My Mountfield lawnmower throttle fluctuates up/down. Starts ok, but never runs smooth. i noticed the throttle control lever pivot spindle is rocking back and forth, causing the butterfly value to rotates slightly. i was told this is due to a dirty/blocked carb, but after stripping this down and cleaning all the jets, there has been no improvement. i'm now led to believe as this is a fixed throttle model, and there is some type of balance mechanism that adjusts the throttle position as the engine warms. maybe this mechanism is faulty and causing the choke problem?
any ideas how i check this? i've taken the engine off the mower but it seems the only way to get at what’s behind the throttle spindle is to take the bottom of the engine off. i can do that, but would first like to know if i'm going in the right direction, and if so is this repairable.
engine RS100 - 100103651027 on a Mountfield SP414.

sean, February 2013

Check gaskets either side of carb and make sure the seal is nice and tight.
My RS100 was hunting and I cleaned the jet boxed it up and it started hunting so I retightened the carb and it was fine. I guess it was getting too much air :-)

colin, August 2021

just fixed my rs100 mountfield from hunting revs remove carb
remove plastic idle screw remove black plastic plug which was located under screw and found small O ring had broken off .throW O ring fit plug without O ring .jobs a good one

ERIC, September 2017

When I prime my mount field lawn mower which has a rs100 motor, I press the prime button a couple of times, the lawn mower starts briefly then cuts out. This happens every time. I have replaced the air filter, and the gasket seals.
Someone mentioned the diaphragm might need replacing but I'm not sure where this is?
It is a mount field hp414.

Andy w 4303, April 2015

A governor is attached to the throttle linkages. This is basically a control system designed to keep the speed of the engine constant. When you run into long grass, the blade and engine slows down. The fins on the flywheel or some other mechanism normally blow air onto a vane which is linked to the throttle plate. The force of the vane is balanced by a spring. When this force is reduced because the flywheel runs slower, the spring is able to force the throttle open further, letting in more fuel, giving more power and compensating for the loss of speed.
Check to make sure nothing is sticking or rubbing in this arrangement, restricting movement. If the throttle control is fixed, there shouldn't be any linkage from the controls on the mower to the governor/throttle linkages.
Speeding up and slowing down of the engine can also be caused by an incorrect fuel/air ratio and you may need to adjust this (if a screw is included on the carburettor). Some carburettors dispense with this adjustment and the ratio is fixed.

Reference: http://eugbug.hubpages.com/hub/Why-Wont-That-Lawnmower-Start

Eugbug, April 2013

Just a thought because this happened to me. If the fuel tank sits below the carb, the carb has to suck its fuel up from the tank. If you separate the carb from the tank you will see a plastic pick up tube with a mushroom end. Inside that mushroom is a very fine filter that becomes clogged up with gunge and all those little bits that fell into the tank when you filling it last year.
When the engine starts it runs ok-then rapidly starts to run rough as it struggles to get fuel past the filter.
Cleanout and flush the tank and gently clean the filter-ideally with compressed air to blow all the passages clear.
As I say it worked for me.

GB in Wales, February 2013