teapot wrote:
I have a Sovereign Classic 35 lawnmower with a Briggs and Stratton

FWIW I have just had a mower fuel problem that turned out to be caused
by 'old' petrol. according to the mower repair shop modern unleaded
petrol goes off after approx 12 weeks. Mine had been serviced in the
autumn and came to use it two weeks ago filled it up with last years
petrol - no water or contaminants - and would start but not run.
Apparently this is a common fault and some engines like my Honda are
more prone to this than others.