Is this thing a stock HP500? Is the intake the stock Dart? Those carbs usually had staggered jetting in them! Is it fat at idle or after its on plane? Have you leaned out the air fuel mixture screws? In is leaner and out is richer. You will be somewhere around 1 1/2 to 1 3/4 if its a stock carb from the all the way in on the air fuel screws. It may have a bad power valve, thats the reason for asking, if you run the air fuel screw in does the idle change and does it want to stall?