Mountfield mower Briggs & Stratton motor - Start well then begins to 'hunt' almost stopping before reving up again only to start the cycle again. It appears to be using a lot more fuel than previous.

Bernard, June 2009

in my experience with mowers 95 percent cert u need a new diaphram kit in carb this is typical symptoms and have changed loads diaphram shrinks over time allowing excess fuel in cost about a fiver this only applies to models were carb sits on top of tank diaphram sandwiched between the two and easy to change

clive, July 2009

Change the carb diaphragm. Guaranteed fix.

phil_saunders(a)bigfoot.com

Phil Saunders, June 2009

Hi John - Yes I'm sure it a carburettor problem. My mower has a very simple 'on top of tank' one, but there isn't any adjustment screw only those screws that attach the carb to the tank! Could my trouble have anything to do with the vent pipe that seems to connect the carb to the exhaust side of the engine? May be I should try cleaning the filter (petrol?) even though it appears clean.
Cheers, Bernard.

bernard, June 2009

Hi Bernard
Black smoke from the exhaust is caused by excess petrol being burnt, i.e a rich mixture; blue smoke is oil burning, due to excess engine wear. I guess the plug is black and sooty, therefore there is too much fuel getting into the engine. Now then - depending on which engine/ fuel system you have....is the fuel tank below the carburettor? If so you'll find a single spring loaded screw on the carburettor which controls the fuel mixture, and to set it, you have the engine going as fast as it can on full throttle, and you turn the screw anticlockwise until the engine starts to stutter a bit, then turn it back in until the engine runs cleanly.

John, June 2009

Thanks John - Yes, there is black smoke from the exhaust, and the plug is oiling up. I have checked the filter and it appears quite clean. Can you suggest any thing else?

Bernard, June 2009

If there is any sign of black smoke, check the air cleaner element for dirt or other contamination.

John, June 2009