No Throttle Response in 4th 5th Gear

[Jalket]

Hey guys, new member here. i recently got my hands on a GZ250 that was sitting in a garage forever, 2002 model 1100 miles.

Long story short, couldn't fixed it, paid mechanics a few hundrede bucks, got it running.(the problem was missing vaccum on petcock) . The new problem that has arisen i lose power the bike starts bogging down in 4th or 5th gear and i can even feel it even in lower gears that its struggling.

One of my Coworkers said its the Seals and too much air is going in, another one said my bowls stuck.

I don't want to pay another couple of hundred bucks for something i can do at home. Please Help!!!!