Sticky rings

[suzuki27]

Just took the top end off a CR125 '75 and the exhaust side of the piston wall/skirt has light vertical scoring. There is a well circumscribed dark burn area there also. The rings above this area are stuck flush with the piston. Just got the motor so I can't report what it felt like when it happened. The crank "feels" OK. It still turned over smoothly on the kickstart. Anyone know what has happened and the likely cause?
I've never seized a 2 stroke, just the more costly 4 bangers!

[Lozza]

Commonly called a 'stuck ring'  if you get the ring out OK you can V the ring groove in front of the exhaust port.

[suzuki27]

Hi Lozzacan you tell me whats needed to V the ring grooves? Cheers.

[Lozza]

A V shaped needle file

[suzuki27]

Any ideas about what might have caused it to have the rings pushed in flush with the piston/groove and stick? Piston is scored below the rings- vertical light scoring. Bore feels and looks good. I am going to rebore it etc but wanted to avoid it happening again. Doesn't look like a lack of lube.

[Lozza]

You have a carburation problem

[suzuki27]

Thanks Lozza.

[Lozza]

I posted that from my phone short story is you have some 'soft' part throttle detonation + heat is enough to hammer the ring groove and close up, trapping the ring. Honda RS125/250 suffer badly from this, V-ing out the ring groove and a 0.7mm ring stopped it in 125/250's

[suzuki27]

Looking at the stator plate it was advanced about 4mm from the standard mark. Was told that a 1-2mm advance on these is about as far as you can go before detonation becomes a problem.
The motor came with a GEM reed ,34mm carb and DG head. Some sort of port work has been done and it was running 2 head gaskets. Not sure if they took some off the barrel or too much off the head!
Your answer sounds on the money Lozza.

[Lozza]

Selling the DG head reverting to the CR head with a sorted squish band will be a good start. It's a really hard concept to get into words but the practicality is (with a RS) you lift the cylinder at the end of each day and check the ring is not trapped. They still sometimes even with the groove V'd out.