sounds like porting,pipe reeds carb jetting to me.
I will echo this statement. If your jetting is proper, change the pipe. The 38mm probably isn't helping things on a 67.5mm bore. I run a 39mm on a 330 and sometimes wish I had a 38mm.
Originally Posted by american opel
You are correct on the gearing. Your small gearing is best for getting more rpm's at a given speed vs bigger gearing. I run a 13/38 myself for the tight woods.