Topic: Jetting CR31 carbs on a CB750F1

[Mark M]

Just waiting for a set of smoothbores to arrive, can anyone suggest a good place to start on tuning these.
The engine is standard, 4 into 4 race pipes (with minimal baffles) and as you probably worked out no airbox just the velocity stacks that come with the carbs. They had been set up for a z750 so I'm expecting they will be set up on the rich side for my use.

Anyone any suggestions that may save me too much messing about.

[MRieck]

They came with 120 mains and ,I think 62 pilots. Leave the mains at 120, put theYY-8 needles 2 notches down from the the top and turn the air screws out 1 turn. Start from there.

[Mark M]

Cheers, I suspect I'm going to playing with these for a while before I get them just right.

[Mikeshonda750F]

where did you get the carbs at? What extra is required to fitting them? Would be interested in a set for my F-0 750 which should be the same part number as your F1's.

[Mark M]

I got them on ebay, there was a guy in Germany selling some CR33's but it's realy just a case of luck.
They normally go for serious money, but I got mine for £255 mostly because he didn't mention Honda in the listing
so it didn't attract much attention.You would normally be looking at 3-4 times that much.

Fitting - well i've seen some fitted to a K head with what looked like F2 rubbers. I'll work that out when they arrive.

[Sam Green Racing]

Hi guys,

Mark just to confirm Mike Riecks starting figures are spot on. I run the same set up on a K useing this jetting with four open megas.

Mikeshonda the carbs are about £500 plus here in the UK. If you are States side Mike should be able to help as he has a set on his Million Dollar CB. As regards fitting we use K2 rubbers on a K5 motor.You might also consider a carb steady from the frame to prevent any frothing in them.

Good luck to you both and Hi Mike.

Sam Green  UK

[Terry in Australia]

G'Day Mark, are the CR31's adjustable for carb spacing? The reason I ask is I just test fitted my Mikuni 34mm RS flat slide smoothbore carbs (I thought they were 31mm when I "inherited" them:) to my "Magic by Mike" head only to discover that while the outside two carbs (either side) align with their respective spiggots, the centre pair of carbs are spaced out about a half inch too wide.

These carbs came off a GSXR1100 engined race bike, and the carb spacing is spot on for my Suzuki GS1000S (8 valve) head, but to make them fit a CB750 head I'd need to somehow close up the spacing between the centre carbs about a half inch, which will apparently require not just some adjustment in the racks, but shortening of linkages etc? Reason I mention all this is that I'm pretty sure that Kawasaki carbs have the same spacing as Suzuki's, so unless yours have slotted racks, you might need to do some mods to make 'em fit? Cheers, Terry.  

[Matt at PSB]

31mm CR Specials are too large by a large margin for a 736cc SOHC CB750. You need 29mm CRs. Your bottom end power response will never be manageable. All the tuning in the world will not help.

Matt at PSB

[Mark M]

Sam,
The carbs have some long mounting tubes on them, which if I fit these to some K intakes will make the intakes about twice the length of the OE setup, my feeling is this will be too long. Where I have seen them mounted the carbs screw in mounts have not been altered but shorter rubbers (F2) have been used.
How are yours mounted?

Terry - yes fully adjustable - haven't measured them up yet, will have a play this weekend.

Anyone suggest a UK source of jets/needles as the ones currently in there are much too big.

       

[Sam Green Racing]

Mark,

To be honest they went straight on. F2 rubbers onto stock K5 stubs.
What Mat at PSB was saying is true in some respects.
The spec sheet for the carbs state 31mm for 750 to 900cc if my memory serves me correctly, so if your motor is stock at 736cc they may be on the big side, but on the other hand our motor is 736 and drives clean throughout the rev range.
The reason we bought them in the first place was because our stock carbs had seen better days and next year the bike will be out to 836cc with cam and head worked.
Parts for them can be obtained from Allens Performance R&D Ltd, Unit B9, Moorbridge Road, Bingham, Nottingham, NG13 8GG. you can also email them on sales@allensperformance.co.uk.
Where in the UK are you? I am in Cheshire.
Hope this will be of some use to you.
Good luck.  Regards Sam Green. UK.

