Why does my John Deere L2548 (Scotts) lawn mower backfire and rev up and down.

Your answer was the BEST re: Valve Adj. TDC! Ttotally fixed the problem. THANK YOU! Now the Lawn mover is backfiring and reving up and down. It started doing this once we ran out of gas while driving it. We have a OHV engine. Our mechanic cleaned the float bowl. Any suggestions? Thank you. sue.

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