Topic: Converting CB650sc CV carbs for use on CB750 project

[cafebob]

So I am trying to convert a set of 1985 CB650sc carbs for use on a CB750.  I have heard a rumor that they are almost a bolt on, and decided to find out for myself.  I received my set off of ebay today. (they cost more to ship then to buy!) 

Below are some pictures of how they fit on a 78 super sport head.  Believe it or not they went right on, almost like they were made to fit.  I used the super sport rubbers. Check it out.










I have tracked down the source of the rumor, and have a email in to see what, if any rejetting needs to be done.  I will update this thread as i get the motor in the bike.

Oh and for anyone who wants to get a set.  A set from a 1983-1985 CB650SC (Nighthawk) should work.

bob

[Jerry Rxman Griffin aka MuthaF'er]

Bob,

Whats the advantage here? I'm not familiar with those carbs.

Jerry

[Pinhead]

Hmm, VERY interesting! That means they should also fit on the '79 CB650! Keep us updated on advantages, pitfalls, the whole shebang!

[cafebob]

well the main reason im doing it is so i can use them in my turbo project.  I'm going to run the turbo to blow through them, and the CV carbs are nicely sealed, come with float vents ready for pressure signal, and they also have dyno jet kits ready for them.


But in general...
"Slide (CR) carburetors are lightweight and cheap. But have the disadvantage that at low RPM, WOT and under load the carburetor dont "see" enough air speed to mix the right air/fuel ratio in the venturi area.

CV-carburetors are relatively heavy and expensive. Their great advantage is to deliver allways an relative adequate air/fuel mix at any engine speed : A depression triggered device adjust the right air speed in the venturi area to perform the adequate mixing of fuel and air.

This all allows for a smooth throttle response for your  bike.

CV carbs are pretty much standard equipment on any moderen no FI bike.  I don't know of one that comes stock with CR carbs these days. 

Plus I just like to mess around, and try to see what I can make these old bike do. 

And to top it all off, they can be had for cheap.  I paid 10$ for mine!

bob

[Geeto67]

Factories use them not because you get the most performance out of them but because they are controlled by atmospheric pressure which allows them to "adapt" to enviornmental changes. pound for pound you can make more power with a conventional carb, but it wont always be as smooth. I can see the advantage in a turbo application where air fuel has to be really precise, but for a stock NA motor I don't see the advantage.

The overarching down side to CV carbs is that they have the worst throttle response. It used to be a more common mod on DOHC cb900fs to put the 750 carbs on them to increase throttle response. I have a cb750f and a 900f and with the stock cv carbs if you punch it sometimes you are actually waiting for the carbs to catch up with the throttle. Most of the DOHC guys can't wait to get rid of their stock cv carbs.

I would be interested to see how these stack up against stock cb750 carbs. Personally I think the 78 cb750F carbs are better in every way than those 650 cv junkers you have on there but it would be nice to know some stock numbers. 

[JLeather]

I'm very curious about how your blow-through setup turns out.  Any more progress on the motor side of things?  How are you planning to plump the turbo to the carbs?  Manifold design is much less important for your blow through than my draw-through system.

[squirley]

I have been looking into switching a cb550 over to cv's for the same reason, to enable a blow through turbo instead of a draw through.  Any idea on what may fit a 550? Just thought I'd ask.

[MRieck]

Factories use them not because you get the most performance out of them but because they are controlled by atmospheric pressure which allows them to "adapt" to enviornmental changes. pound for pound you can make more power with a conventional carb, but it wont always be as smooth. I can see the advantage in a turbo application where air fuel has to be really precise, but for a stock NA motor I don't see the advantage.

The overarching down side to CV carbs is that they have the worst throttle response. It used to be a more common mod on DOHC cb900fs to put the 750 carbs on them to increase throttle response. I have a cb750f and a 900f and with the stock cv carbs if you punch it sometimes you are actually waiting for the carbs to catch up with the throttle. Most of the DOHC guys can't wait to get rid of their stock cv carbs.

I would be interested to see how these stack up against stock cb750 carbs. Personally I think the 78 cb750F carbs are better in every way than those 650 cv junkers you have on there but it would be nice to know some stock numbers. 

GeeT
       Actually those CV's are light years ahead of those 900/750F carbs. Those things (900/750F) have the large, heavy aluminum slides. Sealing and slide action were often compromised especially on bikes with high mileage. The carbs pictured use a rubber diaphram attached to a slide. The good thing about them is engine vacuum dictates how much fuel goes into the engine. You really can't "ham fist" them like CR's or early CB carbs. I'm sure DynoJet still makes a carb kit for them with good needles too. I have gotten excellent throttle response and performance from later model CV's on many different bikes.

