280C "cough" when shutting off.

[cefalu]

I am experiencing a strange mini backfire when I shut my '74 280C off. Right after the motor comes to a rest after shutting it off, it gives out a little cough through the carb.

I am using a Holley economaster, but it is brand new NOS. It did the exact same thing with the previous Holley too. In fact that's why I bough the new one because I thought it might be the carb. So I am thinking it's not the carb anymore. My timing is 10 deg BTDC. It's the M110 DOHC USA version engine.

Otherwise, the car runs perfect, as good as fuel injected. Perfect steady idle.