Topic: 26mm CR Specials on a CB400F

[davesprinkle]

Hello everyone.  I've been lurking on this site off and on for the past several years.  I've gleaned a lot of good information from the collective wisdom here, so I thought I'd give back a little by describing the mods required to get a set of 26mm CR Specials to work on my CB400F.

First of all, the engine specs -- 53.5mm big-bore, displacement 449.6cc, stock cam, stock headers, Bub "conti-alike" muffler.

Secondly, it's been pretty well documented how the 26mm CR carbs don't really fit the CB400.  I was aware before I purchased the carbs that they would crash into the tank, for example.  What I wasn't aware of though, was just how poorly the carbs would run in their as-delivered configuration.  I managed to solve both of these problems.  Let's start with the tank collision.

The problem with the 400-intent CR rack is that the tight bore spacing of this little bike doesn't allow enough room between adjoining carbs to fit the cable-attachment mechanism.  As a result, Keihin mounted the pulley and cable bracket on the end of the #4 carb, hanging it out where it (A) is ugly, and (B) crashes into the tank.  I've read that other 400 owners have raised the tank to solve the problem.  For aesthetic reasons, I decided not to do this, and instead reworked the mechanism so that it would fit between the #1 and #2 carbs.  Here is a pic of the as-delivered cable-attachment mechanism:  [Edited 24 January 2009 -- pics are now at bottom of post, sorry, don't know how to include inline with text?]


I won't bore you with the all the details of the work required to make the mechanism fit inside the carb rack; suffice to say that it was significant.  This is not a trivial job.  The short list is: make a new pulley, modify the existing cable bracket, shorten the linkage shaft, fabricate a new collar for the 2nd return spring, fabricate a new idle-stop, fabricate an idle-speed adjuster cable and bracket, etc.  Here is a pic of the new pulley.  You'll note that one of the pulleys has already had its full-throttle and idle stop tabs welded in place.  You'll also note that the pulley isn't round -- its profile is a linear cam, a duplicate of the as-delivered Keihin profile.

[davesprinkle]

OK, next is a pic of the new mechanism installed on the carb rack.  The top carb rack hasn't been modified -- the lower carb rack has the new cable mechanism.  In this picture, the mechanism is installed between the #2 and #3 carbs.  I found that this location crashed into the frame, so I moved the mechanism in between the #1 and #2 carbs for the final configuration, where it works nicely for cable routing.  You'll also note that I split off the 2nd return spring from the pulley, rather than combining them as on the OE setup -- this is one of the secrets to making everything fit.  Also, note the extended pulley shaft.  I later trimmed this shaft so that it terminated inside of the #4 carb body and pressed in a new cap as on the #1 body.

[davesprinkle]

Here are a few images of the final pulley configuration as well as a side view of the cable attachment:

[davesprinkle]

Here is a shot of the right side of the bike with the tank mounted.  Look, Ma, no cables!  And a couple pics of the taillight and monoposto seat.

[j-conn]

pppprreeetttyyy

[davesprinkle]

OK, next the jetting specs.  If you want to run open stacks, set the carbs up as follows:
Slides: 4.0 (2.0 as delivered from Sudco)
Needles: Y0 (P151 as delivered from Sudco)
Clip position: #3 from top
Main jet: 100 (105 as delivered from Sudco)
Main air jet: 150 (220 as delivered from Sudco)
Pilot jet: 65 (remarkably, this was what Sudco delivered)
Airscrews: 1/4 to 1/2 turn out

If you want to run pod filters, use the following:
Slides: 3.5
Needles: Y8
Clip position: #4 from top
Main jet: 98
Main air jet: 150
Pilot jet: 65
Airscrews: 1/2 turn out

