RE: 4 strokes
You're describing a very typical rich idle condition. The engine is "loading up" at idle, excess fuel puddles in the inlet passages, and then finally slugs into the engine, putting out the fire. Continue to lean the idle mixture needle (or open the airbleed a bit further if its' an airbleed type).
It can help to use a tach when setting up a 4 stroke. Really, tho, it's the same as a 2 stroke, except you'll have to check the valve clearances from time to time. To set the idle using a tach, slowly close the idle screw until the RPM's peak (at idle). Go VERY slowly. Once you've found the idle peak, rich it just a hair until it will accelerate cleanly after idling 30 seconds or more. Test by running WOT for a few seconds, idle for 30 or more, then back to WOT.
A 4 stroke can idle as nicely as a 2 stroke, but they speak with a different accent. Once you've burned a couple gallons of fuel and got those needles tweaked, your idle will come down more. The Magnums can take quite a while to break in enough to idle really well, but they'll get there!
J