Interesting piston damage.

[VTNewb]

Hey guys,
Second post, talked to you all earlier about an older KDX, but I ended up with a 1996 KTM EXC 250 instead... BUT this is about something completely different!

1986 YZ125. $350.

It has OK compression, bout 120 lbs. I know it isn't super high spec but it's plenty to get the bike running.

Just for fun we pulled the head. The piston is brand spanking shiny new. It looks like the PO did a top end job, couldn't get it running (has no airbox and carb was missing important pieces) and sold it to my buddy for $350.

Problem: Looks like teeny debris got squeezed between the piston on the exhaust port side and the head. Very very very tiny marks, about 10 appear on the piston and head in this location.

I suspect the PO blew it up, took the same piston that is very very minorly damaged and put new rings on it, and never got it started.

So, for a rat bike on a budget, would you resurface the head and get a new piston?

We plan on trying to bump start the thing in about a week once he has his airbox and filter bits in.

Thanks guys.

 

[m4i2k2e2]

is the cylinder damaged?

 

[VTNewb]

is the cylinder damaged?

Nope, not at all. Thumpertalk tells me it's detonation damage. Does this seem correct?

 

[m4i2k2e2]

what did the spark plug look like? was that damaged? did you find debris? when you pull the plug tell me what it looks like.

 

[VTNewb]

what did the spark plug look like? was that damaged? did you find debris? when you pull the plug tell me what it looks like.

Looks brand spanking new. No debris found.

 

[VTNewb]

After looking at a bunch of figures and doing a bunch of reading I'm inclined to say it is detonation. Thanks.

 

[m4i2k2e2]

but what caused it? if you replace everything and it happens again, its gonna get expensive.

 

[VTNewb]

but what caused it? if you replace everything and it happens again, its gonna get expensive.

As far as I know, detonation is always caused by the same thing. Low quality gas (or mixture). I'm betting the P.O. used 85 pump gas and weedwacker 2 stroke oil.

 

[ohmthis]

You should also check your timing on the bike. To far advanced timing can cause that. Make doublely sure you jet the bike correctly and I would go through the whole fuel system. check for clogged petcock, hoses, Varnished carb. this could lead to a lean condition too.

 

[BigRedAF]

As far as I know, detonation is always caused by the same thing. Low quality gas (or mixture). I'm betting the P.O. used 85 pump gas and weedwacker 2 stroke oil.

Not always the case...

A lean condition will cause detonation too. If the bike was running fine before it went to hell then I would pay close attention to the dry side crank seal. When it goes bad the bike sucks air on the intake stroke and the engine goes lean big time.

If it leaks there can also be a oily film under the left side cover because premix will seep out on the compression stroke. The crank area gets pressurized when the piston is on the way down forcing the fuel/oil/air mixture up the transfer ports for combustion.