I'm having this problem with my 250rr today.
It feels hesitant, it bogs out when I open the throttle and stalls.
It wouldn't then start and ride, so I had to get the recovery mechanic out.
I suspected the spark plug, as I had this issue with an old KTM 250exc and it turns out the spark plug was fouled.
We cleaned.it.with a wire.brush and some wd40, put it back in and the bike ran like a.charm...for all of about 5 miles.
It then started.bogging and cutting out again, much to my dread.
It would start but it wouldn't run right and it would rev all the way out when I opened the throttle, or it would bog down and cut out.
When I had an old KTM 250 exc, this turned out to be some obscure fault with one of the crank case seals where it was leaking gearbox oil into the combustion chamber.
To replace the seal, the cases would have to be split and have a new one pressured in. I sold it as I just wanted to ride.
Could my beta 250rr have fallen victim to the same issue? If so, I will not be happy and may even quit 2 strokes.
I run 100 mil 2 t oil to 5 litres of fuel, btw. That makes a ratio of 50:1 if my maths is correct.