Topic: Keihin CR "Special" carbs- starter circuit info

[scondon]

  I had bought a nice set of used Keihin CR's from a member here a while back and had finally got them ready and on the bike. They ran pretty well at half throttle and above but were pretty bad at idle to 1/4 throttle. No worries, I thought, I've been through this with stock carbs plenty of times. Synch carbs, clean pilot and main jet passages, check float levels, check for vacuum leaks, recheck ignition/plugs/wires, etc.....  Everything checked out fine but still #3 pipe was barely getting warm. After staring at the parts diagram for a while I finally had an idea what might be causing this.

   Seems the "choke" on these carbs is not the same thing I am used to on the stockers. It's actually a "starter" circuit with it's own fuel/air supply and runs independent of the pilot or main circuits. When the choke lever is lifted it raises a starter valve in each carb that allows air in from openings on the side of each carb and also pulls copious amounts of fuel from the "starter" chamber in the float bowls. It then dumps this mixture through a rather large hole in the carb throats and straight into the engine.

   What was happening to me was that fuel had dried in the area that the starter valve seats. The starter valve is similar in shape to the plunger in a master cylinder and is pushed down by a spring in order to fully seat against the carb body, just above the emulsion tube leading from the starter chamber. Dried fuel(gunk) was preventing the valve from seating and was keeping the starter circuit open. #3 wasn't heating up at idle because it was choking on fuel.

    I dropped all four float bowls and sprayed tons of carb cleaner up each emulsion tube while working the choke lever up and down. Now when the lever is closed, so are all four starter circuits and the bike idles well at 1100rpm.

     There was not a lot of gunk removed, meaning it does not take much to foul these circuits up. Figured I'd post this in case anyone else has trouble getting a good idle to 1/4 throttle with their CR's.

    Now I FINALLY get to do some proper carb tuning

[scondon]

Slid to page three in just 16 hours. My but aren't we a busy site   One more go 'round before this slides to oblivion.

[MRieck]

 This is VERY GOOD INFO!!  The guys at Sudco would say....."we haven't heard of that before" or "You're the FIRST ONE to have a problem". All that BS and they are pros at it!!! This should be under FAQ etc. Good diagnostic stuff......period!
  Mike

[scondon]

Thanks Mike. At first glance those starter circuit air nipples look like overflow ports or "atmospheric" vents. I may follow your route and get some rubber caps for them to shut them off while the weather is still warm and the choke is not needed.

   I'm really impressed with how smooth the engine runs when making power with these carbs. Not a bit of vibration at 7000rpm and above. And even with my open exhaust the bike sounds quieter with all four cylinders burning even and efficiently

[jhasewhite]

I just acquired a 1976 cb750f that has CR special carbs.  I am having a tuning nitemare!!! I know Cb750 have some "character". but this thing is unridable!

 I am a semi-experienced motorcycle mechanic, but still can't tell what is wrong with these touchy cr specials.  The only thing I know for sure is that the idle/slow circuit is waaaay to rich, but when it comes on to the needle it acts really lean (cuts out "rich?" and the revs hang up,"lean?") uugh. I cant get the thing to rev high enough to come on to the main jet (rich?)  Sudco has been no help to me at all. Does anyone have a similar setup? I am running a Kerker 4 into 1 with a very free flowing comp baffle, stock engine, 29mm cr specials, dyna 2000 ignition...

Here in the city of San Francisco, it costs around $650 for a dyno jetting session, plus the cost of jets and tuning labor...  I'm too poor for that.

Any racers out there have some proper jet size/tuning guidelines for me?

I seem to have run into a D.P.O.- a dip #$%* previous owner.

Thanks for your help, I really want to ride this thing

-jhase,
SF,CA

[MRieck]

I just acquired a 1976 cb750f that has CR special carbs.  I am having a tuning nitemare!!! I know Cb750 have some "character". but this thing is unridable!

 I am a semi-experienced motorcycle mechanic, but still can't tell what is wrong with these touchy cr specials.  The only thing I know for sure is that the idle/slow circuit is waaaay to rich, but when it comes on to the needle it acts really lean (cuts out "rich?" and the revs hang up,"lean?") uugh. I cant get the thing to rev high enough to come on to the main jet (rich?)  Sudco has been no help to me at all. Does anyone have a similar setup? I am running a Kerker 4 into 1 with a very free flowing comp baffle, stock engine, 29mm cr specials, dyna 2000 ignition...

