2005 YZ 250 bike has no power in the powerband motor starts

Check the compression. Sounds like there is a lot of bypassed gas.

HI. REMOVE THE POWERVALVE COVER ON THE SIDE OF THE CYLINDER.
MAKE SURE THE ARM IS CLIPPED ON TO THE POWER VALVE ITSELF

START YOUR ENGINE AND AS YOU REV YOUR MOTOR YOU SHOULD SEE THE ARM ON THE POWER VALVE MOVE. IF NO MOVEMENT YOU WILL HAVE TO REMOVE THAT SIDE ENGINE CASE AND CHECK CHECK VALVE FUNCTION....CHEERS

i have a 92 rm 250 an i just had it fully rebuilt an it laggs in power band an splutters an feels guttles

Sounds like it's trying to sieze if it's fluctuating and won't wind up. You may have to pull your cyledar back off and mike it to see if it is out of round and within specs.

plug

go to bike bandit for all parts.

clean out the entire airbox along with really clean out the air filter, use gas to clean the filter then rinse it out with warm water.iv been doing it for years, dont let anyone tell u otherwise, check the reeds, replace if needed, clean the powervalve. clean the carb. i dont no wat else could be a matter with the bike, unless its in the cylinder like no compression of broken or worn piston rings, who noes. get curious check **** out that away u dont have to waste ur time askin us. u no