[Growster...]

We have a Mountfield HP 414 motor mower, (hand propeelled), and there is no throttle control, just literally an 'off' or 'on' bar-operated control.

Just recently, I've noticed that while everything is cutting as it should, there's a tendency for the engine to continually rev and fall back slightly. It never used to, and sometimes, reverts to normal steady pitch engine noise.

This seems to me to be a fine tuning exercise but I'm not sure how this can be achieved, as I cannot see any linkages where adjustments can be made!

Does anyone have any ideas please?

[Gwiz]

Hello Growster, is that model one of those that hasthe red primer button for starting? if so, it sounds like you will need a new diaphragm . It goes between the carb and the fuel tank, costs about £3 to £4. While its apart, clean up the fuel pick up filter on the end of the pick up tube and clean out the fuel tank too.