running but not well

had posted recent problems with fuel mileage. theres more to it. im only getting 15 miles per gallon. bike will backfire if i let off the throttle at 4k or more. idles fine. at highway speeds 55 plus she doesnt do so well. keep in mind i am 235lbs. in 4th and 5th gear i cant gain speed unless im going downhill. if i open up to full throttle in 4th or 5th it seems to bog down. if i ease into it exremely slow it does ok on flat ground or downhill. i can achieve high rpms in 1 2 and 3 but 4 and 5, 4k is the best. any advice would help. thanks

recently cleaned carbs blindly. never touched a set of carbs before. runs better than it did before though.

has the bike been tuned lately? I'm not an expert, just started getting into working on bikes, but it sounds like the timing might be out on the bike, that sounds like the same problem I had with my bike before I managed to get it properly in tune.

have u checked the usual stuff?
clean air filter,
clean plugs,
timing,
points if ya got em,
clean fuel tank,
valve clearance

I think a full tune up is in order, also checking the compression and valves, and advancer in addition to things already said.
also what gear is on the bike, wonder if someone has changed it out

pretty confident that the bike is bone stock except for the superbike bars that i put on today.