out of the engine in a gooey mess. You WILL know if you are getting too much oil in the engine via the big cloud of blue smoke which follows your boat.

Good luck with it.

[Bluebird]

Oil pump is made of brass and probably not malfunctioning.
Ther are some one way valves in the oil lines that can cause oil to overflow into the carbs.
Small and hard to see. Look close.

[JimmyG]

Thanks everyone, I will continue to plug away...