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Author Topic: Jetting CR31 carbs on a CB750F1  (Read 8482 times)

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VMAX1970

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Re: Jetting CR31 carbs on a CB750F1
« Reply #25 on: July 10, 2005, 02:11:57 AM »
all you need to then is put a 836 kit in then they would love the 33mm
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Offline Mark M

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Re: Jetting CR31 carbs on a CB750F1
« Reply #26 on: July 11, 2005, 01:42:12 AM »
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.....   
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Offline Sam Green Racing

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Re: Jetting CR31 carbs on a CB750F1
« Reply #27 on: July 15, 2005, 02:40:25 PM »
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
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Offline Mark M

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Re: Jetting CR31 carbs on a CB750F1
« Reply #28 on: July 18, 2005, 08:39:26 AM »
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.
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Offline Hockers Choppers

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Re: Jetting CR31 carbs on a CB750F1
« Reply #29 on: February 05, 2007, 04:11:12 PM »
Quote from: VMAX1970 on July 10, 2005, 02:11:57 AM
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?
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Offline Sam Green Racing

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Re: Jetting CR31 carbs on a CB750F1
« Reply #30 on: February 05, 2007, 05:22:22 PM »
31s Hocky :D :D :D :D

Sam ;)
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C95 sprint bike.
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Offline Hockers Choppers

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Re: Jetting CR31 carbs on a CB750F1
« Reply #31 on: February 05, 2007, 05:30:35 PM »
33's too big with 836 cam porting etc???
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The thing about common sense is that, it's not that common.

1978 CB750F SUPERSPORT 850cc, cam, porting, dyna ign, cr29's lotsa elbow grease and $$'s. Worth ever penny!!  (S0ld)

05 VTX1800F darkside
cb750 k1

Offline Sam Green Racing

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Re: Jetting CR31 carbs on a CB750F1
« Reply #32 on: February 05, 2007, 05:39:34 PM »
31s are about right upto 900cc.

Sam. ;)
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C95 sprint bike.
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CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline Terry in Australia

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Re: Jetting CR31 carbs on a CB750F1
« Reply #33 on: February 05, 2007, 07:59:45 PM »
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. :) 
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Jetting CR31 carbs on a CB750F1
« Reply #34 on: February 05, 2007, 11:20:29 PM »
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?

« Last Edit: February 05, 2007, 11:33:02 PM by RxmanGriff »
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Offline Terry in Australia

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Re: Jetting CR31 carbs on a CB750F1
« Reply #35 on: February 09, 2007, 04:30:25 AM »
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. ;D 
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I was feeling sorry for myself because I couldn't afford new bike boots, until I met a man with no legs.

So I said, "Hey mate, you haven't got any bike boots you don't need, do you?"

"Crazy is a very misunderstood term, it's a fine line that some of us can lean over and still keep our balance" (thanks RB550Four)

Offline Geeto67

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Re: Jetting CR31 carbs on a CB750F1
« Reply #36 on: February 09, 2007, 11:43:12 AM »
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. 
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Offline Terry in Australia

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Re: Jetting CR31 carbs on a CB750F1
« Reply #37 on: February 09, 2007, 03:15:34 PM »
Quote from: Geeto67 on February 09, 2007, 11:43:12 AM
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.  :'(
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I was feeling sorry for myself because I couldn't afford new bike boots, until I met a man with no legs.

So I said, "Hey mate, you haven't got any bike boots you don't need, do you?"

"Crazy is a very misunderstood term, it's a fine line that some of us can lean over and still keep our balance" (thanks RB550Four)

Offline Geeto67

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Re: Jetting CR31 carbs on a CB750F1
« Reply #38 on: February 10, 2007, 06:53:27 AM »
Quote from: Terry in Australia on February 09, 2007, 03:15:34 PM
Quote from: Geeto67 on February 09, 2007, 11:43:12 AM
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.

 
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