Models: All engines
The most common cause of engines ‘hunting or surging’ is stale fuel evaporating, leaving residue on the main jet or dirty fuel causing partial blockage of the jet. If the motor runs smoothly on partial choke, this usually indicates main jet restriction. **Other causes can be worn or cracked early insulators or poor sealing gaskets
Models: EX Engines
If you find the EX Series engines run smoothly on idle or slight throttle
but begin to hunt or surge at high revs, check the condition of the carburettor insulator. If it is damaged on the right hand side near the blower housing this will cause the engine to hunt or surge as it will provide increased air flow across the back of the carburettor vent which will upset the fuel level. In this instance, a replacement of the insulator will be necessary
