Topic: Tuning Kiehin CR26 carbs for CB550

[FunJimmy]

My recent upgrade to Keihin CR26 carbs has been very successful. I’m really impressed at how easy the bike starts and how smooth is runs around town in city traffic. The carbs needs a bit of work on the idle circuit to improve stop light behavior, but for a race set-up these are great.

The engine is mildly built with Yoshimura 0.020” over pistons yielding a slightly higher compression. I don’t know the exact CR, but the raised dome on the pistons is only a couple mm, so not extreme. Mike (Mrieck) provided the port work and Webcam 58a camshaft with Kibblewhite valve springs complete the top end. Intake air is filtered by K$N pods and the exhaust gasses disappear through a Kerker 4 into 1 exhaust system with Kerker 1.5” competition baffle.



I’m going to spend a bit of time on the dyno to get the jetting well sorted, but I’ve got a few questions that I need some help with first.

Current jetting is:

#60 pilot jets
105 main jets
#220 air jet
Air screw open ¾ turn
YY0 Needle with clip on 5th slot from top

I’m not familiar with the Keihin enrichener (choke) circuit, but even the slightest use of it causes the bike to die. What does that tell me about the jetting?

Is popping on deceleration normal or is it an indicator of jetting issues?

Thanks
FJ

[MRieck]

Looks very nice. Popping would indicate a lean condition in the idle circuit or a possible vacuum/exhaust leak. If it didn't do it before I'd say the air screws can be turned in a bit to richen the mixture. This will probably help your idle as well (though the idle quality of the CR's leaves something to be desired). I'd recommend making a remote idle adjuster which makes it easy to adjust idle secondary to very hot days etc.
 The CR cold start circuit is not very good IMO. It is jetted very rich and ultimately requires cleaning as the plunger sealing surface gets crap on it and pulls unwanted vacuum.
 Once again...very nice work. I like the manifold plate.

[classic bike tuner]

yryo and gerard uses  70 idle jets in there race 500 

but its a 29 cr carb

[turboguzzi]

Nice on ya, 26 would be a good size for a street 550 indeed, kudos for not falling into the bigger is better trap.

i think you could make the attachment solution lighter but if mike likes it, who am i to argue?

changing jets on CR's is so easy that its definitely worth playing with them, preferably on a dyno.

one tip, small dellorto main jets cost a fraction of the original Keihins and work just as well.

i run very closed down air screws on the 29's but i think that on your 26's you could open up a bit. maybe thats the reason its dying on you with choke on.

[FunJimmy]

Nice on ya, 26 would be a good size for a street 550 indeed, kudos for not falling into the bigger is better trap.

i think you could make the attachment solution lighter but if mike likes it, who am i to argue?

changing jets on CR's is so easy that its definitely worth playing with them, preferably on a dyno.

one tip, small dellorto main jets cost a fraction of the original Keihins and work just as well.

i run very closed down air screws on the 29's but i think that on your 26's you could open up a bit. maybe thats the reason its dying on you with choke on.

Thanks TG

I'll be spending some time on the dyno later in the week and will post results.
Great tip on the Dellorto jets.

Those adaplers look heavier than they actually are, but a bit lighter would be nice.

[Doctor_D]

As Mike said, the starter-jets are far too rich on these things out of the box.  On my (now Branden's) Cr29's, I felt I could have gone down a few steps.

Try 35 starter-jets, should be closer to correct.  Alternately, you could get really lean ones (20's?) and use index drill bits to open them up bit at a time until they're right - something that seems well below your impressive fabrication skill level.

[bochnak]

Hi,

What jetting did you end up with? I'm working on a 550 with CR26's and they have #50 pilots installed. The CR book states that #65 should be used as a baseline. I have ordered #60's & #65's.

Currently, the air screw can be closed and it will still run. Best setting is 1/2 turn out. Bogs like crazy at low RPM's. I put tape over intake and it greatly improved.

I have also ordered 35 starter jets, currently 45's.

This thing is running LEAN!

[FunJimmy]

I had an amazing opportunity to spend some time at Twinline Motorcycles with CR Master, Doctor_D and Jeff, AKA Tower.
These guys know CR carbs and dyno tuning like no one else in the PNW.

When I arrived my CR26 Carb jetting was: 

Pilot 60
Main 105
Needle YYO
Clip #2 (5th slot from top)
Air Jet 220
Air screw open ¾ turn


Post dyno tuning:

Pilot 65
Main 105
Needle YY8
Clip #2
Air 220 Inside 230 Outside
Idle Screw 1 turn

These jetting changes combined with a little ignition timing made big differences in throttle response and crispness.
Idling was a different deal. The bike just wouldn't hold a steady idle and required 1500rpm+ to stay lit, but that was found to be cam timing and an intake leak.
Not a fault of the carbs.


<a href="Not a valid vimeo URL">CB550 Interceptor - Dyno Tuning[/url] from FunJimmy on Vimeo.

[turboguzzi]

49hp is really nice for a 550 street bike,

what cam is in there? funny how you still have the dreaded dip at 6K (my assumption, i see mph...) even with your semi hot state of tune.

if you could run stacks between the filters and the carbs it might help mid range.

[Kemp]

Great video, did you use a sniffer in the exhaust for A/F? what rpm were you turning at peak HP? do you have a dyno run just hp, torque and rpm?

your peak power is very similar to Brent's 591cc street bike that has similar mods but stock carbs and makes a bit over 50bhp if I remember correctly.

the 591 I built has similar mods but no dyno time as yet. planning to get there this winter with a set of stock and then bored out carbs to compare which should be interesting to see differences

Interesting that stock 550's put out about 36bhp and that once you get around 50 at the back wheel these bikes become real beasts, lots of fun!!

[bwaller]

After the little problems were corrected I'm pretty sure Jimmy's engine would have produced in the low 50's, right where you'd expect. It'll be interesting to see what his further work offers, I hope it pays dividends!

As Kemp said this one of mine made 50.7 (at about 9K) & 34.1 ftlbs with the Web 58a & stock PD carbs. I think this cam must be very similar to the CB650 stick. Seeing as how I never had much experience with the 500/550's, I am pleasantly surprised how much fun the bike becomes especially with weight removed as well.

It seems relatively easy to get around this 50hp figure in this engine. Making big gains beyond this point takes more effort.