On Wed, 11 Sep 2013 16:36:15 +0200, JBL2527 wrote:

The powered lawnmower has a Briggs and Stratton engine, normally
bullet-proof. A couple of weeks back the bolt holding the fuel tank to
the body worked loose and the tank was wobbling.

Body of the engine or the underside of the carburetor?

I had similar when one of the flap valves that are formed from the
gasket between the tank and carburetor got stuck/misplaced. Suggest
you take the whole carb/tank assembly off the engine and check that
the gasket between tank and carb is in the right place and the flap
valves are OK.

--
Cheers
Dave.