Topic: Air Cutoff Valves

[Frankenkit]

Wondering whose bikes have these, and if they have noticed a major difference in carb performance between carbs that have these and carbs that don't?

http://www.cyclepages.com/ProductGroupDisp.aspx?ProductGroupID=4001251

They come out the side under a plate.

[Soos]

Well.....


I have not run a set of carbs on my cb650 that has these for a while now(over a year).


CB750 carbs don't have them, and I liked them better than the stocker cb650 ('79/80) carbs. they have neither the air cutoff diaphragms or accel pump.
I am currently running a set of CV carbs off a kawasaki(600cc ninja).... No cutoff diaphragms on them either... or an accel pump for that matter.


And difference....
stocker carbs   - 26mm venturi mechanical linkage
750 carbs        - 28mm venturi mechanical linkage
kawasaki carbs - 32mm venturi CV carbs


So comparisons are somewhat skewed..... and I REALLY like the kawa carbs I have been running.
CV has the advantage of being able to go WOT and not kill the motor.
These carbs barely fit the '79/80 heads and is a MUST you use the carb isolators that go with those carbs... but they drop in as a direct replacement for the CV cb650 carbs.
(not too sure on the airbox side for a CV cb650 motor though, as my cb650 has the older style head and airbox)


However compared to the stockers, the 750 carbs also have many advantages.
better throttle response, easier to change jets, and they space perfectly with the '79/80 cb650 heads.
They also have the choke linkage ON the carbs.... no cable as well as problem free when set up properly as opposed to the '79/80 cb650 carbs with the little black plastic part on the carbs for the choke that is prone to break with a too hearty pull on the choke cable.



Supposedly(i'm no engineer) the air cutoff diaphragms are necessary on the cb650 carbs (CV and non CV***MY BAD, JUST RE-CHECKED< NOT ON THE CV CARBS*******) due to the lean carb design.
They help reduce the lean mixture caused when decelerating with the throttle butterfly closed fully.

I have read about someone bypassing these internal passages and compensating for the lean mixture on deceleration by richening the idle air/gas mixture.
Not too sure how effective this is though.
Considering these cb650's run a tad lean to begin with, doing this sounds absurd to me.
The risk/benifits are not equal IMO.


There are carb options out there, just few and far between.



l8r

**edited for correction on the CV carbs**

[Soos]

As fart as performance....



I think they cb750 carbs are a step up, especially with a ported head.

The ninja carbs, well I have nothing to say bad about them. As far as short hop riding, they are better than the cb750 carbs.(better in stop and go driving)
With correct throttle manipulation however, the cb750 carbs are just as good.


Fuel milage was best with the cb650 carbs.
Hit a hair over 60mpg once.
Averaged 45mpg and higher.
Depending on riding habits for that tankfull...




With the 750 carbs, a bit lower, but averaged in the mid to low 40mpg range.

The kawasaki carbs, well.....
with a LOT of high RPM riding, mid to high 30 mpg.
Keeping it within sane limits most of the time, pretty much the same as the 750 carbs.



l8r

[CBJoe]

I have read about someone bypassing these internal passages and compensating for the lean mixture on deceleration by richening the idle air/gas mixture.
Not too sure how effective this is though.
Considering these cb650's run a tad lean to begin with, doing this sounds absurd to me.
The risk/benifits are not equal IMO.

I tried this, but the bike ran worse without than with.  I forked out the dough for the air cutoff kits but I haven't had a chance to rebuild and see how they help. 

Joe

[Frankenkit]

yeah, I just 'fixed' my pinholes with napa's 'plastidip' so we'll see if that does anything... what's the part number on the 750 carbs, just in case I happen to see some... and what jets did you have to run?

[MikeB]

I don't remember seeing these on mine (82 650SC ) 

Hope there not suppose to be there,don't know about running yet, but I did get it started, adjusted the idle, and got good throttle response

[Soos]

yeah, I just 'fixed' my pinholes with napa's 'plastidip' so we'll see if that does anything... what's the part number on the 750 carbs, just in case I happen to see some... and what jets did you have to run?

lemme go check.......


107.5's, but take into account my elevation is nearly 1 mile high.(my rides in the canyons run as high as 9000 ft)
That setup was with my stocker airbox. For me it worked like a charm.

From what I have read every 2000 ft in elevation needs 1 size leaner jet than at sea level.


Paulages might be a better candidate for asking what jets to use with 750 carbs on a 650.
Possibly a size or 3 lower than his due to the smaller bore your cb650 would have?
His 550/650 hybrid is bored to 718cc's, and has a lumpy cam.
I think he is near the upper 120's or low 130's now.



My carb #'s are 7A. Bought them advertised as '73cb750 carbs.
Went for under 25.00 WITH shipping. with the kits, I think I was still in them under 60.00 TOTAL bill.




l8r

[Soos]

I don't remember seeing these on mine (82 650SC ) 

Hope there not suppose to be there,don't know about running yet, but I did get it started, adjusted the idle, and got good throttle response

OOPS!!!
I just re-checked, and they only have a accel diaphragm, no air cotoff diaphragms!!

MY bad.




l8r