FCR's falling on their face when throttle is "whacked" at low rpm's isn't a problem with the carbs. That is the result of poor tuning. I had a set of 39's on my old CBR1000F and had 39's and now 41's on my CBR900RR. I launch my bike around 2500 and nail the throttle. My 60ft. times are in the high 1.40s. FCR's require alot of fuel at the hit. If your bike is stumbling, you need either accelerator pump adjustment, air screw or fuel screw adjustment or larger pilot jet..or maybe all 4. They are awesome carburetors but are a pain to tune (took me a couple years to catch on). On stock motor, hp gains will be small (maybe 2). On a modded engine is where they shine. They flow considerably more than CV carbs. I bought 41's new and they are worth every penny of 1100 bucks I spent on them (back in 2002) Carbs in pic are for a side draft engine (red stacks). Blue ones are for downdraft engines like the RR. Anyone needing help with tuning, PM me, I'd be happy to help.