I have a early 80's craftsman lawn tractor. It's a 12 hp briggs and straton. It will not start with the air cleaner on the motor. If I take the cleaner off it starts, once running I can put the air cleaner back on and it will run like normal.

So killing engine by suffocation. I like to do that to many motors.

The problem there is the carb is starving for air plane and simple. Ran an old outboard boat motor cover off until I fixed the exhaust donut.

Is it running rich when you run it? Is the air filter actually clean ? ( had to ask)

Look though air filter with a light to see if you see light through it. If not is to dirty,

Does it smoke with the filter on?

If I had the All the briggs engine numbers I could teak this better