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The spark plug gap for a Murray 12 hp riding lawn mower should be set at .020 inches. This is equal to .5 mm.
.20
I am assuming that the mower doesn't start.... My first guess would be that you are not getting spark to ignite the gas in the engine. Check to see if you have spark at the spark plug.
Should be 5/8
Spray some ether in the carb. If it fires, it has spark, but not getting fuel. start there.
Without knowing what engine or what kind of mower it is this is just a guess. But, it is possible that your flywheel and/or coil have gotten rusty and are not getting the conact that they need to make the spark. Try removing the cover and sanding the flywheel and coil. If it is a riding mower with a key switch there are many possibilities including anything from a bad switch to one of the safety switches.
RC12YC Champion
Front middle of engine. Easier to do by removing the cover held by four bolts on top of engine. There is a opening in front but to small to work with.
Generally the ignition switch activates headlights on a riding mower and the start position only send current to the starter solenoid. If you turn the key and the engine cranks over than the lack of spark is going to be something else. If it is a riding mower you will have several safety switches to prevent start up under unsafe conditions.
Some cub cadet don't have sparks but can crank.
Valve guides are worn and piston rings are worn crank case oil seeping up to spark plug fouling them change spark plugs or try irridum plugs they sometimes work in engines that consume oil.
does it have spark to the plug if not the part you might check would be the coil or the electronic start module probably the latter if it is a newer mower or of ots older then the coil magneto usually dont go bad could be a bad gap between the coil and the magneto if its an older mower could be a bad set of points if its an older mower point gap could be bad spark plug could be bad as I don't know exactly what the lawn mower is doing hard to tell what is wrong