I should have put this little gem on the tech tips ages ago: Since ratbikers are always putting the wrong carbs on things and then fiddling about for ages trying to get them right here's one quick cheap bodge if your carb is too rich at the idle and low end no matter what you do. Drill a wee small (like small petrol jet sized) hole in the slide about a quarter inch or so down from the top of the carb throat (when the slide is at full bottom) on the side toward the engine. This makes a small air leak that causes the mixture to run leaner just there, but as you raise the slide the hole disappears up the carb and is blocked off and the jetting from there on is the same as it normally was. And if you accidentally make the hole too big you can always clog it up again with chemical metal or something and drill a different one.