I had this happen to me.
On my Honda PA50, I had an engine fouled with lots of crappy
oil, so bad that my spark plug wouldn't spark unless it was revving
My solution - I liberally sprayed carb cleaner into the spark plug
hole, put clean gas (40:1 mix) in the tank, and used a new spark plug.
I put the old spark plug in some carb cleaner (didn't look too bad).
I turned off the petcock, started and ran the engine at a low
speed for a minute or so to (a) flush out the carb cleaner and the
crap it pulled out of the engine and (b) empty the carb.
I then turned back on the petcock, and started her back up.
I adjusted the idle to my liking, and then turned her off.
I let it run at idle for about 5 minutes, then cut the engine.
Once I was satisfied that it was idling correctly with the the 'fresh'
spark plug, I switched back in the old spark plug.
A couple of quick revs to clean off the old plug, and it was good as new.