18 hp briggs on idle it will speed up & down & pop . Very hard to crank sounds like engine out of timing

More likely the carb is ready for a total cleaning or boil. I had one like this, cleaned it twice with no improvement. Had a friend rebuild the carb, no improvement. Instlled an OEM carb and it now runs fine. The OEM from Sears will run you about $120

Check your flywheel key sounds like the timing