Well, its back to the shop

[D3luxe]

I was out riding today and my RM125 decided to go kaput again, for some odd reason.

Engine started making a rattling-type of sound and was bogging out when I gave it gas. Luckily I was close to my shed so I just killed the engine and pushed it the rest of the way. Pulled the exhaust and spark plug out to check a few things out. Spark plug looked fine and had spark, compression was good, and the exhaust side of the piston/intake side of the cylinder both looked to be in good shape.

The one thing I noticed from that was that the left powervalve seems to have a small chip in it, however if anything that should have just blown out of the exhaust, right?

So, on monday I'm gonna take it back to my shop to tear it down and see whats wrong with it, yet again..It's been in there way too many times, and here goes another $50 in gaskets/oil/coolant alone; hopefully anything else I have to replace won't be too much more.

EDIT: The most confusing part of today was it was running great. Started up immediately and had a ton of power to it; then, boom. Any ideas on what to check for when I tear it down? I know the usual cylinder marks, piston marks, piston up/down free play, etc, but is there anything else I should be checking for?

[MXtras]

If everything in the motor looks good check your jets first - sounds like what you could have been experiencing is an overly lean condition with detonation - possibly from a plugged jet? These things happen.

Just an idea.

Otherwise I am sure the mechanical problem will be quite obvious.

Check your reeds and magneto side crank seal, too.

Scott

[D3luxe]

Thanks for the advice Scott.

Seeing as I'll have to pull the carb off to get to the cylinder anyway, I'll check the jets and reeds first. That would make sense since it only sounds off when theres compression, trying to pull from the jets..but would that necessarily make a clinking sound?

We'll have to see what the engine exposes on monday, I suppose.

[D3luxe]

Forgot to update this thread with what I found.

Got it into the shop and tore the top end apart. Found out that the colar of the powervalve actually broke, so the powervalve slipped just enough into the cylinder that the piston was tapping into it, which eventually wore the piston down enough to seize up the ring.

Sooo, $90ish for the piston and $10ish for the powervalve collar..not bad

[DRGSin393]

sorry to hear this but it couldve been alot worse, right?

[D3luxe]

Originally Posted by DRGSin393

sorry to hear this but it couldve been alot worse, right?

Exactly. The sounds it were making made me think that it was a bottom end bearing that blew out on me, so I'm relieved to find its just a simple top end fix.