Holed Piston

You don't have the luxury of running an Air Cooled Machine critically Lean for very long. The heat from the lean mixture accumulates rather than dissipates.
It leads to failures.
You have to know how you are running or you WILL get into trouble.

 

+1 on this(I don't have any AS3 ex) ,,,,may very well be just a hot plug,,,,,,,in any case put a 9 in it,,,,nkg's,,,,for sure,,,,no an's if's or but's,,,,,

what plug was in it? I would almost bet it was not a 9,,,,need to take a serious look into it if it was,,,,,


I agree that the hole on top came from a hot plug,,,,,I believe if it lean seized the piston skirt would have more damage,,,,these look like very heavy pistons an probable took a lot of heat to get the hole in it,,,,an this lead to the lockup,,,,,,hole an a lockup,,,have to say the hole came first,,,which is a hot plug,,,,you should be able to lean sieze the motor without a hole in the piston with a 9 in it,,,,is the angle I would aproach it

bad crank seals may not leak oil,,,(I have to admit I've never had this problem but have seen it),,,get the crank bearings out of it as soon as they get loose),,,,,Dan

 

Interesting feedback on the plugs. I had just changed the plugs and went from an NKG B7HZ to an NKG B7HS because that was all the local Yamaha shop had in stock. It sounds like I should be running an NKG B9xx. When John at NWVCP first looked at the piston he commented that it was caused by the plugs.

The pix shows the piston after I cleaned the melted aluminum off it. There was very little scratching on the skirt but some melted aluminum on the ring grooves. The cylinder showed only one slight scratch which honed out easily so I suspect the siezing may have been due to the metal in the ring groove.

It also sounds like I should keep this bike for the next time I decide I want to go track racing and get myself an XJ for just catching the wind.

 

was it pinging? i dont think the hole in the piston is caused by an incorrect plug. i have seen pistons like that before....usually after they leaned out on a dyno!

 

No Ping, No Knock, No Warning. Just like a woman, got my adrenalin pumping and left me standing alongside the road wondering what happened.

 

we see that quite a bit up here especially with snowmobiles. That looks like a classic case of " the engine sat so long that that seals dried out and we not replaced so it sucked a lot of air and made for a real lean condition which will burn up a cylinder in no time at all if you're horsing it". Maybe bikes are different, but two stroke is two stroke, and summer is hotter than winter here. FWIW.

Dave

 

I had a buddy with a newer 1980 something yamaha two stroke and had a similar issuse when he put new plugs in. Are you sure your plugs were the right heat range?

 

Try a colder plug and please check your ignition timing. It may be too advanced for that cylinder.

Good advice about the crank seals. Smoke may also be from middle crank seals esp. if your gearbox oil starts going low.