Here in the city of San Francisco, it costs around $650 for a dyno jetting session, plus the cost of jets and tuning labor...  I'm too poor for that.

Any racers out there have some proper jet size/tuning guidelines for me?

I seem to have run into a D.P.O.- a dip #$%* previous owner.

Thanks for your help, I really want to ride this thing

-jhase,
SF,CA

Sean lives close by. I certainly don't mean to extend his services but he has  good amount of experience with these carbs. I've worked with them for years but some "hands on" would be better IMO.
  Mike

[scondon]

   I live across the Bay in Oakland and have a couple years experience running/tuning these carbs and a lot of time wrenching on the Supersports getting them to run their best. I can make a trip to SF and evaluate/tune your bike for a fraction of what the shops charge. PM me with your phone # if your interested or just want to talk bikes for a bit so you can get an idea of how to proceed yourself.

Sean C.

Sean lives close by. I certainly don't mean to extend his services but he has  good amount of experience with these carbs. I've worked with them for years but some "hands on" would be better IMO.
  Mike

   Thanks Mike, I feel all teary getting a recommendation from ya

[sohc4]

FWIW, I have just put up the Keihin Troubleshooting Guide and the Keihin CR Owners Manual up for download.

See http://www.satanicmechanic.org/downloads.shtml

Nothing beats hands-on experience and on site support, but it may at least help out somebody...

Cheers,
Axl

[jhasewhite]

Hey thanks a lot everybody, especially the satanic mechanic. I found more info on CR carbs in one day here than two weeks of asking "motorcycle mechanics" for advice.

Hope I'll be riding soon

-jhase

[jhasewhite]

Thanks Sean for the help, the thing actually works now. I put about 25 miles on it since the tune up and it seems to be ok, although the #4 header pipe doesnt seem to be heating up as much as 1,2,and 3. Should they all turn a nice shade of yellow while the engine is on, or are 1,2,and 3 running too lean?  BTW, that extra spring I put on the CR's really made the revs fall directly down to idle without any hangup at all. 

Also, the tachometer cable seal on the engine is bad and seeping oil, do you know the best place to get a new seal?


I should have this thing working well enough for a good ride next weekend.

[scondon]

Thanks Sean for the help, the thing actually works now. I put about 25 miles on it since the tune up and it seems to be ok, although the #4 header pipe doesnt seem to be heating up as much as 1,2,and 3. Should they all turn a nice shade of yellow while the engine is on, or are 1,2,and 3 running too lean?  BTW, that extra spring I put on the CR's really made the revs fall directly down to idle without any hangup at all. 

Also, the tachometer cable seal on the engine is bad and seeping oil, do you know the best place to get a new seal?


I should have this thing working well enough for a good ride next weekend.

   Glad that spring helped Jhase. Since we weren't able to get to the fine tuning stage with your bike yesterday might I suggest that you take a trip across the bridge this weekend and we can hit my favorite "asphalt dyno", Bear Creek Road. Too many stop signs and people in your area to do any performance checks of your carbs under load. You bring the bike, I'll bring the jets and a few tools and we'll call it a friendly ride with plenty of smoke breaks for me

   It did bother me that the last ride you took before I left had 1,2,3 pipes changing color so quickly and I was hoping that it had something to do with the fuel being shut off and bowl levels being low. I would expect some discoloration of the headers during a longer ride, but around the neighborhood is too quick.

   I am guessing that the throttle range that is most affected right now is 1/4-1/2 pull and we won't even know about your mains until your bike hits a "full throttle" road. I would hate to see ya overheat that nice clean motor or permanently discolor that shiny Kerker ( just yet ) so here's what I would suggest.

     Your idle mixture screws are maxed out rich at the moment and I think your mains might be a size or two too small. This may be radical but try raising your needles another two clips and back your idle screws out to 1/2 turn each, and maybe 5/8-3/4 on #4 since it is not as hot as 1,2,3.

   Raising the needles will have an effect on most of the riding you'll do and can make idle mixture a tad richer. I'm hoping that by raising them you will get the mix to the point that idle adjust screws will have a measurable effect which is why I suggest you lean them a bit after raising the needles. You WILL want to check and set the adjustment screws after raising the needles, just the way I showed ya, since the settings I stated above are just a guess.

