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Author Topic: Which size carb for 1000cc?  (Read 2032 times)

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Offline afkrejci90

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Which size carb for 1000cc?
« on: May 12, 2018, 11:13:32 am »
Resurrecting my 1000cc race build.  Have a set of Weber 40 DCOE's I was going to throw on there but getting them fitted is taking more time than I would have thought.  Down the road these will be going on but I want to get this thing going so I am going to throw on a set of Keihin CR's.  That being said which diameter would be better suited for this displacement, 29mm or 31mm?  Head has been ported with larger valves.  I read somewhere on here that 31's throw in too much fuel and foul up plugs, but would this be an issue with the increased displacement?
'78 CB750K http://forums.sohc4.net/index.php?topic=105327.0
'80 CB750C - Sold
'06 CBR600RR

Offline cbr954

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Re: Which size carb for 1000cc?
« Reply #1 on: May 12, 2018, 11:17:57 am »
Im running CR31 on my street 970cc no issues, the CR is very tuneable
03 CBR954RR, 72 750 chopper(970cc
F2 head), 2017 CRF450R, 2001 CR250R, 72 CB500, 79 XR250, 04 CRF50,70's soon to be rebuilt cb750 drag bike.

Offline slikwilli420

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Re: Which size carb for 1000cc?
« Reply #2 on: May 12, 2018, 11:19:04 am »
You won't regret making the Weber carbs work. A superior carb to many others out there. For street use people really like Mikuni RS34.
All you gotta do is do what you gotta do.

Vintage Speed Parts Mashup: http://forums.sohc4.net/index.php?topic=133638.0
Rickman CR Parts Kit Refresh: http://forums.sohc4.net/index.php/topic,154837.0.html
AHRMA CB750 Racer: http://forums.sohc4.net/index.php/topic,158461.0.html
AHRMA Superbike Heavyweight Racer: http://forums.sohc4.net/index.php/topic,173120.0.html
'76F CB750 Patina Redemption: http://forums.sohc4.net/index.php/topic,174871.0.html

Offline afkrejci90

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Re: Which size carb for 1000cc?
« Reply #3 on: May 12, 2018, 11:23:12 am »
You won't regret making the Weber carbs work. A superior carb to many others out there. For street use people really like Mikuni RS34.
A don't doubt that, but to get them to work everything has to made from scratch.
'78 CB750K http://forums.sohc4.net/index.php?topic=105327.0
'80 CB750C - Sold
'06 CBR600RR

Offline RAZZ

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Re: Which size carb for 1000cc?
« Reply #4 on: May 12, 2018, 12:13:02 pm »
Hi take a look at This Weber thread, it learned me a lot, they are time consuming to sync. but alsoluty worth it. Weber carb set-up
https://r.tapatalk.com/shareLink?share_fid=13548&share_tid=68065&url=http%3A%2F%2Fforums%2Esohc4%2Enet%2Findex%2Ephp%3Ftopic%3D68065&share_type=t



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Offline RAZZ

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Re: Which size carb for 1000cc?
« Reply #5 on: May 12, 2018, 12:16:54 pm »
Here some pics of my “Stock” setup


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Offline MRieck

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Re: Which size carb for 1000cc?
« Reply #6 on: May 12, 2018, 12:33:40 pm »
I run RS-34's with 1000cc. Work very well....tuned with an onboard A/F meter.
Owner of the "Million Dollar CB"

Offline RAZZ

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Re: Which size carb for 1000cc?
« Reply #7 on: May 12, 2018, 12:38:51 pm »
Here a set of RS34 for sale: CB750  special, rare, and go fast parts - REDUCED again~~
https://r.tapatalk.com/shareLink?share_fid=13548&share_tid=170441&url=http%3A%2F%2Fforums%2Esohc4%2Enet%2Findex%2Ephp%3Ftopic%3D170441&share_type=t


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Offline PeWe

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Re: Which size carb for 1000cc?
« Reply #8 on: May 12, 2018, 01:49:07 pm »
My CB750 K6 has 71.25 mm pistons. Carbs Mikuni TMR32 works really good but are not cheap.
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to stock K2

Offline scottly

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Re: Which size carb for 1000cc?
« Reply #9 on: May 12, 2018, 05:56:51 pm »
What problems are you having with the Webers?
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Offline MRieck

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Re: Which size carb for 1000cc?
« Reply #10 on: May 12, 2018, 06:17:08 pm »
My CB750 K6 has 71.25 mm pistons. Carbs Mikuni TMR32 works really good but are not cheap.
They are really nice carbs. ;)
Owner of the "Million Dollar CB"

Offline 754

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Re: Which size carb for 1000cc?
« Reply #11 on: May 12, 2018, 06:34:15 pm »
Show us what Webers and manifold you got plus anything you can hook the throttle cable to.
 Usually can be running in an afternoon, if they have been on a  running bike before.
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline scottly

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Re: Which size carb for 1000cc?
« Reply #12 on: May 12, 2018, 06:42:47 pm »
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Offline afkrejci90

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Re: Which size carb for 1000cc?
« Reply #13 on: May 12, 2018, 08:25:13 pm »
What problems are you having with the Webers?

