The 7D is a different animal to a heli carb. Heli carbs are often metered rich in the midrange because of the rpm they’re operated at in relation to the load they’re under. A heli carb on an airplane engine won’t run very well unless you have the ability to lean the midrange enough to clear it up. I put an Enya .60 XF-IV heli carb on my S90 and it ran good on top, but needed to idle higher to be able to clean out the fat midrange you couldnt lean out enough. The midrange was adjustable, but not enough to make it work. The OS 108FSR carb (7D) is a popular replacement for the stock ST carb for two reasons. Many people don’t know how to tune an ST carb properly and aren’t satisfied with a rich midrange that can be tuned out, and the 7D carb has a bigger choke, so the engine often picks up 1k rpm or so.