Could be a lot of stuff, but why look for trouble? Assume it's crud in the carb, best fix, do a very good tear down and clean the carb paying special attention to the small orifices therein. What "usually" works is put an ounce or two (don't know how big your gas tank is) of SeaFoam or equivalent carb cleaner in the gas tank. Run that through and repeat. carb cleaner should eventually remove the varnish, etc. from the fuel circuit including the carb. Make sure that any gas for small engines has Stabil or equivalent fuel stabilizer IN THE CAN prior to fueling. Modern gas has ethanol which does a lot of damage to small engines if not treated. The two products I mentioned I've used for years and swear by, lots of others out there.

Good luck,

Ev