The stock venturi is .196" or 5 mm. That is an ok size, but there is a spigot and needle that restricts the hole to almost half of that. The carb was designed to run reliably, and not so much to outperform other motors IMHO. The motor itself is pretty good. Other .15's that I have are 6mm with a clear unrestricted hole, so the Norvel is quite conservative. In contrast, the LA .15 has a small carb, but a bigger hole (no carb at all) makes very little difference. That is more of a porting restriction for ease of use. It is a good motor though, well engineered and tested.