Bad pipe o-rings causing melted piston?

[Motox125]

Okay, here is the deal. Today, in my third moto, my bike (2000 YZ125) started acting a little funny. It didnt want to pick up RPM, and wanted to stall. It seemed almost as if it wanted to foul a plug. I ride it out anyway, thinking it is just a fouled plug. I pull off the track, and my bike just dies. My dad and I push it up, change the plug, and go to kick it, but the bike has no compression. Late tonight I pulled the top end, and part of the exhaust side of the piston is missing. It looks as if it was burnt off. The entire top edge is very rough. The exhaust pipe o-rings went bad about a week ago, and I havent had a chance to replace them yet. Our theory is that the backpressure into the cylinder sucked air through the exhaust/cylinder junction. There is very strange buildup on the piston. The top and exhaust edge are dark black while the rest of the sides are brown above the ring. Today was overcast and a little cold at the races, so the bike may also have been running a little lean. Whats up here? Could the pipe o-ring cause this piston melting? The exhaust side melted about to the inner edge of the piston ring. Thanks in advance for your help!

 

[Studboy]

I doubt that is what caused your piston to melt.
Something else was at work there.

 

[Motox125]

The piston didnt have too many hours on it. It has 4 races and probably 7 rides. It was replaced this winter. Now that I think about it, could detonation have eaten away at the exhaust side of the piston? Also, there is a tiny scratch in the cylinder, just barely big enough to catch your fingernail on, presumably from piston pieces... how serious is that?

 

[Motox125]

ah, I have another small addition to all of this... this morning I discovered one of the reeds was chipped and cracked from the edge to about halfway down the reed...

 

[CR69r]

in theory bad exhaust o-rings can cause engine damage. i also discovered this one the hard way. your piston being damaged on the exhaust side would logically point to the o-rings as the culprit, and not the reeds. i have never heard of a cracked reed causing engine damage. your piston tells the whole story. pistons don't lie. an experienced race mechanic should know the deal. keyword being experienced and not the 18 year old apprentice at the local dealer.

 

[antont]

Two things worth checking. Had 2000, then 02 & now 03 yz. The 00 piston is trash. The ring pin is located badly. This causes many piston failures, most often ring catch on port which either seizes ring or breaks piston. Seen both many times. Best option, try 02 piston. Porting change may be a problem. But ask dealer.
Regards
Anton

 

[jmics19067]

. Now that I think about it, could detonation have eaten away at

that would be my guess , now the hard part is to figure out why you had detonation.

 

[flynhigh40]

Didnt the 00 YZ125 have plastic inserts in the crank? I think they did so your gonna want to check that out before you reassemble to make sure there not damaged.

 

[Rcannon]

I saw a CR 80 suffer this type of piston damage that wa straced back to a fulty ignition. We replaced the black box. The bike started rnning correctly. Some of the carbon melted off the piston and jammed the exhaust port. The result was one trashed piston and rings.

 

[steven cook]

bike is wey wey wey to lean

 

[snb73]

http://www.smellofdeath.com/lloydy/piston_diag_guide.htm

 

[tcuda499]

snb73, very nice link!!! book marked that one. :cool:

 

[IndyMX]

Today was overcast and a little cold at the races, so the bike may also have been running a little lean.

Sounds like you answered this one yourself.

 

[helio lucas]

i have the eric gorr´s book "mx and off road performance handbook" and the images and text of that link are mainly from the book... so its better to buy the book!!!

it is possible to size the engine with bad o´ring like yours...

but the feeling of the engine like a fouled plug come from the damaged reeds...

my guess is the bad reeds in some manner make that work... not only the o´rings...

maybe from carbon build up from rich mix in the low end... maybe from lean mix in the upper part of the band... maybe both... is quite difficult to guess for me...

 

[2strokerfun]

It isn't always just a single thing that makes an engine go south. O-ring probably not major cause, but coupled with lean running, four previous races on piston, cooler weather, might have helped create the perfect storm for meltdown.

 

[snb73]

Your welcome. I also have a few of Erics books, lots of good info there. I'm just starting to get the hang of jetting, after 2 years of trying. I've learned that the 2-stroke motor has lots of variables, and each one can effect the other. Like said before; check reeds, jetting, good header seal, ect...

Hope this helps, Steve.