Blos Carb
 

Here is a nice carb for running propane or CNG, seems with the correct carb the power is similar to gasoline. I managed to buy one of these, hard to get now. I plan on using mine to introduce vapor into engine for improving gas mileage. I think it will be great for adding supplemental gases to the intake. :D
http://firebird.fm.interiowo.pl/blos/