Why is gas leaking into air filter at low throttle?

Gas is entering the air filter housing, or the air filter? Which are you referring to here?

Hi MOWERMAN2468,
I bought the mower used. It needed a battery charge. So while that was going on, I checked the oil and took a look at the air filter. The oil was clean, but a bit over the full mark. I attributed this to the fact it had not been cranked in some time. The green foam filter on the outside of the metal/paper filter was clean, and the air filter housing was dry. Everything looked okay. The gas tank was empty and free of debris. I Filled the gas tank about half full and then started it. It cranked with no problem, it did spit and sputter for a few seconds, but seemed to catch up quickly. It responded to the throttle increase and decrease. I engaged the blades and started mowing. Then it seemed like the engine would bog down. Like it was flooding itself, then burning off the extra fuel. I thought it might work itself out, thinking there might be some trash in the carburetor. It didn't get any better, so I stopped, and checked the air filter again. It seemed like not enough air was mixing with the gas. When I removed the covers to check this, that's when I found an almost "black" green foam filter, and the housing holding gas around the breather. I had to wipe it up with a rag. I got it dry again, and cranked it without the filters (which I understand is not recommended... ) Anyway, I ran it for a few minutes like this, and increased and decreased the throttle. I saw gas "spurt" out onto the dry housing... yikes. I turned it off and have been reading repair documents since. I could not believe how much gas was gone in the tank.

Well the Black on the filter is probably motor oil. This would have come from the breather assembly. The breather assembly is usually a rectangle shaped "box" that is connected to the air filter housing by a black tube. So follow the black tube from the air filter to the engine and you will find the breather assembly. Now, you did not indicate any smoking from the engine. Usually you will have a "white" smoke from the exhaust as this occurs with a faulty breather assembly.
And as far as seeing fuel spurt up without the air filters in place, this is common, as long as it is just a small amount, which I am sure it was, or it would not be running properly.
I think you have a breather assembly problem.

Thank you MOWERMAN2468,
You are right, there was white and sometimes black smoke. I retrieved the owners manuel for my MTD Yard Machine, and I will check the diagrams for the breather apparatus. Do you think it is clogged, or will I need to replace parts?

The breather assembly is easy to find. The black tubing leaving the air cleaner housing to the engine is where you will find the breather assembly. Usually a 5/16" socket and a few seconds and it is off. Be sure to get a new gasket as well. This is a simple repair. Good luck with this task. As far as the black smoke, that is caused by fuel. Remember white smoke equals Oil burning, and black smoke equals excessive Fuel burning (flooding). And the black smoke is likely caused by the air filter being oil soaked, causing a choking effect on the carb. Which in turn added more fuel to the system, causing slight flooding, causing the black smoke.

Right on! Thank you so much! I will let you know what the results are.