Topic: carb options for 77-78 750F?

[scondon]

    Is anyone running carbs other than the stock Kehin's on their 750F2? I have 2 sets of stock carbs that have run on two different bikes, three different engines, Dyna and points ignition, three seperate exhausts, etc... Both sets of carbs have been rebuilt(needles,pilot,gaskets),cleaned,synched, and endless fiddling with mixture screws. I have been unable to eliminate the "popping" from exhaust and "flat" acceleration from a stop. I am putting together a 836 motor and would like it to "be all it can be".

[Terry in Australia]

G'Day Sean, well a few years ago I had a Montesa 250 dirt bike, it came with a horrible Spanish made Bing 32mm carb that I wanted to replace with a Mikuni 36mm race carb, but like your dilemma, the O/D of the Mikuni was much smaller than that of the smaller Bing, so what I did was machine up an alloy "slip ring" to go over the Mikuni's "spiggot" so that it was a nice snug fit in the rubber manifold tube, I even machined in the "notch" that locks into the rubber.

I used some epoxy (araldite) to seal the slip ring to stop air ingress between the carbs spiggot and the slip ring, and it worked fine. If you like I'll try to make some "Slip Rings" for your favourite carbs so they'll fit into the F2 rubbers? I'll need you to send me an F2 inlet rubber (only ones I've got are on my bike) but I've got spare K carbs here to measure for the slip rings. You'll just need to use your "K" series throttle cables and airbox to complete the deal, and it should run sweeeeeeeeeeeet! Cheers, Terry.

[scondon]

Thanks Terry. I do have that set of "K" carbs which at least has the 40 slow/120 main jets. I'll send an inlet rubber with your "box-o-stuff" this Saturday.
      A local shop owner said to try some Ninja 900 carbs. He gave no gauranties, just said they "might" work. Anyways, I got a set of "junker" '84 Ninja carbs off ebay just to measure them against the F2 carbs. The O/D matches but the spacing is a "tad" off. I'll see if I can get'em to fit when I get the engine together which, by the way, is FINALLY painted and ready for assembly. The Ninja carbs would, at least, have a lighter throttle and hopefully better performance characteristics.
      Anyone else tried, or heard rumor of, carbs from more modern sportbikes on their SOHC?

[Terry in Australia]

No worries Sean, problem with the CB750's engine was carb spacing, compared to the early aircooled Z900/1000 and Suzuki GS750/850/1000/1100 which were all the same. The "Ninja's" had a narrower engine though, so it's possible that you'll make 'em fit, but it can be a frustrating situation, for sure. off topic for a sec, can/does your print shop have the capacity to print computer images on adhesive exterior grade vynil labels? Cheers, Terry.

[Geeto67]

I have a set of 76 cb750F1 carbs I am using on a 78 right now to sort things out I like them much better than the f2 carbs. Eventually I want to go to the cycle exchange setup :

http://www.cyclexchange.net/Carb%20System%20Comp%20Page.htm

While I hear it is techincally less efficient I am more comfortable with mukunis from my expirence with kaw triples. I suppose it is possible to use individual mukunis with a 1969 style cable (or custom cable). You would need to find out what size would work but I would start with those off an s3 400 and go from there.

[scondon]

Thanks Geeto67. Those Cycle Exchange look mighty "tasty", though maybe a bit much for the bike I'm building. I'm keeping the link in case I ever get around to building that Weekend Racer I'm dreaming of. I also prefer the earlier carbs. With the '78 carbs, it's nice to be able to pull the choke and walk away while the bike warms up but that's about it. Those pressed in slow jets have caused me nothing but grief, and no, I have not been able to "gently" pull them loose for cleaning.

     Off topic: Sorry Terry, We only have old fashioned printing presses that go "ka-chunk ka-chunk". What size/application are you looking for?

[Bobby]

I went down this road last year and I received a ton of info from the old Greenspun forum.  I am still vacillating between my "tuned" '75 carbs for my '78 motor and the $500 carb system offered by Cycle X.  My CB750 'Project X' is so out of control that  I have blown my budget, alienated my wife, lost dear friends and jeopardized my job, I'll probably go with the Cycle X system.  That being said, here is my old thread:

http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=00CWsd

[scondon]

Thanks for the thread Bobby,and yeah, I feel your pain(haha). All I was "planning" to do was rebuild the engine. That was 6 months ago. My biggest "mistake" was having the frame powder coated. Since then, EVERY part has had to be cleaned, painted or it would look like a turd on satin sheets. The latest indulgance of my "insanity" was to purchase one of "crazy" Monty Campbells' X-pipe exhaust systems which, come to think of it, would look pretty sweet with those $500 Cycle X carb sets. AAAAAARRRRGGGGHHHHH.

