When you put the Demon on did you check the float settings? If the floats are letting too much fuel into the bowl, it will flood itself and run erratically, which in turn will cause the vacuum to go nuts. Check all the carb settings, not just the idle mixture. Also, by any chance did the engine back fire through the carb when you were getting it going? If you so, you may have popped a power valve which could also cause some of your symptoms. A timing change from 10 to 8 BTDC should not affect a rich condition.
Hope this helps, good luck with it.

Pat