My rb30 had the same stuff happening when I bought it. I put it on a Dyno and was getting a whopping 30kw at the wheels. The Mechanic fiddled with the second stage butterfly. He found it was stuck shut. The engine immediately gave us 70kw. He advised me to take the carby off and have a go at the second stage with wet'n'dry. I found that the butterfly had worn a groove in the bore. I smoothed the butterfly and the bore. It took me quite a few goes to get it right, but persistence paid off. It has had no such problems, for over 12months.