Got a riding mower Huskee 21 hp Briggs intek that was sitting up for a couple of yrs.Got it out to sell it but it will not start. Have put in a new battery ,fuel line and filter, taken it top off and cleaned flywheel and coils. Even carburetor off cleaned the bowl and blew out everything with air hose. Will run long enough to burn gas out of carburetor but won.t pull gas from the tank. Does have fire to plugs. What am I missing? Does carburetor need to be taken apart and cleaned. Mower is only approx 5 yrs old and was only used 2 yrs.