Topic: Current carb set up is holding my bike back....

[sparty]

Guys,

I believe that my current carb set up is holding my bike back.  For those unfamiliar with my café’ project, let me catch you up on what I have done to the mill:

Head ported and polished
836cc piston kit
Heavy-duty valve springs
Racing top valve spring collars
Hotter Camshaft
Amco 45 valve guides
New exhaust valves
New intake valves
Heavy-duty cylinder studs
Heavy-duty cam chain
New cam chain rollers and slider
New connecting rod bearings
New main crankshaft bearings

The PO installed ’78 carbs on the ’75 CB750 K2.  They are currently adjusted just a tad rich and the number 4 carb just won’t synch perfectly even after the entire carb rack was unracked and rebuilt.  I thought about putting on the Cycle Exchange two carb set up, but I am concerned about air flow.  I also debated finding and installing newly rebuilt ’72 carbs.  But most recently I considered going for broke and opting for a set of CR carbs from Sudco.

I was wondering if you or someone you know is running this set up:



Are they worth the $700 clams they require?

Sparty

[Terry in Australia]

G'Day Sparty, well (via Mike Rieck) I just bought a set of new 29mm Keihin CR's from Sudco, but my 836cc engine is still aways from being finished, however for the guys out here in Oz who race CB750 based "Post Classic" racers, they are the ants pants. I would have preferred 31mm CR's, but at the time they weren't available in the US.

I've also got a very rare set of dual twin throat Dellorto 40mm carbs, and a set of 34mm Mikuni flat slide carbs, but I'll use which ever set works best on the 1166cc engine. Cheers, Terry.

[sparty]

Terry,

I was thinking of getting the 29's as well.  I don't intend on racing my bike, would the CR carbs be overkill?  I am sure that they would help open up my 836cc's.

Sparty

[Terry in Australia]

Terry,

I was thinking of getting the 29's as well.  I don't intend on racing my bike, would the CR carbs be overkill?  I am sure that they would help open up my 836cc's.

Sparty

Buy 'em mate, with all your other mods it'd be a shame to run the stockers, and the CR's will "unleash the beast", ha ha! Cheers, Terry.

[Jerry Rxman Griffin aka MuthaF'er]

Watch out for Terry, mate. That monster engine he is building will require a 2 Four barrel set up  .

My debate would be whether my engine could handle the CR31's vs the CR29's. I believe they cost the same. With the head work, (depending on how much was done and whether or not you put in larger intake valves) cam, and 836 perhaps it can handle the 31's. I'm collecting pieces for a 900 engine and I plan on using the 31's. I'll probably replace the carbs on my current bike but I'm not sure what it will handle. 810, race cam, springs, 33.5mm intakes on the new head (not extensively ported) to be installed. I'll probably use the 29's on this. What the hell - it's only $1500. 

[Terry in Australia]

Watch out for Terry, mate. That monster engine he is building will require a 2 Four barrel set up  .

My debate would be whether my engine could handle the CR31's vs the CR29's. I believe they cost the same. With the head work, (depending on how much was done and whether or not you put in larger intake valves) cam, and 836 perhaps it can handle the 31's. I'm collecting pieces for a 900 engine and I plan on using the 31's. I'll probably replace the carbs on my current bike but I'm not sure what it will handle. 810, race cam, springs, 33.5mm intakes on the new head (not extensively ported) to be installed. I'll probably use the 29's on this. What the hell - it's only $1500. 

That's it mate, buy 'em both, I like the way you think! 

But seriously, the 29mm carbs should be fine, the "hot ticket" for the "small port" GS1000 Suzuki's are Mikuni or Keihin 29mm carbs, so for a 736cc or even an 836cc engine they should more than adequate. Cheers, Terry.

[dusterdude]

airflow?

[Terry in Australia]

Yes!

[SteveD CB500F]

Is that YES (lots) or YES (please) ??

[MRieck]

Terry,

I was thinking of getting the 29's as well.  I don't intend on racing my bike, would the CR carbs be overkill?  I am sure that they would help open up my 836cc's.

Sparty

The 29's will work well for your set up. Sudco was out of 31's a little while ago though that situation may have changed by now.

[dusterdude]

whats wrong with the twin mikunis?

[sparty]

whats wrong with the twin mikunis?

Duster,

There is the possibility of restricted air flow to the filters because your(my) legs, with out forward controls, would be in the way.  I am not sure if this is true, but.... it has been suggested.

Sparty

[dusterdude]

i suppose that is possible,but there are ways around that.

[Terry in Australia]

I'd be more concerned about getting my enormous balls sucked into those bell mouths, ha ha! 

[MRieck]

I'd be more concerned about getting my enormous balls sucked into those bell mouths, ha ha! 

Your wife could solve that problem Terry . Seriously, I'd be more concerned with the long overall intake tract length with that Mikuni set up.

[Sam Green Racing]

Sparty, the CRs are worth every penny. We have a set of 31s on the CR750 and they made a big difference, extra 3bhp.