[cafebob]

Wanted to post an update on these carbs.  Bellow is a picture of them after soda blasting, rebuilding, and detailing.  These are the same set of carbs in the first pics.




Link to the whole gallery so you can see good before and after.

http://s239.photobucket.com/albums/ff137/cafebob/cb650sc%20carb%20project/

Right now I am running 40 slows and 122 mains with no air box.  Bike pulls very nice. I am building a intake for them now, so the jetting is sure to change once it is done.

[Caferacernoc]

Nice.

[Pinhead]

Indeed. I'd be interested in the effect on gas mileage...

[cafebob]

the bike they are is an 836 with a lowered CR, so my numbers probably wouldn't mean very much....

[Laminar]

If the throttle slides are pulled up by vacuum, won't the boost pressure push them back down?

[cafebob]

you have to give them boost signal, and use a rising rate fuel pressure regulator.  Basically you still get a vacuum in the plenum, relative to boost pressure.  CV carbs are considered ideal for blow through turbo application.  Lots of reasons why. 

[bert96]

 I have bought CV carbs from a cb700(nighthawk) to install on my K4,the only ''problem'' i see,and i think you will have the same,is to balance those carbs.I think i will have to make hole in the intake manifolds.

And Honda don't make diaphragms for these carbs anymore 


Bert

[bert96]

 I have bought CV carbs from a cb700(nighthawk) to install on my K4,the only ''problem'' i see,and i think you will have the same,is to balance those carbs.I think i will have to make hole in the intake manifolds.

And Honda don't make diaphragms for these carbs anymore  


Bert

[Soos]

Too bad those cb650 DOHC carbs aren't actually 32mm's.... From this site: www.bikez.com/motorcycles/honda_cb_650_sc_nighthawk_1983.php
they are listed as a actual 29mm venturi carbs.





l8r

[cafebob]

83 and 84 where 29, 85 are 32.  http://hondanighthawks.net/650.htm about 1/3 down.  Next time I take mine off, I will take a picture with my caliper in there.  They played games with where they measured the venturi on these bikes.  Get a 85 set.

As to balancing i have been experimenting with a new method.  I bench balance them using 2 different drill bit sizes and 1/8 throttle and 2/3 throttle open.  This get things close.  Then I let the bike idle for 10 mins, and use an infrared thermometer on the pipes, and fine tune the temps.  I can get the pipes to within 5 degree F. Then go ride around the block a few times, come back let it sit for a few mins, and re check temps.  Mine now stay withing 5-8 F.  You can get a very nice IR thermometer for about 20$ on sale (usually 50$) at harbor freight.

You need to pick the same spot on each pipe to be accurate.  Its probably not as good as using gauges, but it gets the bike very close.  Like I have been saying, this bike idles and rides better than any bike I have set up with the stock carbs.

[Soos]

Good to know!!
(and another 650 page to bookmark, thnx!!)


I've got a 32mm set of ZX600 CV carbs on my '79 cb650. What a difference in the upper RPM's!!
The throttle response at low RPM's is not what it is with piston slide carbs, but the trade off is worth it IMO.
Smoothness all the way.
Cannot "bog" the engine down....


The upsides WAY outweigh the downsides.


l8r


P.S. what throttle cables did you use?
The stocker 750 cables?
Inquiring minds wanna know!
Glad to see others opening new territory on carb options.

[cafebob]

The stock 750 cables work.  One works unmodified as the "puller", the other cable needs to have about 1.5 inch of cable housing removed to work as the pusher.  I have mine out to be remade for me as I kind of screwed it up removing the cable housing, and it binds now.  I haven't noticed any difference using just the puller.

Oh, you do need to bend the cable mount on the carb about 15 degrees off center so that the cable does not hit the frame.

My low end response got a little better when I put my prototype plenum on the bike.  I am going to try putting some restrictions in the pod filters and see if that helps improve the low end.


Smoothness is the right word, now doubt.

CB

[cougar]

Hello cafebob; I'm in the process of getting a Yamaha Seca 650 turbo bike with the intension of transferring at least the carb/turbo system over to a cb750 (SOHC). What would you think of this situation? Do you forsee any major problems with this project. Your thoughts and opinions are appreciated.   Thanks,   ...cougar...