cb750 carb spacing

R

rhavindog

learning curve? what learning curve?
Hey guys ive got a question regarding carb spacing on the various years of cb750. Im having some problems with the box-o-carbs that came with my basket case project bike for the carboncb project. namely broken parts and of the dozen no matching casting numbers. so the question is does anyone happen to know the spacing of the various years?. im looking at putting later model (1980 ish) carbs on a 1975 sohc stock engine. any ideas or suggestions?
 
1979 and on changed the CB750 considerably. That was after Honda changed the 750 to DOHC. If yours is a SOHC, I think you would better off looking for a set of those carbs.
 
yes, I know, thats the usual reply. but this isnt going to be a stock rebuild so that answer doesnt help. the flow characteristics wont be an issue as both have similar volume, jetting can solve any minor issues. my question only concerns the fitment. how far apart are the intake ports? has anyone here done this? does it require fabrication of intake boots or can it be done with odd year boots?
I dont like to go the easy route, which is why im cutting up and redesigning my frame, hand molding my tank and seat from carbon fiber, bending a custom stainless "hidden" exhaust etc.
 
There is no good reason to use later model CV crabs in place of the early slide carbs and besides, they are really hard to jet correctly. There's a challenge and there's making work for the sake of it.

I do enough of that to recognize it. If you really want to change carbs, think about early GSXR or fuel injection. Get a set of GSXR600K1 throttle bodies and make a real difference. They are 38mm bore and are nicely spaced and easy to change. Get a Microsquirt and see what you come up with.

Hope you are a certified welder if you plan on modding the frame.
 
Thanks teaz, that helps a lot. the plan for this bike is pretty ambitious ill admit. I came from cars, this will be my first bike build. Im cutting the rear of the frame and dropping the top down 2 inches and angling it upwards. this will move the rear shock mount point just short of two inches but more importantly lower the seat in the frame as Im not a little guy at 6'4" and around 250ish. it will also give it the more modern look im after. a similar mod to what was done to the gulf oil cb550 one of the regulars here built. I will be tigging the frame if (and thats a big if) I can get my tig working properly. if not i will tack and take it to a pro.....im not as invincible as i was in my youth so no compromise in safety.
Ill have to look at injection. Ive built them before for cars with fabbed plenums and rails but never with multiple throttle bodies. sounds intriguing tho. thanks for the tip.
 
I'm interested in the spacing as well I am looking at going efi and I found several sets of gsxr throttle bodys for $50 a pice local and would like to use them as that is what I am using for my yamaha build

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"1975 ish" what is the budget for this project? If unlimited go for the FI setup. GIXER shit may be cool, but why go through all the shit it takes, The "80's" CV's won't be friendly at all without Sheldon from Big Bang to help you out... find you a perfectly fine set of Keihin round tops and with a few adjusts most likely you can bolt them up to a fresh set of rubbers and ride off... even with pods. I may even have a set ::)

As for welding... Unless you plan to detach the head stock or just plan to do some hobby mods, you don't need to be certified, you just need some skills and verification by someone before getting structural (if it breaks you crash) Teazer means well but sometimes the place gets anal. And my advice is likely frowned on by the class elite... Haaaaah!

:eek:
 
Errr. You do realize that the OP posted that back in 2011..... The thread resurrection question does take us back around the same loop though.

And as you can tell, I have no problem suggesting an outrageous solution to test how serious the OP is and to start people thinking outside that proverbial box. :)
 
Tune-A-Fish said:
"1975 ish" what is the budget for this project? If unlimited go for the FI setup. GIXER shit may be cool, but why go through all the shit it takes, The "80's" CV's won't be friendly at all without Sheldon from Big Bang to help you out... find you a perfectly fine set of Keihin round tops and with a few adjusts most likely you can bolt them up to a fresh set of rubbers and ride off... even with pods. I may even have a set ::)

As for welding... Unless you plan to detach the head stock or just plan to do some hobby mods, you don't need to be certified, you just need some skills and verification by someone before getting structural (if it breaks you crash) Teazer means well but sometimes the place gets anal. And my advice is likely frowned on by the class elite... Haaaaah!

:eek:
Click to expand...
I would use he microsquirt controller and the idea behind he gsxr throttle bodys is to use the tps and map. Then I would use a gm o2 sensor and coil I'm not sure what I will use for a oil temp sensor

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I talked quite a bit with a guy about putting a EFI set up on my bike. His, all said and done was about a grand. Not all that much more than a set of CR's.
 
Do you need a in tank pump and what battery does it take? I know the latest MX bikes run a super small battery but only a few have a starter and no lights.
 
Do enough of the duffers that build a "cafe bike" (you know, paint everything satin black and racing red highlights) want EFI? I developed a single throttle body and individual throttle body setup using a Delphi ECU. We even built a prototype COP ignition system that plugged into it. It used an external pump for a number of good reasons. The majority went to Harley guys as the "cafe" guys wanted retro.
 
What would one of those systems run me?

I wouldn't be opposed to doing another 750 just to run an EFI set up.
 
Would be intersting, might need some fancy engineered air box also, I doubt you can just toss a set of spray tubes on, map it and go racing. Other than reliability and to make it so little boys can tune dirt bikes with micro sd instead of fuel and jetting, some of the 450's lost HP switching to FI, but the sport bikes gained a ton with those air management systems under the tank.

Then again, I'm not a genius I just pay attention so as my thing says... If I'm wrong... Oh Dang.
 
Well, a box for one, would be easier than for four.

Wonder how easy it would be to make work like a truck, with a longer intake and a big filter..
 
I would be using a external pump and puting the ignition under control of the microsquirt. The bike is kick only right now and all lights have been switched to led other than the headlight which is a h4 automotive bulb so that should bring the power consumption to a more reasonable load I would then add a 7 or 8 position fuse block to run everything

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