I hope someone can help with this
Right got the top end off again. I found that the piston could just catch the power valve in the one position. Possibly due to carbon build up but I will check this out. (Thanks Knackered for the reminder. It was clear when it was rebored 12 months ago)
Here are some photos of the piston 2 showing indents on the edge of the crown and one showing a slight score on the powervalve side. The bore is fine with no marks.
I have no real experience of 2 strokes.
Should I buy a new piston or just clean it up and fit new rings?
I want the bike to be right but do not want to buy a piston if it is not necessary.

I think you need a new piston and rings.
Looks like the damage to the top of the piston and trapped the rings in casing blow by, thats why theres burning on the side of the piston.
Jon

Looks like a new piston .

Eddie can you post some photos of the bore. Particularly the side transfers.

From the top


From the base


There is no noticeable step on either end of the stroke and any lines shown are faint and cannot be felt even with a nail.

I assume the ports have a small chamfer - I can't really see in the photos.

Have the rings been catching the ports and flaking off the piston crown

Can you remind us of the details of the engine Mk wise. Different bits

Barrel and head are Mk3 1.5mm overbore done at Stan Stephens 18 months ago. The piston and barrel have done approx 8,000 miles trail riding since then. I doubt if Stan chamfered the ports so that is something I can do before I fit the new piston. Bottom end is a Mk2. Big end and mains seem OK.
I'm also going to rough the bore up with some wet and dry to help the new rings seal. Any other tips??

I would agree, re hone the barrel, 1200 wet and dry , IMO, too de glaze it ,radius of all ports ,and power valve lip, wash,meticulously, with a quality, de greaser, new piston, rings , bearing, clips, and wrist pin, check, and clean the power valve, while it's off, and then check clearance of piston to p/valve, in open position.
Then re assemble and run in thoroughly, before any 12 hour Trail Rides
You might want too go one position richer on the needle valve, for running in, ditto 1/4 turn in on the pilot screw, or some open cell foam in the air box, will do the same job, but across all the rev range, IMO.
Mike

Just completed a test ride
Bore honed with a glaze breaker, took the edge off the ports and made sure that the power valve clears.
New piston, rings, and small end bearing.
I've started working through the wiring and have a fork seal, front calliper and wheel bearings to do before starting the serious testing.
Thanks for the help and advice

Eddie, the old small end bearing, did it show any evidence of fatigue? i have just looked at your first photo you posted up, it looks suspiciously, as though something has been sucked up, and out of the transfer port and, deposited above the piston, thus giving those indents in the crown of the piston. If the bits don't stay in bedded in the crown, they are blasted out with the spent gasses.Did you check the reed valve screws were all still located
Hopefully i am completely wrong.
Was the big end bearing in good condition, no up and down, or significant lateral movement.
The big end, also being a caged needle roller, would be the other culprit to let go, though this is normally, catastrophic, and obvious immediately.
Christ,I sound like the prophet of doom, sorry mate, hopefully, all cobblers, and she's good to go for tears to come.
Mike

OOPS Freudian slip there. Sorry Mate
and she's good to go for tears to come.
Mike

FMtB has a point re checking reeds etc., but I don't see any marks on the piston apart from the edges. The only way I can see those happening alone is when you are flirting with seizure; but I am known to be wrong and no experts at college for 2 weeks .

Have you done a plug check when running at 1/4-1/2 throttle . Say - up a hill .





Mike - it's years .

Little end bearing was fine, I changed it because I had a spare one form the last engine rebuild so I thought better safe than sorry.
I will do the plug chop just to be sure.

A friend has suggested that there may be a problem with the rectifier. He has had a couple go on him where they can almost stop working and then start working again when the bike has cooled down. He was getting a dim neutral light and spark but not enough for the bike to run. Once the bike cooled it would run OK. I have a spare so I'll check that works and put it in my bag, If the bike stops again I will swap the rec over and see if it runs.

Any favourite running in procedures??

He has had a couple go on him where they can almost stop working and then start working again when the bike has cooled down. He was getting a dim neutral light and spark but not enough for the bike to run. Once the bike cooled it would run OK.
Eddie, IMO those are also symptoms, exhibited by the generator, when that's about to go down.
I had mine rewound by West Country Windings, and thus far, no further problems, although, LTs said they have never had a Magneto, go down !!!!!
Running in is best achieved with, with a gradual increase, in the load, and the revs, so don't let it labour or over rev.
Mike

The tip i was told for MX bikes was run it steadily at temperature for 15 mins then let it cool right down, do that 3 or 4 times then you should be ok for longer runs. but you must let the engine cool completely inbetween runs. might help but when i ran my Jawas with new pistons i just ran at low revs for a few hundred miles.
Basicly, take it easy and dont stress the engine>