A few notes:
-- Sudco in general has no idea what will work on these bikes.  Take their advice with several dozen grains of salt.
-- Sudco will try to tell you that the YY-series needles are the same as the Y-series needles, but that YY-series needles are better because they have 7 clip positions vs only 5 on the Y-series needles.  They are correct about the number of clip positions, but they don't understand that the relative locations of the clip grooves differ between the 2 needles.  The Y-needles will go 2 clip positions richer than will the YY-needles; thus clip position #7 on a YY-needle corresponds to clip position #3 on a Y-needle.
-- You will not be able to get a stable, low-speed idle on with these carbs on this little engine.  I can only get the bike to idle at about 2K or so.  Any lower, and the bike will die as engine vacuum falls and the idle circuit loses authority.  I worked my ass off on this problem, but couldn't solve it.
-- These carbs are extremely sensitive to airscrew setting.  The airscrews affect the 1/4 throttle region, but have little effect at idle.  I have personal experience that the above settings will work OK.  If you change the airscrews in an attempt to get a better idle, you'll likely find that (A) idle quality will improve only slightly and (B) you'll pick up a misfire at 1/4 throttle that will require a different needle diameter to correct.

[MRieck]

 Excellent work Dave. Sudco has no idea....at least when you talk to them....about CR's, RS's, FCR's etc. I have dealt with them for years and had better luck calling Mikuni USA etc . I really don't get it. They do however control the flow (sole distributor) of aftermarket carbs into the USA....that's the reality.
 I agree about the air screw settings and would add they do not have to be the same for each carb.
 It only took Sudco about 15 friggin years to get the CR 29/31 750 carb adaptors close to right in regard to OD and length. I had modified mine in the past with good results.

[Terry in Australia]

Beautiful work Dave, well done! What did you use for an idle speed adjuster cable and bracket? I really bneed to replace the piece of thick walled fuel hose I'm currently using with something more aesthetically pleasing.........

[davesprinkle]

Excellent work Dave. Sudco has no idea....at least when you talk to them....about CR's, RS's, FCR's etc. I have dealt with them for years and had better luck calling Mikuni USA etc . I really don't get it. They do however control the flow (sole distributor) of aftermarket carbs into the USA....that's the reality.
 I agree about the air screw settings and would add they do not have to be the same for each carb.
 It only took Sudco about 15 friggin years to get the CR 29/31 750 carb adaptors close to right in regard to OD and length. I had modified mine in the past with good results.

Now that you mention it, I recall that I turned down the od of the carb spigots so that they would slip more easily into the intake boots.  Don't recall the exact dimensions, only that the wall-thickness is now 1.5mm.  The OD is still larger than stock, but with a little lube and effort, they'll go on.  Also, I purchased new carb boots.  I couldn't get the new carbs to slip into the hardened rubber of the 32 year old original boots.

[davesprinkle]

Beautiful work Dave, well done! What did you use for an idle speed adjuster cable and bracket? I really bneed to replace the piece of thick walled fuel hose I'm currently using with something more aesthetically pleasing.........

The cable started out as an FCR adjuster cable.  I got mine from Sudco.  I told the guy on the phone that it would be used for a CR Special.  He responded that they would make a modification so that it would work properly with the CR.  I don't know what they did -- something to do with the thread pitch maybe?  Anyway, when it arrived, it was too long for my application.  I cut the cable, made a new end for affixing the knob, made a swaging die, and then swaged on the new end in a hydraulic press.  The cable bracket is a fabbed piece.

[paulages]

hey dave- beautiful work. glad to see you made it to the forum. i recognize that workshop of yours...    i met you a couple months ago in front of my shop by the white eagle if you recall. is the 400 on the road then? we should go out for a ride sometime and you should take the 718 for a spin. i feel like i owe it to you. it's quite better tuned than when you saw it, but still not totally dialed. i'm making an appointment to run it on the dyno at Ron's for a bit and get the jetting dialed in, but it runs strong without hesitation as is.

not too sure how many people will do this mod, but threads like this should get put somewhere on this site where this great info won't be lost, in my opinion.

[bwaller]

Well thought out Dave, meticulous. Nice ride too, what front have you grafted on there?

