My riding mower starts but shuts off within 5-10 seconds. I have a simplicity regent 12 1/2 hp riding mower. I already drained fuel (added fresh fuel), change spark plug, fuel filter, checked fuel lines and cleaned the carburetor reservoir. There was some gel in the carb reservoir. I guess my next step would be to clean out the carb? Any other ideas would be helpful! Thanks!

From what your saying you have already covered everything else ,so now you need to take the carb off and clean it good,
Luck.