The Keihin FCR carburetor FAQ.
How the heck do you say Keihin? Most folks say key -in.
Do I want 39mm carbs or 41s? 900SSs get 41mm carbs. 750s get 39s. You can put 39s on your 900SS but if you ever start really trying to get power from your motor, you’ll need the 41s.
Sudco, the US Importer of the carbs and who assembles the kits, says they sell 4 or 5 41 kits for every 39 kit. Also, the 750SS racer that Sudco was developing their kits on ended up running FCR41s. Switching from 39s to 41s got them a few more hp on top and added slightly to mid range power.
Some shops and Ducati gurus will say that FCR41s don’t belong on a 900SS. Well. if they are your only local support and you might need to turn to them for help, then you’d better get the 39s. Why?
If you turn to them and you’ve got 41s, with their closed mind, they will not be able or willing to make the 41s work right.
Ca Cycleworks customers have warned about some special deals on the Internet for what would appear to be a full FCR kit, when said kits are without instructions, throttle, or cables (and at least once the manifold adaptors, too!) This requires another $80
$160 of parts to properly install these incomplete kits. All FCR kits sold by Ca Cycleworks are shipped complete with carbs, cables, throttle, spigots, airbox adaptors, instructions, and air filter. All Cycleworks FCR kits are verified to ensure the proper baseline settings. For those who purchase incomplete kits used or from other vendors, Cycleworks can supply you with the support you need: proper replacement parts shown below, copies of Sudco instructions, and Ca Cycleworks install hints.
Note that when buying FCRs from private individuals or from eBay, you may need to purchase manifold adaptors, calbes, or throttle, adding expense to the purchase.
What about midrange? I’ve heard that FCRs kill midrange. When referring to Ducatis, this is misinformation . The whole reason that this wives tale came about is from when FCRs were first introduced on the US market and people put them on GSXRs and other 4 cylinder Japanese bikes.
Instead of calculating the best size for their application, they’d put 39s or 41s on a street 750 motor, when 35s are appropriate!
Gas laws say that the larger the venturi, the less velocity with an equal airflow. Engine tuners know that less velocity means less torque. The reason for huge aftermarket racing carbs is typically to allow for more velocity at high RPM with throats much larger than most OEM carbs.
The same heads and same rpms mean that the airflow would be about the same and the reasoning that the carbs so dramatically harm velocity and torque on a 900SS are totally ignoring the intake tract, valves, and cylinder size. A Ducati 904cc V-Twin has cylinders with insane stroke and 452cc displacement each, while a 4 cyl 750 has 187.5cc cylinders with terribly short strokes. Further, the Japanese 4 cylinders also have more valve area with their 4 valve per cylinder heads.
The Ducati has more displacement AND less valve area, meaning that the velocity through the intake tract remains high and the 900’s great midrange torque is amplified by FCRs.
I’ve put FCR 41s on essentially stock 900s and was able to power-wheely in second gear at 40mph at 4200 rpm. You need a 39T rear sprocket for that trick. And when the revs pass 5 grand, you have to back out of the throttle so you don’t flip over.
Having a lightened flywheel guarantees 2nd gear wheelies.
Actual customer e-mail! Will I get tickets? Oh yeah you will. Consider this your warning, cuz you won’t get one from the coppers! Sorry, Cycleworks won’t subsidize your legal defense, just take the $$$ you save from buying here and save it now. -)
I want the kit but already have a KN air filter, can I delete it and get a discount? Yes, I’ll offer a $18 credit to delete the filter from the kit. In order to get this credit, you must add the Delete KN filter from kit n/a DU-0900 $-18 item to your shopping cart at California Cycleworks. I previously had all FCR kits drop ship directly to the customer, but sometimes Keihin / Sudco are not setting them to their own specs!
Now, all carb kits ship to me for calibration then they ship to you.
If I got canisters and the carb kit, would the carbs be jetted already? Bascially, yes. Since the carbs come slightly rich (an appropriate conservative setting), a few hp can be found with some dyno tuning. You may not feel it much though, as dyno tuning won’t help torque too much – it’s already a lot higher than before.
You get the big grins from all of the increased torque. If you want to diddle with your carbs, here’s a useful tuning guide. A lot of useful tuning info is also in Sudco’s catalog: needle selection 151k tuning graphs 110k and slow air screw chart 135k .
Please note that if you ARE going to diddle with your FCRs, keep two things in mind. 1) RPM is meaningless. As shown in the charts, each circuit in the carburetor is about throttle position.
2) You’ll never feel if the carbs are jetted too rich. You can pour enough fuel into the bike to make 110 hp and it’ll feel almost as good as with optimum jetting. But, go and make it too lean and it’ll feel like someone hit the kill switch.
Example: if the needles are too lean for 1/4 throttle delivery, you’ll be toting along at a steady speed at 1/8 throttle and give it some gas to speed up and it’s like someone literally hit the kill switch. Then as you give it more gas, you climb out of 1/4 throttle and hit the 1/2 throttle range where the main jet starts helping out the needle and BAM you’ve got massive power.
I’ve got what I call the 70/20/10 rule. 70% of the people I sell carb kits to bolt the carbs on their bike and wheely into the sunset with a maniacal giggle never to touch their carbs again. 20% of the people will love the increased power but want to diddle about with the fuel screw or slow air screw to get idle just right. Note that these two adjustments are easy to get to. The last 10% of FCR buyers seem all fubar’ed.
They can see the potential for the carbs, but something just isn’t right. They just need to take their carbs to a local shop with a dyno and pay them to make the bike and the carbs play together nicely.
The only modifications that I have seen to require rejetting have been polishing the ports in the head and the Staintune spaghetti system. I’m not sure if going to a 944 or 984 kit will require rejetting, but suspect it would. The stock mufflers flow as good as any slip-on on the market while still attached to the stock header.
The stock collector is so restrictive that the performance from mufflers is almost evened out.
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