Hello, my 1997 Yamaha ZJ 250 had a recent change in performance. One day it was running fine with good acceleration and adiquate power. Now it lacks power and acceleration and will not reach a speed necessary to shift pass 3rd gear. The bike does not start as easy as it used to, and makes a repetative rnnnn rnnnn rnnn rnnn sound at minimal throttle while lacking power and acceleration. Usually the bike gains plenty of power and accelerates consistantly and smoothly. Does any one know what the problem may be? Thanks SW.