As I understand the question, the throttle won't stay put at high speed. The throttle lever has a friction washer to keep it from moving and this can wear over time or someone may have mistakenly lubricated it. The best solution is to replace the throttle assembly--probably about a $30 item. You can also try removing the throttle assembly and then peen the rivet that it pivots on. This may tighten it up enough to keep it from moving. You can also drill out the rivet and install a machine screw with a lock nut so you can tighten it to the desired level. If the friction material is too badly worn then a new throttle assembly is in your future.