My 8hp did this and ran like krap, I removed the carb, emptied the bowl of existing gas...and sprayed carb cleaner into every orifice and into the bowl, turned the throttle to make carb cleaner shoot through the throttle body a whole bunch of times

then reinstalled the carb...pumped the bulb to fill the bowl...added water and it fired right up and ran like a champ

If you attempt this be careful not to break the little plastic throttle linkage connectors

Also take out and test that thermostat just for fun while you are messing around.
This worked for me.
Lou