[docjohn]

Spitting or coughing back through the intake is usually due to the mixture going lean, unless there's crossfiring on the HT leads under load. Are you on a dissy with carb and vaporiser or injection?

 

[docjohn]

After you've eliminated all the usual suspects, it does sound ominously like throttle icing or what used to be called carb icing. Cool and very damp weather with small throttle opening on cruise are classic conditions for it. In some ways it's surprising as there should be plenty of heat around there. Anyway, worth a thought once you've eliminated stray sparks, dirty fuel, low fuel pressure, lean calibration and whatever other forum members suggest.

Just one other thing, and it's an expensive way of solving a one off problem, but I've found that having wide band lambda sensors in both manifolds has been invaluable in diagnosing problems with a Holley.

Good luck

John