Forgot to mention, you could take it apart and check for debris under the valves, I had some what looked like fine sand under valves in mine that was allowing fuel back out to the intake. That discovery led me to remove and flush the gas tank. I discovered it was the sock over the inlet tube that was disintegrating and causing problems down stream, good thing I have an inline filter before the carb. I installed a Robbmc 1/2 inch pickup/sending unit and a pre-pump filter.

Imagine, a pickup sock that falls apart after only fifty years!