[tjurek51]

Here's the base for this project; a 1980 Honda CB750F Supersport.



it has 13,400 miles on it. was restored two owners ago and generally a great bike. I've been riding it now for about 5 years. and yes, I don't put too many miles on it each season.

For those who followed my previous build of a 1982 CB900C, you know I pretty much did everything NOT by the book. That project was a budget build and ultimately turned out ok. it was a great learning experience even though I had to take alot of sh!t, er...heart felt criticisms for the things I did.

For this project, I plan to do things much differently. First off, please note this will NOT be a budget build. I intend to make some pretty cool modifications to the engine and overall look and feel to bring this to a much different place. Still planning it out, so stay tuned, if you're interested. if you have any ideas or recommendations, please let me know.

PS. another poster to a different CB750F build said this, "...I just wish people would stop ruining motorcycles in the name of art. If you are going to tear into a perfectly good motorcycle, do it to make it go faster." To be clear, making it go faster is exactly what i'm going to do!

 

[jcw]

I'm too lazy to search. Can you post a pic how the last bike turned out?

Nevermind. I remember... the carb tuning thread. How's it running???

Here's the base for this project; a 1980 Honda CB750F Supersport.

View attachment 95989

it has 13,400 miles on it. was restored two owners ago and generally a great bike. I've been riding it now for about 5 years. and yes, I don't put too many miles on it each season.

For those who followed my previous build of a 1982 CB900C, you know I pretty much did everything NOT by the book. That project was a budget build and ultimately turned out ok. it was a great learning experience even though I had to take alot of sh!t, er...heart felt criticisms for the things I did.

For this project, I plan to do things much differently. First off, please note this will NOT be a budget build. I intend to make some pretty cool modifications to the engine and overall look and feel to bring this to a much different place. Still planning it out, so stay tuned, if you're interested. if you have any ideas or recommendations, please let me know.

PS. another poster to a different CB750F build said this, "...I just wish people would stop ruining motorcycles in the name of art. If you are going to tear into a perfectly good motorcycle, do it to make it go faster." To be clear, making it go faster is exactly what i'm going to do!

 

[brad black]

methinks turned out ok is a fairly flexible figure of speech.

 

[tjurek51]

The CB 900 project carbs required a larger primary jet. It seems to be running well now. if you consider stable at 110 mph, with power to spare running well. I guess I do...

 

[tjurek51]

First question for the group; I've seen inline 4 carbs swapped out for a 2 carb system, with aluminum intake manifolds, etc. what's the pro's and cons of this, from a performance and efficiency standpoint? better or worse than the inline 4 carbs?

 

[TrialsRider]

Pretty sure ANY number of carburetors can work, the goal is to achieve the optimum fuel to air ratio at all times, provide effective atomization of the liquids with the air and the rest of the system is designed to flow that fuel air mixture into the combustion chamber effectively. One carb per cylinder works pretty good at that task, one carb and a supercharger for a whole bank of cylinders works pretty good too! ymmv.

In the case of your bike, imho you have 4 of what I would consider to be marginal CV type carburetors, those are the epitome of carburetor evolution and complexity to approach the low pollution emissions your bike is suppose to be spewing, they sorta worked pretty good when they were brand new and fuel injection was not an option. All you guys are doing when you refit over-sized jets and free flow pods & exhausts on the stock CV carbs is reversing the pollution control aspects the thing was designed for <- to economically manufacture a motorcycle that runs as lean as physically possible throughout its operating range. :| a street bikes operating range is not wide full open throttle, it's not constant idle and it's absolutely Not blipping a non-loaded engine, it is good even throttle response all the way from idle to top speed in any given gear.

Does your bike currently give you good even throttle response all the way from idle to top speed in any given gear and does it return to mid-range or idle nicely, or does it hesitate, lag, pop and fart all over the place?

 

[woodsman]

https://rocket-garage.blogspot.com/2017/02/freddie-spencer-cb750f-superbike-tribute.html

 

[CaTacL1sm]

Here:

I pulled prices from Dynoman in Texas because they're a good business that I've used before.


DYNOMAN PERFORMANCE - Motorcycle Performance Parts

Stage 3 *750**Package:*

-*Wiseco K985*Piston Kit (890cc)
- DP*Cylinder Sleeve*set
-*WebCam #110*Hardweld Camshafts**
- DP EP1 Extra Plate*Clutch Kit
- KPMI Racing*Valve Springs
- KPMI Shim Under*Cam Follower*set
- KPMI*Titanium Retainer*set
- 2x DP 9.48mm*Valve Shim Kit
-*Viton*Valve Guide Seals
- CFM-20*Head Gasket
- Viton Coated RCS*Base Gasket

$2500

DYNOMAN PERFORMANCE - Motorcycle Performance Parts

CR carbs (with the big bore kit I'm not sure what size you'd want)

$1200

DYNOMAN PERFORMANCE - Motorcycle Performance Parts

Kerker exhaust

$1100

So at least $5k worth of parts for the motor, and that's not including machine work (you'd be crazy not to include a port/polish etc while you're in there).

