I had just finished rebuilding my 250r, and had just finished the break-in. I was finishing my jet runs, (still slightly rich) the quad felt like it ran out of gas. It had fuel but when I kicked it over, there was no compression. I was so choked, I thought I had burned down the piston. [img]i/expressions/face-icon-small-mad.gif[/img]

When I tore down the motor, there were a bunch of crater's in the piston and head. Upon further inspection we noticed that the porcelain was missing from the spark plug's end. It must have cracked off the electrode, and bounced around until it scored my piston.

Luckly it did not damage the cylinder's bore, because i'm at 270cc.

Has this happened to anyone else? Is there something I can do to stop it? The plug was an NGK B8ES, the plug I have used forever.

Thanks for the help.

 

Never heard of that before.....

 

yeah i've never heard of that before either. i've always used NGK B8ES also and never had that happen. so do you have to rebuild it again?

 

I would have a tendency to think something else got into the cylinder and busted the porcelain from the spark plug. Severely over advanced timing causing pre-ignition can some times crack the porcelain but not likely to have it come apart. Porcelain crushes pretty easy compared to an aluminum piston and head. I would keep looking for another source of something foreign entering the combustion chamber.

 

Good point Specta. Look for ANY melting on what's left of the spark plug electrode, or top of the piston, especially right underneath the spark plug, and around the locating pin for the rings (the only steel in the piston). If you find this, the cause was the two stroke's two deadly sins, pre-ignition\detonation. They can make your plug electrode go away completely. If this isn't the case, it's also possible that other foreign matter entered the combusion chamber ie. pieces left from a previous failure in the bottem end or exhaust, or a newly fragmented part. It's much more likely something like this happened than your spark plug simply broke and was consumed by the combustion chamber. I HAVE seen porcelain cracked on spark plugs by the installer, if the plug is difficult to get at and was tightened at an angle without a spark plug socket, however on those occasions the plugs usually just arc electricity (short out), and don't break apart completely. BTW and not that this applies in your case, Bombardier has been using plugs on some of their sleds that come pre-gapped, and CANNOT be gapped or it is possible the electrodes could break (the material is very brittle). They are denoted by a C in the NGK number like: BR9ECS.

 

I was thinking that, something may have gotten in. Still I just can't see it, I have been through the complete motor. The electrode was not damaged, the end gap is still good. If something was big enough to break the porcelain, could it or should it have damaged the electrode or ground.

I think I will split the cases again to see if there is something down there, and make sure it's spotless. Also I better start inspecting the plug after gaping in case I do some damage to them. I got away lucky this time with just a piston. There was not melting of the piston, the scratch was on the intake side by the left "port", i'm not sure what there called.

As for timing I only went with a bigger bore, and replaced the carb TMK38mm. Pipes are the same, CT mid-range and FTZ high rev. So timing should not have changed.

Thanks again for you help.