I would look to the mixture first. 30 degrees advance should be fine. Normally with alcohol, you can run more timing before bad things happen. The fact that it revs the first time makes me think the timing is right. It could be too lean, or it also could be too rich and and it is loading up and when you advance the throttle again, there is too much fuel and it backfires. I would pick a direction and go with it and see what happens.
Has the conversion changed the fuel consumption?