been fightin with the same prob 4 ages, tried yout trick to no success...sigh..am currently polishing valves to get a better seal, even tho iv spent over 6 munths tuning and rebuilding the motor and shes still not running right theres no way in hell that ill ever part with it, my parent think im crasy becos i sit up till 5 am working on it, they just dont understand, anyway, do you think a rebore would help? with new piston and rings, iv got a 1980 xt 250 and the motors never had hte head or barrel off, its don nearly 500 000 km