I have a yard machine with a 16.5 twin briggs.I put a new coil on it and still don't have contenous spark.it will spark one time when it starts to crank then of and on while cranking.I checked safety switches and they seem to be OK.I don't know where to go from here and don't want to throw money at it.

Sounds like coil is going to ground ,
Remove blower housing and unplug the little black wire going to coil and see if it starts , if so see if the wire has a diode in it ,if so replace wire.
Luck