i had the same problem with my beta (2006) i dont no what it had been through before i bought it but after a couple off months it developed a wizzing sound and wouldnt run properly after cleanin the carb air box air filter ect it ran ok but still the wizzing sound we took the flywheel off every thing ok but notice wudwuf key was broken so we put an new on in but still was wizzin so went to rpm and we took it completly 2 bits and the piston ringswere knakerd and the piston was black cost me dad 450 quid but then got rid of it good ridence and a surprise prezei a shiny new sherco ive had it about 1year 4 months never let me down