I have been riding my HS-powered bike for several years now. Originally, I rode with a direct chain drive, but performance was marginal. I finally broke down and bought a SBP shift kit and added a Shimano 3 speed hub and my takeoff and top end improved, but the engine was still anemic, even though it is dependable, quiet, and easy to maintain. Last week I bought a SBP free flow air cleaner and even though it is pretty narrow, it was a tight fit. The engine obviously was running too lean so I started researching how to re-jet. I finally found a go kart website with excellent instructions for drilling out the stock jet on the stock carb. I know you have heard a lot of advice about how to get more out of these little engines. Do yourself a favor. Get a set of wire drill bits with a range of #60 to #80. Remove your carb and pull the jet. Carefully drill out to .260 (#70 or 71) and install a free flow air filter. Attached is a picture of the moped air filter I used. I fabricated a mount from a 3/4" chrome flange for a handrail for a boat. It is 45 degrees so it allows the cone filter to clear the center post. I centered the center hole for the flange over the stock filter and marked the mounting holes. I drilled the mounting holes to match the pattern of the carb. The same bolts hold the carb and the flange to the intake port. The chrome flange was $4 from a marine salvage shop and the cone filter was $11 online. The engine gained noticeable horsepower and the filter can now be changed by simply removing the hose clamp. With the SBP filter, I had to remove my engine to change the filter. The difference this modification made is amazing. It's still not a screamer, but at least it's not a crawler anymore. I don't need to over rev the engine because of my gears, so I don't expect serviceability to suffer.