What signs show when piston is on its way out?

[Arven]

Hi guys,

What do you guys do to tell if your piston is getting warn and needs a rebuild???

My YZ125 had a rebuild when i first got it, Before then it wouldnt start easily, and after the spark plug was cleaned or replaced, it would start, run for a few seconds then die... wouldnt go again for another few hours untill it dryed out and a new plug was put in and it did the same thing again...

Now about 30 - 40 hours use later, its doing the same thing. So i'm guessing its the same problem... Not enough compression. But when it happend the first time, no one knew what the problem was! so just checking as to what you guys do to tell when a rebuild is needed???

Thanks for the info...

[Motu]

It's a bit hard to tell with a 2 stroke - but hard to start with a drop in power is a good indication,they also get a bit rattley.Crank seals is a possibility too,sometimes they get smokey as they burn gearbox oil then too.But these days a 2 stroke lasts longer between top end rebuilds than a 4 stroke - my DT230 has done 25,000km without being touched.

[NordieBoy]

When a mechanic pops the top off and makes that "sucking through teeth" noise and says "the piston's fucked".

Get it looked at.
Pulling a 2-smoke head apart is quick and easy and may only cost you a gasket and an hour - to find out it's something else...

[Arven]

Yeah i dont think its the seals, thats what they thought it was last time and after a hefty checking over it wasnt that...

I dunno. I hear the YZ's should be rebuilt ever 40 hours or so... They are pretty highly strung little 2 Strokes!!

[Arven]

Haha yeah, i think the best way to check the piston is to take off the expansion chambour and look through the exhaust port... If you see lots of black lines and scratches etc in the piston.. Its fuc*ed.. :P

Thats what happend last time anyways! :P

[F5 Dave]

Assume it is fairly recent model (within last decade).

Do a search on the net, there are heaps of articles I bet on pulling a 2 stroke apart. Best you learn how, make sure the bike is clean so dirt isn’t falling off the frame into the engine.

The powervalve should be checked for proper operation & then measure the piston, but most likely the rings will wear before the piston does, you can check rings in the bore with a feeler gauge.

Carb could be dirty & sparkplugs have a limited life, but you should change the rings every so often so you don’t wear the piston & bore as quickly.