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Does anybody user FCR on his bike ?

3535 Views 12 Replies 8 Participants Last post by  cbr900racer77

Are anybody use FCR carbs on his bike? Are they so good as theirs price and what need to be done to make them work properly ?

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Hi Guys,
just a side track, the red things caught my eye and I remembered that my carbs have a similar metal thingy that just sits in the valve which can be removed....I was wondering whether there are special functions to the hollow metal cone.. the reason I asked is because since it is easily dislodged my mechanic have removed it from the valves? Any ideas??
The red things are velocity stacks, and their purpose is to manage the air entering the carb. Many design characteristics determine how they function.
A long velocity stack gives good low-end midrange performance, but hinders top-end somewhat. Conversely, a shorter velocity stack will give better high rpm performance, but won't deliver the goods in the midrange or low down.
Other factors concern the radius of the bell opening, length of the stacks for the center cylinders vs. the outer ones, etc.
The flatslide carbs are directly connected to the throttle cables, so when the rider whacks the throttle open, the slides go fully open. At high rpm, this is okay, but at mid and lower rpms, the engine will stumble & fall on its face because there isn't enough intake velocity generated by the engine to mix and atomize the fuel with the incoming air. Open the throttle slowly at low rpms, and all is fine. It takes practice & discipline to use them properly.
Flatslides can also be problematic to setup to fuel properly for street use. You're likely going to become very familiar with carb removal and installation while trying to get them to work how they should. If you like to tinker and relish a challenge, you're in luck with these jewels.
At full throttle, compared to a typical OE constant velocity carb the flatslide has only the needle obstructing the airway. A CV carb has the needle and throttle butterfly, which degrades airflow.

Hope this helps.
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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.
That seems like an unusually low rpm to launch from at the dragstrip. But hey, if it works for you, so be it.
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