I too had the same problem and it was most definetely fuel boiling in the carb after shutting the engine off. I lowered the float levels and installed a half inch plastic spacer purchased from Jeg's( I preferred the black color rather than the typical redish phenolic spacer ).
JEGS CNC Poly-Billet Carb Spacers - JEGS
I have not had an issue since. I was running the cooling fans manually for around 5 minutes as a temporary fix until I installed the spacer and still do so when I park it in the garage.