You can drill a small hole in the plug in the top of the carb body and use a small sheet metal screw to pull it out. The idle port adjusting screw is underneath the plug. When I had my GN400 I´d also read where you could get some small flat washers and stack them underneath the c-clip that´s on the one groove in the needle that controls the main jet to richen the mixture. As for swapping carbs, I would think that just about any carb of the same size would work. 36mm, if memory serves me correctly. I´ve got a Sportster now and have swapped carbs about 4 times now, and they´re all about the same size. I´ve had butterfly carbs, a vacuum-piston one (like the Mikuni on the GN400) and currently an Edelbrock that is a flat-slide carb.