Help 04 Cr 125 Frying Pistons

[momurder]

ok so i got a 2004 cr 125 i just bought it like 2 months ago and first ride after i bought it i fried a top end. so i took it home changed the gas (i figured thats what it was) and put a new top end in. and then it ran great for about 2 weeks and now i just fried another top end.
people keep telling me its my gas but im goin 36:1 so thats not it. and i know its not my crank because that was rebuilt about 3 or 4 rides before i got it. and someone said maybe a bad crank seal?

 

[_JOE_]

Could be a leaky crank seal. Pop the ignition cover and see if you can see any oily residue near the crank. If so the seal is shot, if not I would have a leak down check done to make sure they're good. It's less than the cost of a piston. While you're at it pull the carb out and clean it, paying close attention to the main jet. It may be partially restricted. Also check the number to make sure someone didn't mess with jetting and end up too lean. If you need more help just ask.

 

[sharky243]

If the carb is clean, the jetting where it should be, and no crankcase air leaks, It's posible that the CDI may be staying advanced. Take a look at the spark plug, check the burn. You want to see a nice tan or light brown colour. If it is whitish, it's too lean for some reason (ie. an air leak ?). Did it seize ? Look at the piston to see how it failed. Maybe there wasn't enough piston to cylinder clearance. Also on the CR's, it's important to purge the air from the cylinder water jackets. If you don't, the coolant may not circulate and cause a hot spot. Also check to see if the stator plate is where it's supose to be. Maybe someone messed with it trying to advance the timing. As far as weather you have enough oil or not, an oily residue at the tip of the silencer indicates that there is enough oil. If it's dry then there is not enough oil. 36:1 should be plenty. How long do you let it warm up before your ride ? I wait until the rads are hot on my bike.

 

[2strokerfun]

If ... the jetting where it should be,....

That might be a big "if"
What's your plug look like?

 

[rmc_olderthandirt]

What ocatane gas are you running? I don't know what the compression ratio is on your bike but many MX bikes require premium gas (91 octane) and if you run regular (87 octane) then you could suffer detonation, which could destroy the piston pretty quickly.

As others have said, check the jetting. It might have been jetted for a higher elevation and now it is running too lean. Running lean will result in high cylinder temperatures which can lead to failure.

Rod

 

[momurder]

well im out of town right now so i cant get into checking any of that for the moment.

i usually let it warm up about like 5-7 min and not just at an idle i rev her a bit. and i do have a little residue not a drip just a little ring

and my plug had a bit of like a carbon residue or something like that. didnt look too good.

and ive been runnin 89 octane (i know not too good) but i plan on bumpin her up to premium from now on.

and also my friend has the same bike but an '01 and his has been doing the same thing but his crank is about to go.
but thanks for all the help i cant wait to go actually find out what the hell it is. ill keep you guys posted as i figure it out.
thanks again.

 

[whenfoxforks-ruled]

If your plug is not silver and missing parts, your bore has issues. Its either past the service limit or out of round. What was the skirt clearance? The other thing it could be, is the piston scored by the exhaust bridge?