I think that the nitropropane will work because its designed to work with gasoline instead of the alcohol, I'll look for some on my local motorcycle shop and give it a try!!! about the diaphragm swelling I my self haven't seen any att all yet.
The other product I will like to try is VP'S propylene oxide which is an additive for gasoline racing once I have time to go for some shopping I will buy it, I've been busy building all them Ryan's Rebels that people have ordered, so it will be some time before I'll go shopping.
As for the rich settings You will need to open the low end as well as the high end needle enough to make it run smooth what I do is open both needles about 3-4 turns start the engine adjust the high end and then the low end at full throtle until you find an equilibrium point where the idle and high end works best, so good luck with your experiment I hope you are enjoying it as much as I am.
Bill Pomplun.