ive got a 73 200, mine acted up for a bit , i traced it back to the idler plunger, it has a bend in the arm ,if that bend isnt stable it falls and vibrates , i wrapped a plastic zip tie around it and hung it from the idler arm loosely and that took care of it. i dont know if thats youre problem but , you could also look at youre adjusment screw on youre carb, it could be chockeing it out. i dont know tech terms ,but it does sound minor for what its worth.