In the past three years I have learned a lot of what there is to know about '78 750F's, but just a little about motorcycles in general. There is much to know about carbs beyond cleaning and adjusting and I consider myself a novice in this area. That said, can anyone tell me what the CR kehins are? and where they, or Mikuni's, can be found( on other bike models? performance shops? ) Thanks!

[Bobby]

Hmmm...  I messed around with 28mm Mikunis Flat Slides on the CB750's (from a Kaw Z1-900 drag bike).  Definitely some "choppin' and a fabricatin'" to make them line up.  I never finished that project as I found a sweet set of Keihns.  Kawasakis and Suzukis are good donor bikes, but the transplant is not for the timid and is not endorsed by the Surgeon General.

I remember a guy claimed he mounted 4 29mm "dirt bike" Flat Slide carbs on a CB750F and after locating a 4-1 throttle cable, rigging a rack and some re-jetting, he alledged that it ran "Like a scalded dog".  However, I never saw this machine.

That being said, there are always a bunch of Mikunis listed on eBay.

All things considered, I'm using my '75 carbs until I scatch that winning ticket for $500.

[dusterdude]

geeto,the cycle x deal is way cool,i may try that myself and bobby go ahead man dont worry about the bs

[Lumbee]

...hey Scon...I feel u'r pain...I'm dealing with 77/78 carb woe's right now.  Carbs from my 78F just seemed to quick working after sitting up for a week, while I swapped a motor.  Is there a difference in the carbs on a 78F & K model?  I thought they were all the same on the 77/78's regardless of model...

[TwoTired]

The basic design appears to be the same.  But, the carbs are set up differently due to the difference in exhaust systems and perhaps the cams in stock form.

From the Honda Shop manual:
                            CB750 K7                          CB750 F2
Main Jet no.                 #115                                 #105
Air Jet No.                   #150                                 #120
Slow Jet No.                #35                                   #35
Slow Air jet No.          #150                                 #150
Jet Needle-Setting      F2D51E-1                          F2051F-2
Float Height                 12.5mm (0.492 In.)       14.5mm (0.571 In.)

[dzachary99]

I bit the bullet and bought the CycleX carbs, I was feed up with those old !@@^!%@ well you all know. The system is very well put together and Ken the owner is very good to work with. I did  re-learn an important lesson though, a carb is a carb and will always be a carb no matter what it looks like. I am still trying to get it just right. The big plus is Ken will help and guide you every step of the way and all the jets are free for tuning purposes. And now there are only TWO to tune. I will keep you posted as the tuning progresses.

Good luck with the wife, mines just now starting to talk to me again.

God Bless,
Doug

[Bobby]

Right On Doug!  You have inspired me!  I'm gonna...  I'm  gonna...  Ummmm...  I better ask me wife.... 

[CHUNG]

Hey Dude, I'll be in the mother lode in two weeks, gotta make the pilgramage to the mountains and smoke the piece pipe with me? The Cycle Exchange setup looks very intreaging. That's one 34mm carb per pulse. Should free that puppy up a bit?

[MRieck]

Thanks for the thread Bobby,and yeah, I feel your pain(haha). All I was "planning" to do was rebuild the engine. That was 6 months ago. My biggest "mistake" was having the frame powder coated. Since then, EVERY part has had to be cleaned, painted or it would look like a turd on satin sheets. The latest indulgance of my "insanity" was to purchase one of "crazy" Monty Campbells' X-pipe exhaust systems which, come to think of it, would look pretty sweet with those $500 Cycle X carb sets. AAAAAARRRRGGGGHHHHH.

In the past three years I have learned a lot of what there is to know about '78 750F's, but just a little about motorcycles in general. There is much to know about carbs beyond cleaning and adjusting and I consider myself a novice in this area. That said, can anyone tell me what the CR kehins are? and where they, or Mikuni's, can be found( on other bike models? performance shops? ) Thanks!

Keihin CR's are available from Sudco. Unfortunately the screw in adaptors are to small for the F2 and F3 intake manifolds (or "insulators" as Honda calls them". I think the adaptors for a DOHC CB750 or 900 will work. Mikuni smoothbores went out of production years ago.