Sears troubleshooting tracter cranks over backfires but wont start

Check for leaking intake gaskets or leaking head gasket

Those issues will draw in extra air, that will lean out the fuel mixture
or prevent the proper vacuum from drawing fuel into the cylinders,
thus the lean backfire

Need to get yourself the spark plug tester with the clip attached
& says for small engines,to test for a strong spark

There is one also that says HEI on the plug
That one is for vehicles with electronic ignition
& plug gaps of .043 thru .060,not for small equipment

If you have a carb with a float,take the carb apart & clean it
with carb cleaner,then green brakleen & install a new spark plug

For the gaskets,you pretty much have to take things apart to check them

A tune-up on small equipment also includes changing the head gasket, as odd as that seems

If you hear your neighbors mower popping as they cut the grass,
they have a leaking intake gasket or head gasket

When you clean the carbon off a piston & cylinder head you
will think the motor is brand new & starts on the first try