My Murray has a Briggs and Stratton 18.5 hp motor. It has always backfired at the end of the day when I park it for the night. Now the mower backfires constantly. I cleaned the carb., replaced air filter, replaced spark plug, replaced fuel filter, and drained and changed the gas tank. The mower acted better for just a bit then started missing when I was going down hill only. Now it is back to constant backfiring again. What is my next step?

James Nickens, May 2007

Hi James,
Start with the easy/cheaper stuff first and re-check that the fuel level in the float chamber is not overflowing and overfuelling the engine. If you are happy that all is well with carburation and ignition it sounds like this engine is ready for a de-coke, depending on how many hours use it has had since new or last de-coke. WHY? Because I think the backfire at switch off is caused by pre-ignition ie Red hot carbon in the cylinder igniting the fuel sucked in after the ignition is killed but before the engine stops rotating! Good luck!
Mr Fixit.

Make do 'n' mend!, May 2007