The ST .51 is a bored out 46 case. Its light and produces excellent power. However, its a little bit of a bear to tune correctly and takes patience to get broken in and tuned correctly. I found that the I needed to lean the low needle out more than I woudl have expected to get best idle and transition. Also follow the tips on the Super Tigre manufacturer's forum for carb tweaks. Once properly tuned this was a beast and ran very very well.

Duke