what carb is on it ?
some times the powervalve is right on the vacuum limit and the valve can close shutting the fuel off on the p/v circuit.
ideally you want the p/v closed at idle and the mixture screws opened up, so it is the idle mixture scres that feed the idle fuel. you may have the butterflies open too far which start to draw fuel through the main fuel circuit as well.
you can drill holes in the butterflies to fix that.
also check if the vac advance in the dizzy is moving , ie pull it off.
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