[davesprinkle]

hey dave- beautiful work. glad to see you made it to the forum. i recognize that workshop of yours...    i met you a couple months ago in front of my shop by the white eagle if you recall. is the 400 on the road then? we should go out for a ride sometime and you should take the 718 for a spin. i feel like i owe it to you. it's quite better tuned than when you saw it, but still not totally dialed. i'm making an appointment to run it on the dyno at Ron's for a bit and get the jetting dialed in, but it runs strong without hesitation as is.

not too sure how many people will do this mod, but threads like this should get put somewhere on this site where this great info won't be lost, in my opinion.

Hi Paul,  thanks for the kind words.  Yeah, I remember your bike.  Nice work.  I've got an idea about your oil system.  We should talk offline.  And yes, the 400 is running.  Too bad we've lost the sun for another 5 months...

[davesprinkle]

Well thought out Dave, meticulous. Nice ride too, what front have you grafted on there?

Thank you -- it's gratifying to get this kind of positive feedback.  Marzocchi forks, Ducati Pantah, shortened 2".  Brembo caliper, 4-piston, Ducati Monster.  330mm rotor, CBR929.  Ceriani-replica triples.  Stock hub, narrowed.  Excel shouldered rim and stainless spokes with all inner lacing pattern.  20mm hollow axle.  GB500 front fender, with a haircut.  Fabbed clipons and headlight bracket.

[MRieck]

Well thought out Dave, meticulous. Nice ride too, what front have you grafted on there?

Thank you -- it's gratifying to get this kind of positive feedback.  Marzocchi forks, Ducati Pantah, shortened 2".  Brembo caliper, 4-piston, Ducati Monster.  330mm rotor, CBR929.  Ceriani-replica triples.  Stock hub, narrowed.  Excel shouldered rim and stainless spokes with all inner lacing pattern.  20mm hollow axle.  GB500 front fender, with a haircut.  Fabbed clipons and headlight bracket.

What...no Ti???

[tsflstb]

I have a new hero now.  Screw Aquaman.

Awesome work on that 400F.  You've got it all sorted out...motor, brakes, suspension.  Have you dyno'ed it yet?  Are those Works shocks?

That hollow front axle may very well be a Ti piece from TTR400.  I drooled over that for a while when I re-did my hubs.

[paulages]

hey dave- beautiful work. glad to see you made it to the forum. i recognize that workshop of yours...    i met you a couple months ago in front of my shop by the white eagle if you recall. is the 400 on the road then? we should go out for a ride sometime and you should take the 718 for a spin. i feel like i owe it to you. it's quite better tuned than when you saw it, but still not totally dialed. i'm making an appointment to run it on the dyno at Ron's for a bit and get the jetting dialed in, but it runs strong without hesitation as is.

not too sure how many people will do this mod, but threads like this should get put somewhere on this site where this great info won't be lost, in my opinion.

Hi Paul,  thanks for the kind words.  Yeah, I remember your bike.  Nice work.  I've got an idea about your oil system.  We should talk offline.  And yes, the 400 is running.  Too bad we've lost the sun for another 5 months...

still have your number somewhere. i'll give you a call soon. can't ever trust this weather... the leaves won't go away, but we could very well have 60F and sunny next week.

[davesprinkle]

I have a new hero now.  Screw Aquaman.

Awesome work on that 400F.  You've got it all sorted out...motor, brakes, suspension.  Have you dyno'ed it yet?  Are those Works shocks?

That hollow front axle may very well be a Ti piece from TTR400.  I drooled over that for a while when I re-did my hubs.

Yeah, SAE-corrected power is 48hp at the rear wheel running open stacks, 46.5hp through the filters.

Shocks are Ikons.  The shocks were originally intended for a CB750.  I purchased them because they're roughly 20mm longer than the 400-intent shocks.  I needed the additional length in the rear to balance the longer forks up front.  The 750 springs were way too stiff as-delivered; I had to install lower-rate springs to get any static sag at all.  Oh yeah, and remove the stupid chrome spring covers...

The hollow axle is my own design.  It's cut out of 17-4 H900 precipitation-hardened stainless steel.

[andy750]

Excellent work Dave and I agree that this thread/info should be kept somewhere obvious for future reference. We need a technical library with these sorts of mods  . Have you done any top end work or is it carbs only?