[Sam Green Racing]

PS,   If you have seen them fitted to a K motor with F2 rubbers it may have been ours.

[MRieck]

Mark,

To be honest they went straight on. F2 rubbers onto stock K5 stubs.
What Mat at PSB was saying is true in some respects.
The spec sheet for the carbs state 31mm for 750 to 900cc if my memory serves me correctly, so if your motor is stock at 736cc they may be on the big side, but on the other hand our motor is 736 and drives clean throughout the rev range.
The reason we bought them in the first place was because our stock carbs had seen better days and next year the bike will be out to 836cc with cam and head worked.
Parts for them can be obtained from Allens Performance R&D Ltd, Unit B9, Moorbridge Road, Bingham, Nottingham, NG13 8GG. you can also email them on sales@allensperformance.co.uk.
Where in the UK are you? I am in Cheshire.
Hope this will be of some use to you.
Good luck.  Regards Sam Green. UK

   I agree with Sam about displacement in regard to what Matt said. It is also about application as Sam stated. The latest CR's have had the screw on manifolds modified from Sudco(or the factory) to fit early K manifolds. The previous verions had to large an OD- I modified mine, and others, by cutting the OD AND LENGTH of the adaptors. Sudco has FINALLY seen fit to do this on their own ( I DID #$%* to them about but t hey ALWAYS said "your the only one with a problem"......Bull#$%* I say!" The groove for the manifold sealing lip was always to narrow...I opened that up too. These carbs fit nice now(new versions) except for the groove. That goes for 29's or 31's. I hope this helps

[MRieck]

When i say Sudco, or the factory did this on their own I mean they modified both the OD( most important) and LENGTH( duh......) of the screw in adaptors......finally. I really did #$%* long and hard about this to them but I personally think they were going to sell what they had in stock- period. That groove is still too narrow. I can open it up if anyone needs.

[Sam Green Racing]

Mark,

Have you had any joy ?

[Matt at PSB]

When i say Sudco, or the factory did this on their own I mean they modified both the OD( most important) and LENGTH( duh......) of the screw in adaptors......finally. I really did #$%* long and hard about this to them but I personally think they were going to sell what they had in stock- period. That groove is still too narrow. I can open it up if anyone needs.

Auh,... Sudco does not suggest or recomend 31mm CR Specials for 736cc SOHC CB750s. Sudco suggests & recomends 29mm CR Specials for this application. Sudco does offer the option of the oversized 31mm CRs on a 836cc kited & aftermarket cammed "Race" version of the SOHC CB750 engine. This would be a very impratical set up on a streetbike.

Never the less the carb spigots are the following Sudco part numbers: For the 29mm it would be #020 473. I have used these with excellent results on 736cc SOHCs. For the 31 CRs the spigot part number is 020 474. I have not used any 020 474s before because I have never wanted to attempt to install 31mm CRs on a SOHC CB750. Of course four spigots would be required & they are not cheap. Are you in possession on for 020 474 spigots? If not the this could be the cause of some of your difficulties.

Matt at PSB

[Sam Green Racing]

Matt,

Three of us are trying to give advice to Mark who unless you know different hasn't told us the capacity of his CB750F1.
So we don't confuse him let us look back and take stock of what we have said, bearing in mind he has already purchased the carbs.
Also I would take it that the carbs we are all talking about are the modern equivalent of the 31mm CR Keihin that the factory used on its 1970 CR750 the bike that Mark is trying to replicate.
As Mark had already bought the carbs Mike gave advice with a base set up. I agreed with Mike as this is the set up we use on our bike. You came in and suggested that the carbs would not be suitable on the grounds that they was to large a bore at 31mm.
Mikes bike is out to 912 and makes a lot of power, whereas our bike is only a 736 (we think) runs clean through out the range, but makes no more power than the stock carbs.
Mike and I both agreed with your theory if the motor is in fact a 736 motor.
The thing we should all have done is asked Mark for more info on his motor and its intended use.
Come on Mark, give us more info.

Regards Sam Green.

