Simple: You have an air leak somewhere.

There are many places for air to get in:
1. Around the carburetor barrel
2. Through the front bearing
3. Under the carburetor seat
4. Around the backplate
5. Through the head gasket

What nobody's been able to explain to me is why OS engines and engines with OS carburetors shut down reliably. Even OS engines use shielded (not sealed, not airtight by any stretch of the imagination) front bearings. You'd think that if the front bearings were the problem, OS engines should have major shutdown problems too.