What is a constant velocity carb?
Email Mujahideen
The title says it all. After reading about them for years,
it finally occurred to me to ask, what is a CV carb, how does
it differ from non-CV carbs, etc?
--
Robert Allen, r...@sun.com DISCLAIMER: I said it, not my company.
"Traditionalists often study what is taught, not what there is to create."
- Ed Parker, Grandmaster, American Kenpo.
David A. Braun
> The title says it all. After reading about them for years,
> it finally occurred to me to ask, what is a CV carb, how does
> it differ from non-CV carbs, etc?
>
On a "normal" bike carb, twisting the throttle will yank up the throttle slide.
If you grab a BIG handful at idle, you can cause the pressure differential
between the upstream (airbox) and downstream (intake manifold) sides to
equalize to the point where no fuel is sucked up into the intake tract.
TWIST. Burp. Pafff. (How emBARassing!) Stalled motor.
Constant velocity carbs (as the name implies) TRY to maintain a constant
velocity of air thru the carb. When you twist the throttle, a butterfly
valve (close to the manifold) twists in the tract. A skillfully engineered
systems of passages in the carb allows the pressure differential to be
applied to a diaphragm to which the slide is attached. The pressure
differential lifts the slide. The velocity is maintained so...
TWIST. r rr rrr rrrr R RR RRRROAR! ZOOM!
Actually, if you lookit the slide of a CV carb while the engine is running,
the bugger is jumping up and down real fast.
p.s. If yo' bike has CV carbs and you tour, carry a spare diaphragm. They
are (~) cheap, very easy to pack, ~ easy to replace, and will spoil your
Sunday if you get a hole in one of them.
=============================================================================
"Watch this newsgroup for exciting tips and information."
The Nashville Flash - d...@vuse.vanderbilt.edu
Chuck Karish
> it finally occurred to me to ask, what is a CV carb, how does
> it differ from non-CV carbs, etc?
A CV carb is one in which the effective throttle opening is
controlled by a vacuum-actuated piston, rather than by a slide or
butterfly controlled directly by the throttle cable. Usually
the throtle cable controls a butterfly upstream of the slide.
Opening this butterfly changes the pressure differential
between the airbox and the carburetor throat, and the slide
adjusts itself accordingly.
You can tell a CV carb by its vacuum chamber. There's either
an inverted saucepan or a flat tin plate held down by four
screws on the top, holding either a close-fitting piston or a
piston connected to a rubber diaphragm. The close-fitting-
piston design is used by Keihin under license from SU (or
whoever owns SU, Zenith-Stromberg, etc. now). I think Mikuni
makes carbs of both types. SU automotive CV carburetors are
reported to work well on Harleys.
The advantage of a CV carburetor is that the rider can't open
the throttle too quickly and bog the engine; it can mediate
between rider demand for power and engine demand for fuel
mixture in the correct amount and fuel/air proportion.
--
Chuck Karish kar...@mindcraft.com
Mindcraft, Inc. (415) 323-9000
Mr. Bill
>
>Actually, if you lookit the slide of a CV carb while the engine is running,
>the bugger is jumping up and down real fast.
Yeah, definitely wild looking!
>p.s. If yo' bike has CV carbs and you tour, carry a spare diaphragm. They
>are (~) cheap,
I recently paid $45 for one for the Little Honda. I'm not sure I'd call
this very cheap (I like to cry poor a lot 8^). It's the only one I've
ever lost on a CV-carb, after about 88,000 total miles on motorcycles so
equipped (266,000 diaphragm-miles). What are other people's experiences
with longevity/durability of CV diaphragms?
>very easy to pack, ~ easy to replace, and will spoil your
>Sunday if you get a hole in one of them.
