REDBALLS302

Ok so when I pulled my bike out this year it kinda ran shifty. I figured it Judy had bad gas in it so I flushed the tank blew the fuel lines out n changed the plugs. After about ten mins of riding it gets weak and when I twist the throttle it basically does nothing. Pull the clutch in and it revs out around 5 k. If I flip the run switch it picks up snatches me n takes off. But then eventually poops out. Usually the first ten mins it runs normal. Had an aftermarket exhaust but went back to stock haven't rode since I put that back on but not sure if it will make a differ. Any chance. It could need a valve adjustment? Also I have no. Guages so how can I tell what codes its throwing? Any pins I can jump? "