Sudden compression loss

[jrm]

I've been riding 2-stroke MXers for almost 25 years and never encountered this before.

Started my bike yesterday (early 80s CR250) just before the start of my vintage moto. Everything seemed fine, then the bike died. When I tried to restart it, it was clear that the compression in the motor was drastically low. There was some resistance to the kickstarter, but not much. The bike ran fine just earlier in the morning and on the way to the start gate.

The piston seems to move smoothly up and down the bore with no nasty noises, but it won't come close to firing. Head and base gaskets seem OK. Nothing looked wrong when I took a look through the exhaust prot after removing the pipe.

I'm going to tear it down this evening, but has anybody experienced a sudden loss of compression like this? I'm guessing (afraid) it's a stuck or broken ring.

 

[SirHilton17]

yup... I have had it happen when a bearing cage came apart, and peices got up into the combustion chamber, and they squashed the piston, and the piston squeezed the ring, therefore compresion loss...

That is most likely your problem, because that is a old bike, and it probably hasent had new bearing in a while... notice i said probably

 

[Britt Boyette]

I had it happen once and it turned out to be a bad plug in from the CDI box. Too much resistance because of corrosion.

 

[jrm]

Found the answer -- bad case of indigestion after a morning snack of piston pin circlip. Ugggh.

 

[chevyss_98]

i used to have a 79' YZ125, when the compression died on it, it wuz because a chunk of the piston broke off and went out the exhaust port, but this all started because a circlip snapped off for some reason, allowing the wrist pin to come into contact with the cylinder wall

ya, circlips suck....