Big Red lacking power?

[Amater45]

Hello again, I took my 83 big red out for a ride today and i noticed she is lacking power. I was ridding on packed snow machine trails (legal here) and there was only about 1 inch of soft-ish snow. 1st, 2cd gear have good amount of power, but once i hit 3rd and up, if i dont have the rpms up it wont climb a hill in anything more than 2cd. i weigh a good 250lbs, and my exhaust i have is a gutted stock one (which sounds terrible). Any words on what else could cause power loss?

edit: my tires were also a little low, could that have anything to do with it?

[dustrunner]

sounds like if 1st and 2nd have good power than the engine is fine... put some air in those tires and keep the rpm's up and remember its only a 200..