Great looking bike - we need more photos of the front end and from the top to see rider position. A future bike of the month for sure...

cheers
Andy

[Tim2005]

Dave, very interesting info indeed, and a very good looking bike. As you'll see from the photo attached I took a different approach to fitting CR's on my 460, 54mm kit & Yoshi ST-1 cam, by modifying the underside of the tank to obtain the required clearance. I also fitted a different fuel tap takeoff at a slightly different location as the stock one fouled the CRs, not sure how you got around this issue. Interesting jetting notes- to compare mine were

Slides: 2.0
Needles: YY0
Clip position: #7 from top
Main jet: 110
Main air jet: not recorded - better check!
Pilot jet: 70
Airscrews: 1/2 turn out

These were the settings at some testing; I'd like to get the AFR around 12.5-13, I gather that's about ideal. Testing showed up AFR as over 13:1 from 4000-5000 (but very very hard to get a stable reading at those revs) and around 11:1 from 7000-11000rpm. It dropped below 10:1 at 6000rpm for a couple of hundred revs, it used to do that too on the stock carbs so that may be an effect of the cam.  I've since moved the needle clips up a couple and changed the mains to 105s and it is definitely better, though I have not as yet been able to recheck the AFR. What effect did your changed slides and main air jets have?

This is all done with open stacks.  I'm curious you found some pods that will fit, any that I could find would not clear the frame, where were they from & any other downsides to fitting them?  I did look into those so-nice alloy stacks with the gauze mesh but they are nearly 400eur a set. Finally, I haven't had your experience with idle speed at all, mine will tick over pretty happily below 1500rpm.

Cheers


Tim   

[MRieck]

Dave, very interesting info indeed, and a very good looking bike. As you'll see from the photo attached I took a different approach to fitting CR's on my 460, 54mm kit & Yoshi ST-1 cam, by modifying the underside of the tank to obtain the required clearance. I also fitted a different fuel tap takeoff at a slightly different location as the stock one fouled the CRs, not sure how you got around this issue. Interesting jetting notes- to compare mine were

Slides: 2.0
Needles: YY0
Clip position: #7 from top
Main jet: 110
Main air jet: not recorded - better check!
Pilot jet: 70
Airscrews: 1/2 turn out

These were the settings at some testing; I'd like to get the AFR around 12.5-13, I gather that's about ideal. Testing showed up AFR as over 13:1 from 4000-5000 (but very very hard to get a stable reading at those revs) and around 11:1 from 7000-11000rpm. It dropped below 10:1 at 6000rpm for a couple of hundred revs, it used to do that too on the stock carbs so that may be an effect of the cam.  I've since moved the needle clips up a couple and changed the mains to 105s and it is definitely better, though I have not as yet been able to recheck the AFR. What effect did your changed slides and main air jets have?

This is all done with open stacks.  I'm curious you found some pods that will fit, any that I could find would not clear the frame, where were they from & any other downsides to fitting them?  I did look into those so-nice alloy stacks with the gauze mesh but they are nearly 400eur a set. Finally, I haven't had your experience with idle speed at all, mine will tick over pretty happily below 1500rpm.

Cheers


Tim   

I didn't understand the idle issue either especially with a stock cam.

[davesprinkle]

Dave, very interesting info indeed, and a very good looking bike. As you'll see from the photo attached I took a different approach to fitting CR's on my 460, 54mm kit & Yoshi ST-1 cam, by modifying the underside of the tank to obtain the required clearance. I also fitted a different fuel tap takeoff at a slightly different location as the stock one fouled the CRs, not sure how you got around this issue. Interesting jetting notes- to compare mine were

Slides: 2.0
Needles: YY0
Clip position: #7 from top
Main jet: 110
Main air jet: not recorded - better check!
Pilot jet: 70
Airscrews: 1/2 turn out

These were the settings at some testing; I'd like to get the AFR around 12.5-13, I gather that's about ideal. Testing showed up AFR as over 13:1 from 4000-5000 (but very very hard to get a stable reading at those revs) and around 11:1 from 7000-11000rpm. It dropped below 10:1 at 6000rpm for a couple of hundred revs, it used to do that too on the stock carbs so that may be an effect of the cam.  I've since moved the needle clips up a couple and changed the mains to 105s and it is definitely better, though I have not as yet been able to recheck the AFR. What effect did your changed slides and main air jets have?

