I have a riding mower with a CV-12 engine. Sometimes it stops as if out
of gas. If I remove the gas cap and wait a moment, a stream of bubbles
from the fuel line will bubble up through the fuel. Then it will start
quickly and run fine.

I don't see what could be getting pumped up with air except the fuel
pump. It's two check valves and a diaphragm operated by a nylon lever.
The outlet check valve is at the top, so I don't understand how the
pump could accumulate air.

Has anyone else run into this problem?