Check the pics of them>>>>>>
                                             v
Sam.                                      v
                                             v
                                             v

[Terry in Australia]

Sparty, the CRs are worth every penny. We have a set of 31s on the CR750 and they made a big difference, extra 3bhp.

Check the pics of them>>>>>>
                                             v
Sam.                                      v
                                             v
                                             v

Geez that Chris is an old bastard mate, he looks older than dirt! You're looking ok though Sammy, although like me, you could lose a few pounds, ha ha!

[78CB750F1]

Sparty, I run 29mm CRs on my CB750F stock motor and the performance is amazing.  From my understanding, many of the vintage CB750 racers with big bore motors run the 31mm version.

[sparty]

It is confirmed, my current carb set up is holding my bike back.  Today, I took it out for a ride with the intent to run it a few times at WOT  or at least come close to redline in all gears.  It turns out that in first gear I can't get it to redline, because my fuel/air cuts out around 7000RPM.  I had a feeling this would happen.  I haven't run the bike hard since I did the 836CC upgrade because I wanted to be sure it was in proper working order.  Now that it is, I find that I can't make the power I should due to my current carb set-up. The PO put '78 carbs on the '72 CB750 and they just aren't cutting it.  I have got to get those CR carbs.

BTW, my new race exhaust is #$%*in'.  It screams so freakin' loud I need ear plugs.  I can't wait to hear it at redline.

Sparty

[Terry in Australia]

Hey Sparty, hows aboot posting a pic? I want to see that beast! Cheers, Terry.

[Jerry Rxman Griffin aka MuthaF'er]

Sparty,

I definitely will not talk you out of getting those CR31s !! However, have you discussed how you are jetted?
I'd be surprised (not the first time ) if the actual carb sizes were keeping you from going above 7000. Also, do I understand you have '78 carbs now? I don't know much about these especially the fuel pump, but what about the fuel pump guys?
Again, I won't talk you out of spending $750 on a real nice set of carbs that would definitely make a difference..... but it could only take a few bucks to get your current carbs straightened out. Hope I didn't miss anything addressing this stuff already.

[sparty]

Sparty,

I definitely will not talk you out of getting those CR31s !! However, have you discussed how you are jetted?
I'd be surprised (not the first time ) if the actual carb sizes were keeping you from going above 7000. Also, do I understand you have '78 carbs now? I don't know much about these especially the fuel pump, but what about the fuel pump guys?
Again, I won't talk you out of spending $750 on a real nice set of carbs that would definitely make a difference..... but it could only take a few bucks to get your current carbs straightened out. Hope I didn't miss anything addressing this stuff already.

Thanks for the concern.  The '78 carbs are not very adjustable.  The slow jet is pressed in making it very difficult to make jetting changes.  I rebuilt the carbs back in May and they do work better, but the lack of adjustment is why I am going to go with the CRs.

Sparty

[sparty]

Hey Sparty, hows aboot posting a pic? I want to see that beast! Cheers, Terry.

Terry,

I hope to post pics on Wednesday of this week. 


Sparty

[eurban]

Sparty-
You description off your "fuel air cutting out" at around 7K would more likely indicate a need to change your main jets than that your carbs are incapable of handling the needs of your modded motor. . . . My experience with the #35 slow jets on the stock 78 carbs and performance upgrages such as an 836cc engine and pods (also dyna ignition, mildish performance cam, but stock 4-4 exhaust)  filters is that there is no need to go larger.  I actually drilled a set of the #35s out with a micro drill bit to what should aproximate a #40 and used this for my initial post rebuild riding.  Bike ran best with the idle screws just about closed down so I have since put in an unmolested set of 35s and she runs best (good bit nicer than with the drilled idle jets) at about 1 and 1/8 turns out.  In my case anyways there was certainly no need to go richer in the idle jets.  I also ordered a set of (probably about $40) adjustable main jet needles specced for the 77 carbs from David Silver Spares that allows for more precise adjustment than by adding washers underneath the stock 78 needles.  Main jets are of course readily available. You could also shim the spring of the acclerator pump and or adjust the clearance between its actuation rod and the throttle linkage.   However, changing such things as slide cut away and needle profile does not seem to be much of an option with the stockers.  All in all I am sure that the CRs would probably give you more outright performance than the stock carbs.  I do wonder if they actually improve general driveability?  For those using them, what are they really like on a street bike driving around town or on the highway?  MRieck mentions the lack of an accelerator pump and even with the 29mms you are sizing up which may very well actually lower power at the low end of the RPM scale.  They also cost a major chunk of change!!  If you want em cause they will give you the most top end power, and because they look way cool then I would say go for it.  But if it is as you say simply a matter of adjustablity then you might want to rethink!

[Sam Green Racing]

Sparty, the CRs are worth every penny. We have a set of 31s on the CR750 and they made a big difference, extra 3bhp.

Check the pics of them>>>>>>
                                             v
Sam.                                      v
                                             v
                                             v

Geez that Chris is an old bastard mate, he looks older than dirt! You're looking ok though Sammy, although like me, you could lose a few pounds, ha ha!

Guess I deserve that for all the sheep shagging jokes i've told about you 

Sam