I have a Mountfield 46R PD petrol lawnmower - two years old.
Engine has started hunting / chugging and won't run at high speed.
I've changed the spark plug - no change.
Seems to be running rich - plug sooted up.
I've checked air intake - all ok.
Replace the carburettor diaphragm. 20 mins and £2.
phil_saunders(a)bigfoot.com
Phil Saunders
May 2010
carb needs cleaned out or new air filter
paul
May 2010
If you have a mixture adjustment screw on the side of your carb and it's between the carb body and the cylinder try turning it clockwise 1/8 to 1/4 turn at a time and see if that improves things. If the screw is between carb and air filter try turning screw anti-clockwise etc etc.
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Try fixfault.com to fix lawn mowers.