18"poulan chain saw wont rev up

My 18" poulan chain saw will start ok but it wont rev up.while i hold the triger on it runs but wont fev up.when i release the trigger it stalls.it will start back up.but runs at one speed.

Carbs dirty need cleaning. try adding some stabil you can buy at auto zone with fresh fuel it may clean itself out w/o having to remove the carb.

Either the carb has dirt in it and needs cleaning or the muffler is choked up take of the muffler to see if it is clean