Probably need to fine tune the idle circuit/mixture screws, and blow out the air bleeds with a can of carb clean. Something you will need to know how to do, even if you send it out and put it back on after a rebuild.

One thing alot of guys do is crank up the idle speed screws. What ends up happening is the throttle plates open too far, and the carb begins bypassing the idle circuit. Ends up running like crap, loading up, etc.

A vacuum gauge is a useful tool to tune the idle mixture screws. But, you can do it without one. First i would blow out the air bleeds, both idle and main jet bleeds. i'd turn the idle mixture screws all the way in, and then back them out 1.5 turns each. Start the engine, let it get warm. Then take each screw, and slowly turn it in until the idle speed drops. Then back out a 1/4 turn.

If you find turning the idle mixture screws in all the way, doesnt wanna kill the motor, the idle speed screws are probably turning up to high, which you will need to back them off, and re-adjust everything.

Once you get all that sorted out, you can go do some WOT runs, and check jetting. I believe the 500HP engines used staggard jetting. They ran on the fat side stock i believe. So you might be ok with the upgrades, but I'd still check plugs.

I have 540 hp 502s that have a little more cam and cnc dart pro one heads. I use 800 cfm holleys jetted the same as an HP500.

Staggered jets Pri 81/75 Sec 89/93. It takes a little time to tune for good idle, but it can be done as thunder explained. You may have to adjust timing at idle as you adjust idle.

End result will be better idle, but will still run a little fat at speed.

Thanks Guys that was alot of help and good info.

stagered jets
 

Sport...Can you explain "staggered jets Pri 81/75 Sec 89/93."? Thanks

What he means is merc used different jets all the way around in the HP500 carb. Most common jetting would be say 92's in the primary, 96's in the secondary. Where as merc didnt use the same jet size in the primary or secondary on the HP500 setup. The primary got a 81 jet and a 75 jet, secondary a 89 jet and 93 jet.