1980 CB750K with Keihin CR31 Carbs *Problem*

B

Bspurge

New Member
I bought a stock 750 from my uncle a while ago and removed the original carbs, air box, and exhaust/headers. The bike ran fine when I did this but I wanted to put on performance carbs and K&N Pods. So after getting everything synced up the bike runs! My problem that I am having is when I give the bike a lot of throttle (0-100%) the bike bogs down. I can rev the bike up slowly and it works fine but not when you hit it really hard. Looking for ideas on what I could do to get the bike tuned correctly. Thanks guys!
 
You changed the carbs and now it doesn't work right...now there's a surprise. :P OK, I'm a jerk...sounds like you may be running a tad lean, try raising the needle jets or fitting larger mains.
 
on a stock motor the cr31s are so big you may never be able to give it all the throttle at once

you just create zero vacuum with no venturi velocity
 
Any slide carb has to be opened gradually otherwise the engine will just die.
31mm carbs are too big for a stock 1980 CB750 engine, 27 would be better.
 
julian.allard66 said:
Any slide carb has to be opened gradually otherwise the engine will just die.
31mm carbs are too big for a stock 1980 CB750 engine, 27 would be better.
Click to expand...
I'm surprised you suggest the cr27's because the websites all say cr31's are compatible with 80 cb750k... any links to someone who has successfully done the carb swap?

Sent from my SM-G935V using DO THE TON mobile app
 
julian.allard66 said:
Any slide carb without an accelerator pump has to be opened gradually otherwise the engine will just die.
31mm carbs are too big for a stock 1980 CB750 engine, 27 would be better.
Click to expand...
 
Bspurge said:
I'm surprised you suggest the cr27's because the websites all say cr31's are compatible with 80 cb750k... any links to someone who has successfully done the carb swap?

Sent from my SM-G935V using DO THE TON mobile app
Click to expand...
Nope, I was basing it on a 1985 GSXR750 that makes 100bhp has 29mm slide carbs, so I was guessing that a 70bhp 750 would need the next size down.

CR31s would be right for a race tuned motor, the race kit for the GSXR had 33mm carbs.
 
Accelerator pumps are tricky, there's a point where you're opening the throttle too quick for the engine but not quick enough to make the pump work. The Husky 610 with a Dellorto 40mm carb I used to own was bad for this, early GSXR750s it's barely noticeable on.
 
"compatible" means they will fit on and they do

now try and make them run right

not a plug and play item
 
julian.allard66 said:
Accelerator pumps are tricky, there's a point where you're opening the throttle too quick for the engine but not quick enough to make the pump work. The Husky 610 with a Dellorto 40mm carb I used to own was bad for this, early GSXR750s it's barely noticeable on.
Click to expand...
Not argument there, but on a thumper and the SOHC CB750s it makes a huge difference. Honda put them on XLs an Xrs post 78 and it made all the difference in the world. But more to this guys bike, I know the SOHCs ran 26mm rack. Without looking it up, I can't imagine the DOHC Ks would've used much bigger. Going that large is a huge jump.
 
cxman said:
"compatible" means they will fit on and they do

now try and make them run right

not a plug and play item
Click to expand...

It's like the 30mm carb kits for the cb350. Sure... They'll mount up and you can get the bike to run. Good 'effin luck actually getting it to run right though.
 
I had a look in a UK magazine I had handy, says 30mm CV carbs, so 31mm slides would flow a lot more air, they can probably me made to work, but I don't think you'd get much mid range power.
 
29s are a pain to get working on a stock motor

normally you can drop down 2-4 mm when going from a cv to a smooth bore flat slide

32s wont be happy thru the whole range driveability will suffer
 
It's already been said. Over carb'd.
CR29's are pretty common on 750's
I've got CR26's on my ported high comp cammed 550 and they're great.
 
I have an excellent solution to your problem.

I will clean and bench sync a set of round tops to SOHC 750 spec with known jetting using factory parts supply a new set of UNI filters, all new clamps and even toss in a fresh set of cables.... all you have to do is ship me the CR's after you install the round tops.

Deal?
 
With about 13:1 compression, porting and a cam they work great. A gsxr has a totally different engine design. Apples and cucumbers.
 
Tune-A-Fish said:
I have an excellent solution to your problem.

I will clean and bench sync a set of round tops to SOHC 750 spec with known jetting using factory parts supply a new set of UNI filters, all new clamps and even toss in a fresh set of cables.... all you have to do is ship me the CR's after you install the round tops.

Deal?
Click to expand...

Brilliant. Wish I'd thought of that. :-)
 
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