Lawn mower pull start kicks back?

I own a 5 h.p. push mower that pulls fine when the spark plug is removed. However, when the spark plug is in I cannot pull it all the way without it kicking back. I haven't checked the valves so the problem could lie there. If not, what could be wrong? I thought a sheared flywheel key or bent crankshaft would be a possibility but I would think it wouldn't pull at all if that was the case. Maybe timing being off could be it...just looking for some other opinions. Thanks