I have 2 flymos with two stroke tecumsch engines that do this, i suspect that the light weight of flymows causes vibration to wear out parts in the carb. I had one carb with a float valve system develop a leak in the float itself. Information and parts for tecumsch carbs seem to be available online. Try a carb kit overhaul.
I have had trouble with petrol cap vents getting blocked as well, and this is the easiest cause to diagnose so investigate this first.
Another problem is the fuel taps which tend to develop leaks. Without a fuel tap the fuel cant be turned off and will tend to seep into the carb while the mower is not being used, either evaporating and leaving un-burnable oily residues or leaking out of the mower or both. If the petrol tap is working turn the petrol off and run the engine until it stops before putting it away. I don't have the taps fitted any more cos they leaked and i could not find replacements so I always drain my tank now before putting the mower away.
I have also read that this can be caused by a leak of air into the crankcase that only opens up with heat. Old cracked gaskets or shaft seals would be the culprits here.
David Bean, November 2016