    If your bike gargles on gas with the "two clip" raise then drop them back down one. If it runs OK and starts to clear and pull past half throttle then leave it as is as that would be good for a freeway trip to Oakland

   Feel free to give me a call. If you've misplaced my # I can call ya


    I hope you don't mind me posting the pic you sent me. Definitely a sweet looking bike and I can't shake the feeling I've seen it before.

   Jhase's badass  "I'll take one with everything on it" 750F

[MRieck]

 Chrome Kerkers change color fast Sean. The tubes aren't that thick.

[andy750]

Nice bike and I agree about "seeing it before" somewhere....where did you get it jhase? Ebay bike?

cheers
Andy

[scondon]

Chrome Kerkers change color fast Sean. The tubes aren't that thick.

   Yes they do, and mine were all blue and purple within a month   Jhase's were getting that way within a 5 block ride and the bike was popping past half throttle. Never tuned a set of CR's to a stock cam/motor before but with the K&N's and med. comp baffle I'm thinking that 105 mains might still be a bit small.

    We didn't get to do any real testing of the carbs since most of our time was spent with tuning the bike. Valves were all sloppy and ignition was a bit uneven and set in the #1 curve. Raised the idle, popped the needles up a clip, and set mixture screws was about all we could do at the time. Bike revved up well on the stand but one of the return springs was off the carbs so revs hung up making the bike difficult to ride and there was a stop sign every block so nowhere to really check throttle position under load.

   Jhase is pretty savvy and catches on quick. Lots of motorcycle experience, just not with this model. His bike has a stock  '76 750F motor with 29 CR's, pods, med. comp baffle, Dyna 2000, 2.2 coils, stock ngk D8's, 7mm dyna suppression wires. Basically everything you'd want on a hot rod but feeding a stock engine. Am wondering if the 2.2 coils might be a bit much and whether some iridium plugs or "overgapping" of the ngk's is in order. Also, the mixture screws are only 1/4 turn out and anything more is too lean(60 jets). Seven position needles are 3rd the top at the moment (I think, can't remember...Jhase?).

   Posting all this so any of ya "keyboard" mechanics can add yer thoughts

[jhasewhite]

Needles are 2nd groove from the bottom now - #6 position.

I did over-gap the plugs a tiny bit to compensate for that extra voltage. I also heard a rumor that electronic ignition spark looks weaker than with points, so maybe that was a bad idea?

 I raised the needles two grooves and leaned the idle mixture about a half turn and it did make the bike more drivable off idle and up to the point where the needle taper and main jets come into play. It developed a sporadic low toned pop/backfire at idle only. Off idle to almost 1/2 throttle feels great, if not slightly fat.  No matter where I set the idle air screw on the #4 cylinder, the header pipe won't heat up and just doesn't seem to firing as strong as cylinders 1,2,and 3 (could be where the rich pop is coming from??). I pulled the plug and it did look carbon fouled, so I cleaned it with 600 grit sandpaper and checked the gap again. I'm thinking that the starting circuit may be letting some raw fuel (and air) in, so I'm headed off to the Kragen to get some carb cleaner and vacuum caps to block off and clean the entire started circuit (the bike starts halfway through the first kick with no choke anyway... 

I would like to say that it is still rich at idle, but those left three headers seem to heat up and turn yellow pretty quickly.

I'll tinker with it some more and keep the revs low until I can get across the bay to Oakland for some more expert tuning and most likely larger mains from Sean.

[scondon]

 Damn, you catch on quick Jhase   Sounds like a good plan you have. Also check those screw on tips on the spark plugs and see that #4 hasn't jiggled loose.

[Jerry Rxman Griffin aka MuthaF'er]

Sean, etc,

How do you compare 105's in the CRs with 105's in the stock carbs? I'd assume they are the same size?!

Now, CR29's are 1mm bigger than stock 28's. Wouldn't this have increased air flow? Need more gas to match? Stock jets are 105's. Kerker. Yellowing Kerker. Pods. 2.2ohm coils sucking the life outta a stock electrical system. My 3ohms are bad enough and I'll never get that elusive 14.5 theoretical V's from mine. I see lean here especially at sea level.

