Depends maybe on if you have vacuum advance. Timing plate swings on one pivot, off to the side, and if reluctor gap or points are set critically, the gap changes as soon as vacuum level drops when you go into it, and then you misfire. How the carb is ported, how much advance picks up in the canister, and the spring tension against the canister diaphragm all work together.
But I think your reluctor gap is too close or too loose.