[MRieck]

Matt
  Just to throw fuel on the fire. The adators are different secondary to the larger ID of the 31's- nothing else. The 29's and 31 adaptors used are the same OD and length(which, as I mentioned was to large for the K insulators). I cut down the OD on 31's and 29's to 1.533 which is about a millimeter larger than the OEM K/F1 carb spigots. I also mentioned the groove for the internal lip is to narrow- I take it out to .170. The overall length sucked too which i reduced to .6165 after being screwed into the CR carb bodies. Like I mentioned Sudco finally does this( in regard to OD and length) on both the 29 and 31 adaptors. It has been my experience...and many others, that Sudco doesn't want to hear #$%* about anything being wrong. Their favorite line, like most in the industry, is "that's the first time I've heard of that". I can certainly get people to back that up. I personally have installed many CR's, FCR's and Mikuni RS's. I certainly don't know everything BUT 31's can work on 750, as Sam said. given a particular application. What does McGrew run on his CR-750's...31's. Hell I wouldn't recommend 29's unless there was some motor work and that includes some porting to that crappy head as well as 836 pistons amd a cam/springs/ retainers. My beef has always been with Sudco as they are the sole distrubutor for Keihin and Mikuni and don't give a crap about any problem "they haven't heard of". They are full of pat answers. Icould go on about bad float needles for CR-31's etc, etc. but to get back to Sam's point I'd like to hear what Mark is running and start from there. BTW, my 915 made 92.8 HP and 57 lbs with 29's. It now has 31's and is a very practical street bike.
  Mike

[Matt at PSB]

Matt
  Just to throw fuel on the fire. The adators are different secondary to the larger ID of the 31's- nothing else. The 29's and 31 adaptors used are the same OD and length(which, as I mentioned was to large for the K insulators). I cut down the OD on 31's and 29's to 1.533 which is about a millimeter larger than the OEM K/F1 carb spigots. I also mentioned the groove for the internal lip is to narrow- I take it out to .170. The overall length sucked too which i reduced to .6165 after being screwed into the CR carb bodies. Like I mentioned Sudco finally does this( in regard to OD and length) on both the 29 and 31 adaptors. It has been my experience...and many others, that Sudco doesn't want to hear #$%* about anything being wrong. Their favorite line, like most in the industry, is "that's the first time I've heard of that". I can certainly get people to back that up. I personally have installed many CR's, FCR's and Mikuni RS's. I certainly don't know everything BUT 31's can work on 750, as Sam said. given a particular application. What does McGrew run on his CR-750's...31's. Hell I wouldn't recommend 29's unless there was some motor work and that includes some porting to that crappy head as well as 836 pistons amd a cam/springs/ retainers. My beef has always been with Sudco as they are the sole distrubutor for Keihin and Mikuni and don't give a crap about any problem "they haven't heard of". They are full of pat answers. Icould go on about bad float needles for CR-31's etc, etc. but to get back to Sam's point I'd like to hear what Mark is running and start from there. BTW, my 915 made 92.8 HP and 57 lbs with 29's. It now has 31's and is a very practical street bike.
  Mike

OK then, I stand corrected. 31mm CRs & 912cc engine with a lot of head work might work. Your experience with Sudco is vastly different than mine. I find then to be extremely knowledgeable & helpful right up to the point, where they say: "We have never done that before." At that point you are outside of their existing database and you are boldly going where no many has gone before. In that situation help is hard to find. I am sorry I was of such limited assistance to you.

I have not put 31mm CR Specials on a SOHC before. I would hazard to guess that Sudco has not scene over a dozen of these applications over the years. This, IMHO, is edge of the envelope stuff & by its nature very tricky. The problem is that the few people that do these applications & figure it out keep the info secrete. Then the slide, main fuel, slow fuel, air jet, & slide needle numbers do not become part of the general (Sudco) database and the next poor fellow is left in the dark too.

I am organizing the importation of 35mm horizontal FCR flat slide sets of four carbs directly from Japan at this time if you know anyone with an older inline four cylinder with around 900cc displacement or better. They will be around $1,039USD per set.

