Small Engines (Lawn Mowers, etc.): Lawn Mower wont start, craftsman 917, champion rc12yc

Question
My friend brought me his mower (craftsman 917.378941) because it won't start. He replaced the plug but nothing.  After looking it over, I tested the spark at the plug by grounding out the base, it sparks.  There is also fuel in the bowl.  I even tried starting fluid (small quantity but have not even gotten it to fire.  Although it appears to backfire from the intake (not major#.  I worry he got the wrong plug which is not hot enough to psark under compression.  I noticed the plug he put in #FF-20) was not an exact match to the champion RC12YC the specs call for.  Could this be the problem.  If not, what else?

Answer
Jim
Have him install the correct plug as these plugs  have a specif heat range,also have him set the gap at 0.30 Also back firing could mean a valve problem.  I would suspect that the carb has water or other contaminents. Remove the carb bowl and inspect. Clean as needed. Also drain the tank and replace the fuel filter if you find anything foreign in the carb bowl.One other thing may be bad is the coil having intermitting  problems.