Once started my Mountfield SP530's Briggs and Stratton engine starts to "hunt" namely revs rise and fall despite there only being one throttle position.I am reluctant to start major strip down without being sure of problem

Nige Reanney, September 2007

It's the diaphragm! I had the same problem. Be very careful to fit the new diaphragm correctly. There is a gasket on top of the rubber diaphragm. The gasket goes against the carburetor and the rubber diphragm against the petrol tank body. It is very easy to tear the diaphragm when putting in the bolts. I am speaking from experience.

Dennis, October 2009

It is probably the diaphram in the carburettor has degraded and needs renewing. New one costs about £5.

David, June 2009

I have same problem exactly!

Malc, May 2009