All quite true, especially the last... ;^(
Mr. {pestering the shop about my fuse block} Bill
--
Bill Leavitt | lea...@cs.umd.edu | uunet!mimsy!leavitt | AMA#313707 | DoD#0224
Toybox: '89 Probe, '82 GS850G, '76 CJ360T, '69 Impala ragtop, '68 Lone Star 15'
Standard Disclaimer: "Who? When? Me?? It was the booze!!" -- Michael Binkley --
Jonathan E. Quist
>Constant velocity carbs (as the name implies) TRY to maintain a constant
>velocity of air thru the carb. When you twist the throttle, a butterfly
>valve (close to the manifold) twists in the tract. A skillfully engineered
>systems of passages in the carb allows the pressure differential to be
>applied to a diaphragm to which the slide is attached. The pressure
>differential lifts the slide. The velocity is maintained so...
>TWIST. r rr rrr rrrr R RR RRRROAR! ZOOM!
>Actually, if you lookit the slide of a CV carb while the engine is running,
>the bugger is jumping up and down real fast.
Some CV carbs don't have diaphragms. The dashpot (okay, throttle slide
for non-Brit-car enthusiasts) piston has low pressure air ported to
the piston's upper surface; increasing airflow momentarily increases
air velocity, which decreases pressure, which draws the piston up,
which enlarges the carb opening, which reduces air velocity....
I haven't found it in my Keihnin's (sp?), but the SU carbs on my
MG have a damping mechanism to avoid dashpot oscillations. The
damping tube must be periodically refilled with engine oil.
The advantage, according to the Haynes MGB manual, of the CV carb is
that mixture is continuously adjusted for demand. Which brings up
the question: If you have CV carbs and change your air filters
or exhaust system, why should you need to re-jet, since the carb
is supposed to be compensating. (Or is this one of those questions
that should be posted under the subject "Test posting - please ignore"?)
>p.s. If yo' bike has CV carbs and you tour, carry a spare diaphragm. They
>are (~) cheap, very easy to pack, ~ easy to replace, and will spoil your
>Sunday if you get a hole in one of them.
Funny, my wife's gynecologist said almost exactly the same thing
about diaphragms!
--
Jonathan E. Quist INTERACTIVE Systems Corporation
j...@i88.isc.com Naperville, IL
'71 CL450-K4 "Gleep"
DoD #094, Deployed, Operation Dessert Storm (a Flurry of Puddings)
charles stannett
|> piston connected to a rubber diaphragm. The close-fitting-
|> piston design is used by Keihin under license from SU (or
|> whoever owns SU, Zenith-Stromberg, etc. now). I think Mikuni
|> makes carbs of both types. SU automotive CV carburetors are
When I worked for SU-Butec, it was owned by the then-named
British Leyland, who owned vehicle companies like Rover, Morris,
MG, Jaguar and Triumph (cars, not the bikes). I assume that SU is
still owned by the Rover Group, although they might be branded
as Unipart. And yes, a friend of mine -- who also used to work
for SU -- had an SU (I think an HS4) carb on his Sportster.
Trivia items:
1. I seem to recall that SU stood for Skinner Union,
the "Skinner" being a link with a shoe company called Lilley and
Skinner. It would appear that early carbs used a leather wicking
system to vapourize the fuel.
2. I'm a member of the MG Owners Club (as well as owning a Virago),
and they sell chromed dashpot covers for SU's.
3. Can someone e-mail info on "joining" DoD -- if from UK you can
post on the local Ogri e-mail list. (See August BIKE mag, p.11)
Charles Stannett
***************************************************************************
e-mail csta...@axion.bt.co.uk
telephone Ipswich (0473) 645794
Royal Mail Software Technology Division (SSTF G24c)
BT Laboratories, Martlesham Heath, IPSWICH, IP5 7RE, England
***************************************************************************
Chuck Karish
(Jonathan E. Quist) writes:
>The advantage, according to the Haynes MGB manual, of the CV carb is
>that mixture is continuously adjusted for demand. Which brings up
>the question: If you have CV carbs and change your air filters
>or exhaust system, why should you need to re-jet, since the carb
>is supposed to be compensating.
