99 yz250 not reaching full potential

I have a 1999 yz250 that i have had for about 6 months. Ever since i got the bike i have had the feeling, and so have many others that have riden it, that the bike is not reaching its full potential. I would estimate it should have a top speed around 90 mph but the bike only reaches about 60. Also it never hits a power ban which is frustrating. This probably has something to do with it, but when i go to let out the clutch in 1st gear if i dont give it alot of gas or get it rolling it will stall. Besides that, once it is going in mainly gears 1 and 2, the very begining of the gears if i go to give it gas it doesnt respond MOST of the time until i have given it about 1/4 of the throttle. Not usually but sometimes, i will give it gas and it will pick up and start to go only giving it about 1/7 of the throttle but usually it takes a decent amount of trottle before it catches I know the bike has been rebuilt before and i can see there is a jb weld on the top of the head but im not sure if that has to do with the issue. If anyone has some ideas please comment im very curious to fix this thanks!