The lack of a manifold.  Tried machining my own but ran into some problems due to inexperience.
'78 CB750K http://forums.sohc4.net/index.php?topic=105327.0
'80 CB750C - Sold
'06 CBR600RR

Offline 754

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Re: Which size carb for 1000cc?
« Reply #14 on: May 12, 2018, 08:52:51 pm »
Do you have a drawing ?
 If you look at the RC manifold closely..
 You can machine 4 tubes and press them into a plate.  Lictite them in.
 May have to leave the hole in the tubes undersize, then port it, hand finish to get a good matchup.  Some throttle bits you can make or maybe buy.  We can help you with the manifold, but it means a bit if lathe or mill work.
 
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline gschuld

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Re: Which size carb for 1000cc?
« Reply #15 on: May 12, 2018, 09:31:50 pm »
Rc manifold for reference

George


Offline PeWe

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Which size carb for 1000cc?
« Reply #17 on: May 13, 2018, 12:10:57 am »
My CB750 K6 has 71.25 mm pistons. Carbs Mikuni TMR32 works really good but are not cheap.
They are really nice carbs.
Mike, have you tried TMR’s on your CB750 yet? I guess you have a set....
I use CB750 throttle control with wires. I need to be very focused when taking off and riding slowly on first and sometimes second gear. Gear ratio is tough. Full throttle less than 1/4 turn. No sneezing allowed!!

RS34’s are easier to handle?
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to stock K2

Offline Medyo Bastos

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Re: Which size carb for 1000cc?
« Reply #18 on: May 13, 2018, 05:40:05 am »
i have a mpd manifold under the bed  ;D

Offline MRieck

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Re: Which size carb for 1000cc?
« Reply #19 on: May 13, 2018, 07:11:56 am »
My CB750 K6 has 71.25 mm pistons. Carbs Mikuni TMR32 works really good but are not cheap.
They are really nice carbs.
Mike, have you tried TMR’s on your CB750 yet? I guess you have a set....
I use CB750 throttle control with wires. I need to be very focused when taking off and riding slowly on first and sometimes second gear. Gear ratio is tough. Full throttle less than 1/4 turn. No sneezing allowed!!

RS34’s are easier to handle?
I have a set of 32's and 36's here but haven't used them Per. The RS's are pretty good especially using the 77/78K reproduction insulators from Vintage CB. Stock cables work great. I am surprised you were able to use the stock throttle cables for the TMR's. When I mocked it up it looked no good and when I bought the 36's different length cables were specified. Interesting.
 They only problem I encountered with the RS's turned out to be a defective float. #1#3 and #4 were burning nice but the A/F was wonky and #2 was rich. Played with height 3 or 4 times and fiddled with the idle screw/pilot jet etc. Finally bit the bullet and bought a new float and problem solved. The rack of 34's was brand new when I bought them. Go figure.
Owner of the "Million Dollar CB"

Offline PeWe

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Which size carb for 1000cc?
« Reply #20 on: May 13, 2018, 10:48:22 am »
I have photos in my computer home about the cable installation on the TMR’s.  I’m in Spain now close to Benidorm, so no garage nor riding. Sunshine, sea and beers are OK too.
I know that I had to add an extra nut I made a cut in by a saw blade for the wire to use the nut as adjuster or lock nut. Nut is open so not tight too hard, lock with Loctite. One cable is a little bit long, front adjuster at the switch/ throttle is in outest position. Maybe I can adjust at the carbs a little bit more.
Edit: Here photos when I installed them
http://forums.sohc4.net/index.php/topic,61168.msg1660522.html#msg1660522

Jetting sorted out since then
Pilots 30
Fuel screws 1 1/2 turns out (stable idle and less need of choke)
Needles 2:6
Mains 135 (more open pipes 137.5)
« Last Edit: May 23, 2018, 12:17:54 pm by PeWe »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to stock K2

Offline turboguzzi

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Re: Which size carb for 1000cc?
« Reply #21 on: May 13, 2018, 11:54:55 am »
you said race bike... if it's for AHRMA the only CR are accepted IMHO, would say 31

Offline Bill/BentON Racing

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Re: Which size carb for 1000cc?
« Reply #22 on: May 13, 2018, 07:18:45 pm »
I might need that, pic please
BentON Racing Website
OEM Parts | Service | Custom Builds
BentON Racing Facebook
Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
______________________________________
See our latest build 'Captain Marvel' CLICK HERE

Offline scottly

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Re: Which size carb for 1000cc?
« Reply #23 on: May 13, 2018, 09:43:58 pm »
I might need that, pic please
What is the "that" you might need, Bill??
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Offline Bill/BentON Racing

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Re: Which size carb for 1000cc?
« Reply #24 on: May 14, 2018, 03:29:56 am »
Lol, I thought I quoted Medyo,  I'm interested in the MPD Weber manifold.
BentON Racing Website
OEM Parts | Service | Custom Builds
BentON Racing Facebook
Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
______________________________________
See our latest build 'Captain Marvel' CLICK HERE

 

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