Tom, as Mike mentioned, I recommended that he check his distributor, maybe you should do so.
Another problem could be jetting.
Make sure that those idle jets located parallel and on both sides of the fuel inlet are sparkly clean. Probably wouldn't hurt to make sure the passages are clean by shooting some compressed air through the jet opening.
You could also have too big of a main jet or idle jet or too small of an air correction jet.. or who know's.
A visual inspection of the butterflys will reveal if they are opening correctly, another method is to run up the engine and see if both sides are drawing the same amount of air via the air flow meter.
Depending on the octane you're running, 30 degree's may be too much, more like 28 degree's to be on the safe side.
With the engine ""COLD"" re-check the valves and make sure that they are not tight. If too tight, they could be causing the valves to be open during combustion thus causing backfires. But, it sounds more like a carb problem than a valve problem but worth checking.
Another common problem is that the fuel pump isn't pumping enough fuel and you're running lean. Have you checked your plugs and what color is the exhaust residue in your tail pipe???
If your plugs are golden brown or grey, and your exhaust residue is grey you're too lean. These engines seem to live longer if the plugs are dark and the residue is dark.
A slight overrich condition is much better for an air cooled engine that a lean condition. The extra fuel aids in cooling. Granted you'll get more HP with a lean engine but it won't last.