I have an 86 kx 80 my problem with it is it doesnt have enough power sometime it does and somtimes it doesnt up a hill it goes slow and when in nuetral and i rev it hight it high it will not have a constant (someone told me be a might be point if it has it )

Could have low compression or bad reed valves, if you take your hand and move the kick starter is it easy to do? i would do a compression test.

Make sure you carb is tuned properly and you have good compression and spark.

And if that didnt work it could be the bike doesnt have enough power for your weight. also those bikes dont have power valves so they are a bit like light switches, on or off power, try to keep it up in the rpm range, gear it low when going up hills.