Small Engines (Lawn Mowers, etc.): lawnmower, crankcase, spark plug

Question
when start lawnmower oil blows out of muffler which gasket do i have to change

Answer
I'm guessing there is another problem.  Did you recently tilt the mower or does the oil smell like gasoline?

If the oil smells like gas then the carburetor may be leaking.  Gas eventually gets in the oil, thinning it and filling the crankcase.  If this is the case, you need to change the oil, clean/repair the carb and maybe clean the muffler.

I would also recommend removing the spark plug.  With the spark plug removed, pull the rope several times to drain/remove oil from the cylinder.

Let me know if this helps.
Eric