float valve does not cut off the fuel supply and the bowl overflows.
Because of changes to EPA regulations the overflow is inside the carb throat so liquid fuel flows down the carb and into the engine.
Liquids do not compress so when you started the engine the uncompressable liquid finds the easiest way out, usually through the gasket.
This was most likely done a while ago and the hole is just getting bigger till it gets so big the engine will not run.
For liquid fuel to be blown out of the muffler, liquid fuel has to be getting into the cylinder and that can only happen if the float valve is not shutting off the fuel.
Generally the fuel will also leak past the rings & dilute the oil in the sump causing the oil level to rise.
And it is not really a diagnosis, for that we need you to do a lot of tests.
On the web it can be no more than a best guess because all we have to work with are words on a screen.
Check your oil level, is it higher than it should be & smell like fuel ?
Did the mower crank for 1/2 turn then struggle to complete cranking ( trying to compress liquid fuel )
Then spin a bit quicker than normal ( compression lowered cause gasket is blown )
Does it start in 2 or 3 revolutions or did you need to crank it several times for long while before it starts
Is the inside of the air filter housing oily.
Your engine manual will be found here
http://www.mymowerparts.com/about_us.php
