Yamaha RXS 100 - HELP!

Please help! I was a happy owner of Yamaha till yesterday. On a way
back home I was doing 60mph. Suddenly bike started loosing it's
perfomance First was keeping up to 50, then 40 and maybe 5 minutes
later it could do hardly 30mph. When l open the throttle and the bike
is under load it hardly moves, can get to the second gear and 30mph
but it's very weak and jumpy. I already checked transmition oil,
exhaust is not blocked, two stroke oil tested. What else it can be?

 

Spark plug?

 

Doubtful - they normally either work or they don't IYSWIM.

Humm... IMHO, the main things I'd be considering are:

1. It may have nipped up (1) - I've had this on two strokes before
purely because I've used cheap two smoke oil and it didn't flow well
enough into the bore
2. Done a piston ring
3. That old classic, 'shit in the carb'
4. Broken / blocked reed valve - a good excuse to take this out and
replace with a Boysen replica fashioned from a margarine tub lid, if
ever there was...
5. Lump of carbon has broken away in the exhaust and caused a
blockage - seen this before more on an MBX50, and it cut the
performance right down
6. Oil pump has died / cable has snapped, and caused point 1 to come
into play

(1) Partially seized the piston in the bore briefly, normally due to a
lack of oil

HTH

 

Just to let you know! Exhaust cleaned, spark plug checked but plug was
very oily, covered in I imagine two stroke oil. Does it mean that
piston has gone?

 

No