Thanks Centerline and others, for your advice, explanations and pictures! For my 2001 TW200 with ethanol problems, I decided to let the dealer do the carb cleaning (everything inside the carb was bright yellow from the ethanol residue). When I got the cleaned out carb back, I added socket head cap screws to the float bowl for easier access. I drilled out the plate/plug over the idle mixture screw, and changed the setting from the stock 1 turn out, to 2-1/2 turns out. I added 3 x .01 shim washers to the needle. And I removed the stock #126 main jet and replaced it with a #128 main jet. I viewed these steps as a conservative way to increase performance, without trying to approach the bleeding edge of carb tuning.
Wow, am I ever pleased! At startup I need little or no "enricher"/choke to start the engine (note that it still needs to be warmed up before moving off). It is more "ready to go" at any speed, and there's no indication of any troublesome side effects. The engine simply seems to run more smoothly, and to take throttle more readily, at any engine speed.
My only regret is that I didn't do this mod years ago, since it would have made off-road riding a whole lot more fun.
- - - Happy trails, Jim