My '78 Yamaha 15A doesn't rev up to more than 1/3 of maximum under load and makes a 'burbling' noise with throttle fully opened. Starts and idles perfectly and revs up with no problems in neutral. Initially, the problem was intermittent with longer periods of normal operation and the engine then changed spontaneously between normal and faulty operation, i.e. revs went suddenly up or down without changing throttle position. Within the last weeks (few hours of operation)the problem has become permanent. Engine doesn't overheat and cranks over with normal resistance also in forward gear. Pressing primer doesn't bring revs up. <br />Replaced fuel filter, cleaned carb, checked fuel pump and tank ventilation. Plugs looked clean, replaced them anyway. Used fresh fuel and Yamaha oil.<br />What might be the problem?
