318 backfiring

Might be water in the gas, which happens naturally with changes in temperature and humidity.
When the water gets into the bowls it is reluctant to leave.
To prove it, just drain the Primary bowl into a clear glass container and you will see it cling together in the bottom, looking like a slug of some kind.
If you find water, you have two choices, pump the tank dry, or add some acetone which will bind the water to the gas and make it easier for the water to pass thru the jettings.

If water has sat in the bowls she will likely need to have the fuel wells cleaned out.