A decent suspension setup front and rear is between $700 to $1500

If you do it right, it will cost a lot, but it will be amazing

 

[TrialsRider]

... but it will be amazing

at the drag strip.

 

[CaTacL1sm]

With a proper suspension and frame bracing, why wouldn't it perform on the track?

 

[Teazer]

First question for the group; I've seen inline 4 carbs swapped out for a 2 carb system, with aluminum intake manifolds, etc. what's the pro's and cons of this, from a performance and efficiency standpoint? better or worse than the inline 4 carbs?

I believe that they come form Murray's Carbs and supposedly work very well on the street. Murray knows more about carbs than most and I suspect that the kit is better than most will achieve with pods and jets in OEM carbs. I would guess that CR carbs would make more power, but they were never designed to be used on a street bike. That doesn't mean they won't work, but like most things in life will be a compromise.

That said, Mr Honda and his friends at Keihin managed to make modified OEM carbs work on the RCB endurance racers. The OEM carbs are not easy to get right, especially with an aftermarket exhaust without a lot of dyno time.

Here's the link: https://murrayscarbs.com/product/cb750-cb900-cb10001100-dohc-mikuni-carb-system/

https://murrayscarbs.com/product/cb750-mikuni-carb-system/

For a full on Freddie Replica, CR crabs are the biz, but for the rest of us, maybe that's more than we need. Contact Murray and ask him.

 

[CaTacL1sm]

Youd think Murray would use a less crusty bike to show off his product (referring to the first link). That picture makes it look like something someone cobbled together on DTT

 

[TrialsRider]

With a proper suspension and frame bracing, why wouldn't it perform on the track?

It would perform scary fast if you couldn't also put track rubber on it. imho <- & for me the bike would still be too massive to comfortably throw around a track ymmv.


... it looks agricultural

I thought he might have seen a pair of 2 barrel weber carbs on one :/ that's been done before too.

 

[TrialsRider]

...
That said, Mr Honda and his friends at Keihin managed to make modified OEM carbs work on the RCB endurance racers.....

& Did they keep the air box or go with felt pod filters? lol

 

[jcw]

If you do it right, it will cost a lot, but it will be amazing

Problem with building these old bikes is that you spend a lot of time and money, and some decent 17yo rider on a used craigslist 600 blows your doors off.

At least they look way cool.

 

[Teazer]

& Did they keep the air box or go with felt pod filters? lol

Neither. Open velocity stacks IIRC.

Way back in the day the late Leon Moss of LEDAR who was a bit of a motor and carb expert, tried to get a CB900 to run with an aftermarket exhaust and he ended up with external tubes to equalize pressure in the carbs. I don't remember all the details but I think it went from rich to lean and back again as the throttle was opened.

I have no idea if that was all one big pressure differential issue or an exhaust resonance pressure wave timing thing, I have no idea.

That does raise a question as to how owners of cars with SU carbs (more or less the same design) get around those same flow/pressure wave issues.

 

[CaTacL1sm]

Problem with building these old bikes is that you spend a lot of time and money, and some decent 17yo rider on a used craigslist 600 blows your doors off.

At least they look way cool.

True, but pouring money into an old deathtrap to make it slightly better can be so satisfying.

 

[jcw]

I'd love to know what framecrafters did for this chassis.
Tires are clowny. But suspension looks top notch.
I'd ride the shit out of it...

Framecrafters Project Showcase - Bikes and Frames


I would brace the frame (doesn't have to complicated, some well placed proven ones), then figure out how to fit modern rubber in decent sizes to the front and back. 110, 120 front, 150-170 rear. That likely means a fork swap but not necessarily USD. That way you get bigger rubber up front and better brakes, too. Set the geomtery, brace the swingarm and space for the larger rear wheel. Quality bushing for the swingarm, bearings for the steering head.
Update the fork internals if possible, upgrade to nice rear twin suspenders.

Ride it for a while until you get bored of the stock power, spend more money on engine mods. Those are pretty well sorted...

 

[tjurek51]

Trials, I'd have to say no, my current carbs do not give me nice even throttle. idle to mid is a bit of a sputter. WOT gets me to about 90 mph tops and seems solid. though I had one time when suddenly had loss of power going freeway speeds. pulled over, shut it down, waited a minute, restarted it and it's run fine ever since. I've not touched the carbs since I bought it, other than keeping clean, high proof petrol in it and stabilizer in storage.

i'm definitely moving to the flat side CR carbs for the rebuild. want to move to pods and the CVs are shit for that, from my experience. tried it on the 900 project and couldn't get it tuned. I've not worked with the CRs but my understanding is they're better designed for PODs.

but appreciate the thoughts. will stick to the 4 carb set up. I've seen the aluminum billet intake manifolds for the CB750. are they worth it or should I stick to rubber?

 

[tjurek51]

couple of chicken scratch ideas. First Conceptual look;