Jetting Keihin FCRs

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Jun 14, 2007
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Anybody running 35mm Keihin flatslides on a Commando?

I have a pair and love them, but they're running a fair bit rich. I want to pull and rejet, but because of their size, getting them in or out requires dissassembling them partially - which means not a quick and easy on/off job, so once I get them off, I'd like to have the right jets on hand.

I'm thinking of just raising the needle clips a position or two to start, but you know how rejetting if anyone knows what the baseline "as delivered" jetting for these carbs should be - or even better, if anyone's been through this exercise and has suggestions - I'd appreciate it.

I am running them on stock intake runners with K&N pods.

Thanks - Brian

I just bought a new set and it will be some time before the bike is on the road however, I can look at what is inside mine to see if it matches yours. Mine were supposedly jetted for a stock 850 (that's what I asked for).

That'd be great, Jean, that's exactly the sort of baseline I am looking for - many thanks!
Info ... Hframe.htm

for the # refer to the chart or the picture

Jetting Keihin FCRs

The main jet on my carb is a 152 (#27)
The main air jet is a 200 (#103)
The air screw is out one turn (#129)
The pilot screw is out one turn (#32)
The slow jet is a 58 (#28)
The needle is a OCEMR and the clip on the third groove from the bottom (#23)

These carbs are really well made, I'm happy with my purchase, I just hope they run well.

Hope this helps.


PS post your settings, it could help me or someone else.
Thanks Jean, that's very helpful!

I predict you will love the FCRs. I have them on two bikes. I put them on a Ducati 900SS and they TRANSFORMED the bike. No basis for comparison on the Commando as I put them on before ever getting the bike to run, but I like them a lot.
Well, I can confirm I have the same needle and clip position. I just raised the clips to the middle position, fourth from the bottom instead of third. Hopefully that will cure my richness problem - 'cause it sure is a PITA getting these carbs on and off, the fit is VERY tight. I find I need disconnect the intake runners from the head and leave attached to the carbs. Still have to do a bit of maneuvering, but it works. Good luck and thanks again. - BrianK

i would be interested to know whether changing the clip position did the trick.
I have just fitted a pair of FCR's to my 850 i/state so any feedback on successful set up would be great.

I have only done a couple of short runs so far (todays run would have been much longer if a broken spoke hadn't caused a puncture :cry:) but very impressed, mind you my old Amals were well knackered.

I have found everything to fit space wise except i had to drill out the rivets on the bracket the throttle cable adjusters mount to and refit it one hole lower on the mount. Otherwise the tank would rest on the bracket.
I was out yesterday for a good run, although the temperature was in the mid 30s. I don't notice ANY difference in performance. No popping on decel ("the overrun," for you Brits!). But I figure it'll need 4-500 miles before I pull the plugs to check those. I'll report back.

If it wasn't such a PITA, I'd drop the needles another notch right away and see how that works. But it's enough work to not want to do it more times than I need to (although generally I believe in going to the "just a tad too lean" stage and then backing off one setting).
After my broken spoke i decided to get the wheels relaced with stainless spokes, hubs polished, very nice now that all is back together. So finally back on the road today and wonderful autumn day on nice twisty roads - until i ran out of petrol about 10 miles from home.
I have the large Interstate tank and ran out at 180 miles which means under 35 (UK)mpg!!!
I haven't played with the Carb set up yet, and have a couple of other issues to deal with which may be affecting things, but if that is anywhere near typical these sure are thirsty little suckers.
Would be interested to know what mileage others are getting.
I had been getting about 35 MPG with the FCRs. They were waayyy rich. I have now raised the needle clips THREE positions, to slot 2 (they were in slot 5 when I got them). Haven't been far, but they still don't "feel" too lean. I will check plugs in a few hundred miles and report back.
Progress report:

As mentioned, I've raised the needle clips (lowered the needles) THREE slots. Just pulled the plugs - maybe a slight improvement, MAYBE. But still sooty black.

This is just riding normally, coming home, letting it cool, and pulling the plugs. Not plug chops.

