Spark plug looks like new

[vanderwel]

Have 2 stroke 200hp salt water series II. Pulled spark plug out so I can do a compression test and one of the plugs came out like new. Other had slight black and greasy. What would cause a plug to look like new when it been in a running engine? ( I only replaced plugs couple of hour running ago) I would suspect this cyl. Has not been firing. Are there any valves between carb and that could fail Ina closed way?

[robert graham]

May have an internal water leak on that cylinder, so when water flashes to steam upon combustion can have a "steam scouring" effect on that plug, causing it to be totally white. A bore scope into that cylinder may reveal the combustion chamber and piston crown to be shiny clean, with all carbon gone. If that's the case, then remove head to inspect for leak...hopefully may be just a bad gasket and not a pin-hole in metal casting...Good Luck!

[muskrat1]

[QUOTE=vanderwel;74174] What would cause a plug to look like new when it been in a running engine? ( I only replaced plugs couple of hour running ago) I would suspect this cyl. Has not been firing.QUOTE]

Clean plug as in looks new = cylinder not firing.

Check for spark on that cylinder.Might be as simple as a bad plug.

How did plug look that you took, before replacing when you changed them?


Curiosity - What has lead to the compression test?
Selling/Buying?
Poor performace?

[vanderwel]

thanks robert and muskrat,

the reason i went for a compression check is that the engine temp alarm goes off and engine slows to safe RPM. a new impellor has been fitted, new thermostats, new fuel filters, new spark plugs, ran the engine on premix fuel when alarm went to check if the oil injection was a problem, but all this work and problem still exists. hence a compression check to look at the cylinder condition. Im now thinking of a reed valve issue on the cylinder with the clean plug (but i cant find any info on these yet will keep looking though) ive only owned this boat for a couple months so im still learning the ins and outs of this engine. i have notice the engine has a vibration at lower RPM so the clean plug that explains that. Im keen to learn more about reed valves. will keep this forum posted on how i progress.

[rodbolt17]

can we get an engine model number so we know what fuel and ign systems we are disscussing ?
and, what were your numbers?

[vanderwel]

hi cold compression 800 - 900 (5 were at 900 and 1 at 800)
Hot compression all same at 800 - 850 had a look onto top of pistons with torch and they all looked same. rough, sooty, greasy film. no "clean" piston as such. I think the problem might be with a fuel blockage to one cyl.

[rodbolt17]

and we still have no clue as to the fuel or ignition system we are disscussing.
by you language and use of the metric system I assume your not in the US market.
international market motors are sometimes very different than US market.
and thus is the reason I ask for model numbers.

[vanderwel]

yep down under in aussie. 2-stroke, 3 x twin carb engine is 2002? i think in USA they can be called V-Pro.

[99yam40]

And the reason you refuse to post the model # that is on you motor is why

[ausnoelm]

yeah, not 100% sure why you do not post the model as asked, it is no secret, you have some of the best Yamaha people at your disposal for FREE, they asked a simple thing, and so far you have just side stepped it.

[vanderwel]

all sorted losing water pressure from the warped water pump plastic housing. plug issue sorted too (not firing)