This is all done with open stacks.  I'm curious you found some pods that will fit, any that I could find would not clear the frame, where were they from & any other downsides to fitting them?  I did look into those so-nice alloy stacks with the gauze mesh but they are nearly 400eur a set. Finally, I haven't had your experience with idle speed at all, mine will tick over pretty happily below 1500rpm.

Cheers


Tim  

Hi Tim, thanks for the feedback.  A brief comparison of our jetting shows the following:

Slides:  You're running a 2.0 slide; I'm running a 4.0.  I changed out the slide to correct a 1/4 throttle rich condition.  The new slide was effective to solve the problem; however, I've since learned that the airscrew is just as powerful in this 1/4 throttle region, and it's cheaper.  Were I to do this again, I'd probably avoid the expense and hassle of a slide change and just try to make it work with the as-delivered slide.
Needles:  You're running a YY0; I'm running a Y0.  As I mentioned, these have the same profile, differing only in clip groove positions, which leads nicely into the next point of comparison...
Clip position:  You're running position #7 from the top; I'm running position #3 from the top, which yields EXACTLY THE SAME EFFECTIVE PROFILE.    As I mentioned in my previous post, the Y-needles will go 2 clip grooves richer than the YY-needles.  Groove #3 on a Y0 needle is the same as groove #7 on a YY0 needle.
Main jet:  You're running 110; I'm running 100.  This is why you're seeing 11:1 A/F from 7K to 11K.  I was targeting 13:1, which is roughly 15% leaner; hence my smaller jet.
Main air jet:  Sudco normally ships these carbs with a 220; I'm running a 150.  This jet has the effect of tilting the A/F ratio over the speed range under full-throttle condition.  With the 220, the A/F ratio trends richer as engine speed increases.  The smaller air jet helps to maintain a more consistent A/F ratio, although I should say that the effect is pretty subtle.  A/F still trends richer as speed climbs, just not as much as with the 220.
Pilot jet: You're running a 70; I'm running a 65.  Pretty close.
Airscrews:  You're at 1/2 turn; I'm at 1/4 turn.  It's likely that my richer airscrew setting is compensating for my leaner slide and slightly leaner pilot jet.

Regarding the idle speed -- I suspect your larger displacement engine is helping you to get a stable lower-speed idle.  I found that the idle circuit completely loses authority and goes lean as engine speed drops.  I can get the thing to idle at ~1700rpm, but if I target a lower speed, the engine will likely die as A/F goes leaner and leaner.  I found that a richer pilot jet will improve this situation, but only at the expense of making off-idle fueling (1/8 throttle) very rich.  But I'm not going to disregard your success -- I'll throw in a 70 pilot and see if this works better.

Yes, found the same problem with fuel tap location.  I moved mine as well.  I used a Pingel tap and weld-on bung.  Conveniently, it has an outlet fitting the same diameter as the carb.  Pic is below.

Regarding pods -- these are the ubiquitous K&N brand, provided by Sudco.  I'm not sure of the PN.  Yes, the #1 and #4 filters crash into the frame.  To get them to fit, you must crush and deflect the filters.  First installation is extremely difficult; subsequent installations merely troublesome.

I didn't know that aluminum stacks were available, so I had some made.  See the pic below.

A previous post requested more pics of the front end.  Here they are.  (Please disregard the dirt from a ride in the rain...)

[davesprinkle]

Here is a picture of how the pod filters must be tweaked to clear the frame.  Note that the #1 and #4 filters have been bent inward.  By the way, the number printed on the filter is: F020A4.  However, this number isn't recognized by the search function on the K&N site.

[WFO]

Is this bike going to be a daily rider?

[davesprinkle]

Is this bike going to be a daily rider?

A weekly rider would probably be a more accurate description.