The problem arises when you mention "HP" carb. A few years ago a trend was started with the custom carburetors where billet blocks were the rage and all the metering passages in the carb block got "special tuning" by adding more emulsion holes. Holley started losing sales and had to jump on the bandwagon. Long story short, what it all boils down to is the "new" style carbs have been altered in a way that will make them run rich and have every problem you are having. Don't get me wrong, they run great at WOT and such. But, cruising is not their strong point. They will turn you plugs black and gas will eat your lunch. If you want some milage and a cleaner running carb, search ebay and find a used, good condition, 3310 750 vacuum secondary. With very little tuning you can get that carb to run very well on your combo and run much cleaner. Save the HP for the track.....even though there won't be much difference between the two if tuned right.