If it's dry, get a can of Starter Fluid when you buy the plug and spray a bit into the carb's intake. If it starts immediately the carb is not allowing fuel to enter the engine.. some little jet in some part of your carb's idle system is probably clogged.. maybe one of the air passages. Also check for a low float level. That makes it difficult to pull fuel up into the air stream.
Since it's also got symptoms of 4-stroking, my guess is you just have a dirty carb and maybe it's got other issues.. so fuel delivery is erratic.