What I think this tuning technique is trying to accomplish, is make the maximum allowable airflow of the carb match what the engine can actually use. I haven't tried this idea, but i feel it may help. The magnum 1.08 carb with the sleeve installed is a much smaller bore than the ST carb, and i get the same rpm with it that I got with the ST carb. The difference is that the main needle is more responsive, and the low needle is less sensitive. It would be nice to see someone make a sleeve for the ST carb to choke it down to the same diameter as an OS 7D and see what it does. When I ran the Magnum carb without the sleeve I had a less responsive high needle that needed to be opened much more, meaning it was less fuel efficient. I think the intake diameter of the carb is a major factor in engine performance and operation.