Lunar wrote on 09/10/13 at 00:33:56: However, I noticed that the throat of the carb is wet with fuel. So I'm thinking the float is set to high and fuel is getting sucked in at much less throttle than it should.
Too much fuel causes black smoke and rich running...it does not make the engine backfire. The backfire while running is caused by a lean mixture that results when you close the throttle.....and the fuel circuits are closed off while the engine has high vacuum and continues to pull in air....the mixture goes so lean that it will not ignite in the cylinder by the spark plug - but can be ignited by the hot exhaust system (or the exhaust flame from the next time the spark plut does ignite the fuel in the cylinder). When the throttle is closed the only fuel that flows is the idle circuit....and it cannot supply enough fuel to the cylinder when the engine is running above idle speed.
The source of the pop/bang when you shut the engine off is that the ignition is switched off....however the engine continues to turn and pump fuel and air throug the engine and into the hot exhaust system....where it can be ignited.
There is nothing wrong with checking your float level - but it will not do much to change the backfire unless the level is too low.