Hi Ken,

           What comes to mind is the motor is not generating enough vacuum
to pull the fuel in.

When you pinch the return you create such a rich environment is gets enough
fuel to run.

As you know this has to be caused by a mechanical fault of some kind. Valve
timing is the most common.

Try the old rag trick in the spark plug hole and see if it blows out.

Good luck and I'll be watching your results.

John