Small Engines (Lawn Mowers, etc.): lawnmower repair, petrol cylinder mower, fuel cap

Question
Hi,
Hope you can help. I have a petrol cylinder mower with electronic ignition. It starts very well and then runs ok for around 30 seconds before backfiring through the carbrettor then slowing up and finally stalling.

I have taken the carb off and cleaned thouroughly with an airline and even tried a spare carb - no difference. I have tried another plug and even took off the cylinder head to check the valves - am now at a loss, have you got any idea what else it could be?

Thanks for your help
David

Answer
First try loosening the fuel cap slightly to allow the tank to vent.  If this corrects the problem, you will need to clean (or replace) the fuel cap (which has the vent).  If this doesn't correct the problem, the issue could be related to either a loss of spark once the engine gets a little warm or compression loss past a valve once warm.  Check for spark immediately after it quits.  If it has spark, remove the spark plug lead and crank the engine over and listen for a whisping sound from the exhaust for a leaking valve.  Hope this helps.