I finally gave it up! I tried the FCR 41 carb on my xr600r, supposedly the hot set-up. This was not the FCR MX 41, but the standard FCR that Sudco recommends and sells as a kit with cables and bell-housing specifically for the xr600r. Although Sudco maintains that they have jetted it perfectly (50 pilot, 170 main) for the xr600r (with snorkel removed and a medium flow muffler), they are way off. Even with the mixture screw turned all the way in, and using the smallest jet in the "kit" (a 48) it still is too rich to pass emissions testing. The 170 main ran way rich on the dyno/EG analyzer. The carb has no choke, so it's a PITA to start cold, and it is also very difficult to start hot. If you get the bike running, don't stall! The mid-range throttle response was great, and it was hard to give that up, but in the real world, you have to be able to start your bike, and in many areas (e.g. Phoenix), you have to pass emissions. I've now heard that you have to go down to a 42 pilot jet. Who knows where the main should be. The dyno tester in my area told me a 143! I tried that, and the bike was down on power. If I'd known better I would have bought the MX version that has a choke and a hot start button, but Sudco didn't offer it in kit form for the xr600r, nor did they recommend it. The MX carb alone is $560 and comes equipped with several clear pink vent hoses that some have likened to an octopus on top of the carb. I also notice that Xrs Only has pulled the FCR carb and the kit from their on line sales. I'll bet they had a lot of complaints. When Sudco calls this a racing carburetor, that's what they mean — this $525 non-refundable unit is not suited for the real world. I'm back to the stock carb.