I'm at slot one - no more needle adjustments left. So I think I will get some 145 mains and try those next. Once you get past that, I've got tons of Keihin jets so I'm on easy street.... yeah, right! But for labor and hair-pulling.

To be continued.

Jean, how you doing with yours?

Thanks - Brian

PS, wonder if I should play with the pilot screws a bit too? Is that a gas screw, meaning screwing it IN makes it leaner? Still, one turn out seems "in the range" - maybe I have to go with smaller slow jets?

Comments from the peanut gallery and beyond are welcome. - B
Oh, and PPS - I'm getting 34-36 MPG around town - and I'm talking 40 MPH roads for the most part; yes, occasional stop signs and lights, but not Manhattan traffic fer cryin' out loud....
Got me wondering. The FCRs really like the idle turned up to start. Once it catches, you can just about immediately turn it down a bit, then more once it warms.

I wonder if this may indicate that I need a bit more air (via slow air screw) or less fuel (via slow jet or slow air screw)? I'd like to do something about my sooty plugs!
The FCRs really like the idle turned up to start. Once it catches, you can just about immediately turn it down a bit

I think it has more to do with the lack of choke or enrichener circuits.

Since the FCR's are pumpers, what starting procedure are you using? On my 900SS bevel (kickstart) with 40mm Dellorto pumpers, it likes a couple twists of the grip to shoot some raw fuel to start. The 900SS SP (electric with fuel pump) with the 41mm FCR's prefers I don't touch the grip when it is cold. It will fire and idle and once it has begun to warm (1-2 minutes) I can blip the throttle. If I twist it before starting, it will wet foul one or both plugs.
Hi Ron. I have FCRs on both a 900SS CR and my Norton. On both bikes, I use the accel pumps for cold starts and crack the throttle a bit, no accel pump, for warm starts. But both like the idle turned up a bit first. The Norton seems especially sensitive to this.
Checked the fuel screws - Right side was about 2/3 turn out; I left as is. Left was about 1-1/2 turns out; I reduced it to 1 turn.
BrianK said:
Progress report:

Jean, how you doing with yours?

- B

Still quite a ways from riding, the weather is nice now and I ride my pushy as much as I can (gotta keep fit and I have a bicycling trip in June, start in Montreal, ride to Boston, then to New York and ride back home) not to mention the job jar (fun stuff like painting windows, pulling weeds...) which for some strange reason keeps filling up.

Today's scintillating chapter of As the Carb Turns:

Slow air screws were both (!) at one turn out. Turned them an additional 1/4 turn out.

Leaner and leaner....when will it seize?...round and round the crankshaft goes...for now!

Tune in tomorrow for the next episode of As the Carb Turns. Same Norton channel.
A very good manual can you download from:

this guy knows what he is talking ! If you have not a good basic set up, go to a dyno guy,that will save a lot of time and nerve !

I run these carbs on my triumph, and they are great pleasure, cutting like a knive. If you run the carbs without a airbox, a stumble a low revs and WOT is normaly!

Greetings Marco
Thanks Marco, all information welcome!

Based on a very small sample, I may have raised MPG to 39. That's a bit more like it, if it's not an illusion.

BTW, there are no flat spots whatsoever in this bike's powerband (and I do run K&N pods). I have been leaning it out bit by bit, but power has never been an issue. Just MPG and sooty plugs!

Actually, I was off the bike for a couple weeks sorting out a bearing problem (self-induced!) in the rear wheel. Riding my others, all of which are torque-monsters in their own right (but for the Beemer, which has other charms..), but when I got back on the Norton a few days ago, it still damn near snapped my neck when I first snicked into first and opened the throttle. I had forgotten the danged torque peak is at about 1200 RPMS. GOD I LOVE THIS BIKE!!!

Happy long weekend to all. Hope the riding is as good where you are as it has been - knock on wood - here. BritIron meet tomorrow in Oxford, MA, some 50 miles or so from here. Time to ogle the Beezas!
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