I've got an old lawn mower thanks to FatherT 
It's engine is a Briggs & Stratton Quantum XM 35, was a pig to start this year. So took off the float, cleaned it all up, fairly clean anyway, blast of carb cleaner in all its bits, new air filter and put it back together. Problem was the gasket on the air-intake was in a right state, so took it off for now. Re-assembled, without gasket.
Pull cord 3 times to prime, pull again and bang, straight into life.
Runs well, apart from after a while, the revs get very lumpy, it gets so bad it stops every so often. I'm thinking as it heats up, it's letting air in at the intake, more than it should, lack of gasket giving it lumpy hot revs?
Thinking of this:
http://www.ebay.co.uk/itm/Briggs-Stratton-Carburettor-Gasket-Set-fits-Max-and-Quantum-Engines-/221242600987
But can I use a liquid sealer? Or is better off going for one of these kits? 
Check how clean the petrol tank is and any filter in there, as they have a tendency to collect grass cutting over time which as you use the mower migrate to the fuel filter and block it, when the engine stops they are released from the filter and when you start it the cycle starts again.
Thanks, not seen a filter, when I removed the fuel line to the float, it was pouring out nicely, had to stand the feed pipe vertically with some paper towels in it.
As I'm not used to Carbs, I released the bolt holding the float and pondered for a few minutes why it was still draining fuel out of the tiny cap

Tempted to drain the tank actually, as for many years it was left over winter with fuel in it.
Is the filter at the bottom of the tank? I've not seen one