SO, considering I know nothing about CR's but will learn soon I hope, should you try going up 1 or maybe even 2 sizes on the mains to eliminate the yellowing? I know this doesn't alleviate the cold cylinder, that would be a seperate issue. I have 1 pipe discoloring on my Kerker and I'd like to think my valves are set correctly but I may have a mushroom tipped Manley valve creating my issue that gives me a false gap reading. If not, will you work on my carbs too?     

[scondon]

Sean, etc,

How do you compare 105's in the CRs with 105's in the stock carbs? I'd assume they are the same size?!

Now, CR29's are 1mm bigger than stock 28's. Wouldn't this have increased air flow? Need more gas to match? Stock jets are 105's. Kerker. Yellowing Kerker. Pods. 2.2ohm coils sucking the life outta a stock electrical system. My 3ohms are bad enough and I'll never get that elusive 14.5 theoretical V's from mine. I see lean here especially at sea level.

SO, considering I know nothing about CR's but will learn soon I hope, should you try going up 1 or maybe even 2 sizes on the mains to eliminate the yellowing? 

   These CR's are crazy, Jerry. Starter circuit, #60 pilot jets, 7 step needles(older style) makes for some pretty wild tuning choices. The main is about the easiest choice to make and I'd say your spot on with your analysis.

   That dead, not really dead just not burning as well, #4 does sound ignition related and I'll have a look at Jhase's plug gap and plug lead when he comes by, if he doesn't beat me to it. Hopefully it is just a stuck open starter circuit and that can of carb clean he gets might do the trick.


   

I have 1 pipe discoloring on my Kerker and I'd like to think my valves are set correctly but I may have a mushroom tipped Manley valve creating my issue that gives me a false gap reading. If not, will you work on my carbs too?     

   You bet    Sometime in August sound good? Give ya the 3000 mile tune up before sending ya packin' up North

[jhasewhite]

Yep, ebay bike.  It was originally built (painted and fancy parts bolted on but no tuning) by G&G motorsports in Encino, ca. 

40+ hours of work and I'm still sorting out all of the problems and neglected issues that the "professionals" left for me.

[scondon]

 Don't forget to add those center stand bolts and safety pin    Thanks for naming the seller Jhase. Guys, If ya ever want a really nice looking bike but also expect to ride it then don't go to G&G Motorsports. Definitely worth the selling price 'cause of all the nice parts, paint, and chrome/polishing work done, but "ready to ride" should be extended to "ready to ride to the mechanic on the next block and hope you don't die on the way there"

   Pardon me Jhase for havin' a little fun, you definitely saved a sweet machine from those SoCal smokedopers

[andy750]

Sounds like you are getting it sorted jhase and with Sean on board you are going to be flying in no time! Im looking forward to the day I come back to SF and get a chance to meet the man myself!

Have fun with the bike!

cheers
Andy

[bochnak]

Hi guys,

I'm trying to tune some CR26's on a CB550, and was wondering what size pilot jets and how many turns on pilot screw are you guys running.

Currently, the carbs have #50's, air screw can be closed all the way and it will still run. Best air screw setting is 1/2 turn out. Bogs like crazy at low RPM's. I put some tape over intake and problem greatly improves.

The CR guidebook states that #65's should be installed as a starting point. Also, it will not idle below 1800, and may be related to the lean pilots.

I'll clean the choke circuit and I have #60's & #65's on the way.

[beernard]

Thanks everyone,
I have been trying to sort a set of CR Specials for my Triumph Tiger while I am recovering from a Knee replacement. Why??
I also have a CB750 racebike running CR's. Runs fine.
My mental world has put me in the position of chopping up a set of CR's from a GSX1100 racebike to fit my 1993 Tiger. What was I thinking?
The actual chop-up was ridiculous. Now I am trying to tune it. Bizarre when you don't have a clue.
Point being, you all have taught me some good stuff. Long live the forum.
PS this thread is really old, but so what?
PSS Honda Fours Rule.
Regards,

[MRieck]

You can attach clear tubing over the air pick ups for the cold start circuit. It helps prevent junk from getting in there and compromising the sealing disc.

[beernard]

Thanks MReick. Sounds like a good idea.
All the best,
Beernard.