WX 11 Query

Morning all, I have a Villiers WX11 that for a reason I cannot seem to sus out  why the engine has suddenly started to do this,   the engine starts with no problem but after approx.  30 seconds  or so of it running it will spit back through the carb. and the governor arm that connects to carb. top moves to fully down position, after spit back/misfire gov. arm returns to normal running position and engine will run ok for another 30 secs. then same happens.

The engine does this regardless of where choke lever on carb. top is positioned,   I have had carb. off and to bits and cannot see any problem so help please.

Thanks for any help

Well problem solved, the fuel control needle was being stopped from moving up/down to allow the correct amount of flow of fuel into the float chamber by what looks like a very small piece of rubbery stuff, with the fuel tank being nearly empty I suppose that did not help to move this bit of whatever it is into chamber.   So after engine has been running for a while fuel level is too low but when fuel is up to needed level away it goes till same problem happens.

Its now for sale on an auction site, genuine reason for parting with it.