Once again, sorry I was not much help with the 31s. I would love to hear how it turns out. 92.8 BHP & 57 ft/lbs is very serious from a SOHC. At what point does the primary chain(s) become a reliability issue?

Matt at PSB

[MRieck]

Matt
 I'm very interested in the 35 FCR's. I currently run 39 FCR's on my 87 FJ-1200. As you know, Sudco dosn't list this model BUT they do for the european late model. They work (with adaptor work....AS USUAL!!!) I would be more than happy to share info...my FJ-1314  makes 153HP with 94Lbs with these carbs. They are great carbs...period. I'd be in on a set of those....and I'm not bull#$%*ting!

[MRieck]

BTW...thanks for the compliant. Anything over 90 is good for small engines. The cutaway and stuff isn't that inportant BUT needles(the top secret types), their height, etc do matter. Matt, don't let Sudco fool you....for Christ Sakes they have been around since the early 70's. That "British Guy".... And keep in mind.....I never said they were a-holes but unless you were Rob Muzzy they certainly were not forth coming!! Seriously...keep me in mind for those 35's- I'd help do development work (for free!!)

[MRieck]

Christ..it dawned on me. Matt are you the fella from upstate Washington I bought 2 sets of CB-750 valve caps from?? It's got to be.

[MRieck]

.....and if so...a good guy!

[Matt at PSB]

I can work with you on those 35mm horizontal FCRs. I have to bring them in at four racks of four carbs minimum at a time. It takes $1039.00 USD to get them off the dock & onto a slow container ship bound for Los Angeles. Lead-time is about 90 days. Yes express shipping is possible, but costly: To the tune of several hundred additional dollars per four sets of carbs. I have volunteers standing in the wings one more set & we are good to go.
   
I will need to know everything about your bike & how you plan to use it. This goes into the initial carb set up.

Alas, I am not from Washington State. I spent a half a year underground in a gold mine in Wenatchee once, but I do not recall buying or selling motorcycle parts during that stay.

I can be found in the Yellow Pages in Missoula, Montana, USA under “Performance Sportbikes” or you can PM me & I’ll provide full contact info. We accept Visa & Master Card.

Matt at PSB

PS: I have not found a source for horizontal FCRs smaller than 35mm.

[Mark M]

I posted a reply yesterday but it seems to have dissapeared into the either......oh well.

Right I'd like to thank all 3 of you for the input - to be honest I'm begingin to think I'm getting in over my head.
I started this project as a purely cosmetic excersise as I didn't think I could find carbs at a sensable price.

Anyway I now have these to play with - for the moment I'm going to get the bike up and running with the OE carbs.
It's 736 and no tuning work of any kind.

I have a cople of stripped engines (K2+F1) which as both sets of barrels have seen better days are my best bet for a big bore. Then gassflow and valves, a new cam and HD chain for it........this is going to have to be a long term project I think.
Something to aim for over next winter.

On the other hand I might just chuck them on over the weekend and see what happens.
Anyway thanks again for all the input.

Sam - if your bike was on show at Beaulieu this year, with the Historic Endurance Race Team then yes it was yous I saw.
BTW I'm in Chelmsford (Essex) 

[Sam Green Racing]

Mark,

It wasn't our bike at Beaulieu, but I have seen the Historic Endurance teams bikes and I don't recall seeing the carbs we are talking about on any of their bikes. Some of their bikes have the origional CR 31s and if you have bought a set of these for the price you quoted then you have hit the jackpot.

[VMAX1970]

all you need to then is put a 836 kit in then they would love the 33mm

[Mark M]

Sam,
Yeah it was a cople of the bikes they were showing - a Honda Britten and another endurance racer, I'd need to go bafk through my pictures, will post this evening.
No not the old CR31's but the newer chunky type.

I've got the suggested jets but the YY8 needles are currently out of stock, they offered me some YY7's and I already have some YY9's, which flow the most fuel. Is a higher number more or less?

