If you are launching in 3rd and spinning, you will want to work on a few things. Either re-gear or find a better throttle/clutch control combination. Andrew Short (he is a professional motocross racer) is one of the best starters in current mx/sx. He says part of his success comes from 3rd gear starts. He could be full of dookie and trying to throw people off. But the theory is it cuts a shift out and lets you hold hold it in gear longer...but can be abusive on the clutch.
Also you mentioned it has a bog. You should dig into that and figure out the cause. Sounds like you are working on it, but obviously it will help your track performance. Are you lean on the pilot/needle? Have an air leak? Hopefully you are running an open air box.
That does look like a bunch of fun. Do they ever run a full track? Right and left turns?