have a listen

I'm wondering if your float levels are correct. And another possibility is your vacuum is low enough the power valve is opening some at idle, which would only get worse when you put it in gear. Not sure what carb you have? 390, 450, 600. Possibly could have a 10.5" valve? Were you able to read your vacuum with a gauge yet? Some will discount that the PV should not affect the idle circuit, but i have personally seen it be a issue.