The buildup is almost there now and after finding I had crap in my OE carbs decided I might as well play with the CR's as the std carbs. They were fairly easy to adjust and fit onto the F1 tubes, it wasn't until I had them on that I realised the throttle action is now very stiff, they did close nicely before adjustment. So they will have to come off again. While they were on I did manage to get the bike running, but all over the place, racing like mad at first so I probably have an air leek, the PO had drilled and taped the connecting tubes (to aide syncing I guess) so I need to work out a better seal under the allen heads.

With a bit of luck I'll have the niggles sorted by the time the tank comes back from painting and I'll be able to get it MOT'd. Then I can start playing with the jetting.....   

[Sam Green Racing]

OK Mark, we are all talking the same carbs, so I hope that the info you have had is of some help. Good luck.

Sam

[Mark M]

Sam,
Well it will at least run - a bit, now my ignition unit has packed up. With a bit of luck I will be sorted for
1st August as I'm off to the track (Cadwell Park) with my bike club. Then I can have a bit of play with the jetting.
Anyway thanks for all the input guys.

[Hockers Choppers]

all you need to then is put a 836 kit in then they would love the 33mm

Is this for shure? I'm kicking around buying a set. Do the 33's work?

[Sam Green Racing]

31s Hocky

Sam

[Hockers Choppers]

33's too big with 836 cam porting etc???

[Sam Green Racing]

31s are about right upto 900cc.

Sam.

[Terry in Australia]

I always get confused about "what's right" for a certain capacity, I've also got Suzuki GS1000's and the early "small port" heads worked really well with Mikuni 29mm "Smoothbore" carbs, then 33mm Mikuni smoothies for the later "Big Port" heads. (valve sizes remain the same in both heads)

Now my confusion here is that if 29mm Mikuni smoothbore carbs (which flow about the same CFM's as Keihin CR's, to my knowledge)are the "right size" for Suzuki GS1000's, then why do we need 31mm CR's for 836 - 900 cc Honda engines? Cheers, Terry.  

[Jerry Rxman Griffin aka MuthaF'er]

Terry,

My Yoshimura catalogue lists CR31's for their 500 kits also! I figure Pops Y knew what the hell he was doing 

How would the 33's etc bolt up to the head? Will the same manifolds work?

[Terry in Australia]

G'Day Jerry, well I'm not gonna cast any aspersions at the late great Pops Yosh, but 31mm carbs on a 500/4 sounds a little "over the top"? That's one 31mm carb per 125cc cylinder? Most 125's of the era were lucky to be graced with a 22 mm carb?

If someone was lucky enough to find this conversion kit they'd want to hope the inlet rubbers were in good condition, I doubt if they'd ever find replacements! I just won a set of Mikuni 29mm smoothbore carbs on Evil-Bay, they came off a Kawasaki KZ900, so I can't see why they wouldn't work just as well on an 836cc engine?

Maybe it's that "Why own a .38 when you can have a .45" thing? I'd love to see what size jets they're running? Cheers, Terry.  

[Geeto67]

Reading this thread gave me a chill. Since it was first started, Matt at PSB passed away. He was a regular on the cb1100f.net board but seldom here despite owning a '75 cb750F. 

[Terry in Australia]

Reading this thread gave me a chill. Since it was first started, Matt at PSB passed away. He was a regular on the cb1100f.net board but seldom here despite owning a '75 cb750F. 

Geez I'm sorry to hear about that Geety, Matt seemed to have a wealth of knowledge, rivalling our own resident expert, Mike Rieck! RIP Matt. 

[Geeto67]

Reading this thread gave me a chill. Since it was first started, Matt at PSB passed away. He was a regular on the cb1100f.net board but seldom here despite owning a '75 cb750F. 

Geez I'm sorry to hear about that Geety, Matt seemed to have a wealth of knowledge, rivalling our own resident expert, Mike Rieck! RIP Matt. 

he was amazing with bikes. His SOHC was pretty high mileage if I recall (like close to 100,000) and he was a pretty helpful chap.

not to sound incensitive but he was also the best place to get FCR carbs for old hondas, he sold them cheaper than anyone else and they were already set up to bolt on. Haven't found them since.

he passed away in a motorcycle accident (hit a car head on) in july 2006.