Your shifting problem could very easily be due to the throttle issue, yes. You already know the throttle is an issue so fix that first then go from there. A very likely culprit would be a throttle cable that needs to be lubricated or replaced, but that's not the only thing that could cause that.
When you talk about "tightening the screw for the clutch", are you saying you adjusted the clutch? You don't just "tighten" it, there is a procedure to adjust it.
85 Tri-Zinger 60
85 ATC250SX
86 ATC250SX
87 ATC250SX
02 XR650L conversion
84 ATC 480R