The problem is that the carburetor doesn't get any real
feedback from the engine as to whether the mixture is correct.
It has to assume that a particular pressure differential
between its intake bell and its throat corresponds to a
particular mass flow of air through the carb.
Consider two ways that this can go wrong:
- The air cleaner is less restrictive than expected, so the air
pressure in the airbox is higher than expected and there's
more mass moving through the throat at the same pressure
differential (air speed).
- Individual air cleaner elements are installed to replace the
airbox. Intake restriction is reduced dramatically and the
tuned length of the intake tract is changed. The reflected
pressure wave from the closing intake valve causes the
incoming charge to stop in its tracks at certain RPM, causing
a flat spot. (In fairness to the carburetor designers,
jetting probably can't cure this one.)
Alastair Young
>> The title says it all. After reading about them for years,
>> it finally occurred to me to ask, what is a CV carb, how does
>> it differ from non-CV carbs, etc?
>> Robert Allen, r...@sun.com DISCLAIMER: I said it, not my company.
>>
>
>On a "normal" bike carb, twisting the throttle will yank up the throttle slide.
>If you grab a BIG handful at idle, you can cause the pressure differential
>between the upstream (airbox) and downstream (intake manifold) sides to
>equalize to the point where no fuel is sucked up into the intake tract.
>TWIST. Burp. Pafff. (How emBARassing!) Stalled motor.
>
Amal Concentric Mk1.5
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Alastair Young Systems Supervisor (SMTS) _
European CAD Developments Limited aka EuCAD )/___
EuCAD/Cadence mail: alastair +44-31-225-3434 __/(___)#####c ARIEL NH
Full Email Address: alas...@eucad.co.uk / /\\|| \ / \ NVA927
9 Wemyss Place, Edinburgh, Scotland EH3 6DH \__/ ---- \__/~~~~~ 1956
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
--
Paul Thompson
>p.s. If yo' bike has CV carbs and you tour, carry a spare diaphragm. They
>are (~) cheap, very easy to pack, ~ easy to replace, and will spoil your
>Sunday if you get a hole in one of them.
If you get a hole, the diaphram won't open, and you'll have to limp around
on a fraction of the power your bike should be making. As to being cheap,
the diaphram for a little 250 Rebel costs $70 from Honda! (Ouch)
--
Paul Thompson Apple Computer DoD #47 Would you remember a 1-line .sig?
Laszlo Nemeth
|> d...@vuse.vanderbilt.edu (David A. Braun) writes:
|>
|> >p.s. If yo' bike has CV carbs and you tour, carry a spare diaphragm. They
|> >are (~) cheap, very easy to pack, ~ easy to replace, and will spoil your
|> >Sunday if you get a hole in one of them.
|>
|> If you get a hole, the diaphram won't open, and you'll have to limp around
|> on a fraction of the power your bike should be making. As to being cheap,
|> the diaphram for a little 250 Rebel costs $70 from Honda! (Ouch)
R75/5 bing diaphram cost $15 ;-)
Of course I haven't replaced mine recently so your $ may vary.
Laszlo Nemeth
las...@boulder.colorado.edu
"Nothing is as over rated as a bad f*ck or as under rated as a good sh*t."
"hey - my tool works (yeah, you can quote me on that)." From el...@Sun.COM
"Flashbacks = free drugs."
"Data for president in '92! Troy for Vice president!"
DoD #0666 UID #1999
Paul Blumstein
+
+The advantage of a CV carburetor is that the rider can't open
+the throttle too quickly and bog the engine; it can mediate
+between rider demand for power and engine demand for fuel
+mixture in the correct amount and fuel/air proportion.
Not to mention that it makes it easier for the manufacturer to meet
emission requirements with a CV carb.
The disadvantage of the CV carb is that it uses a mixture enrichener
instead of a choke. That requires more finesse to use so that you
don't foul your plugs.
============================================================================
Bureaucracies turn conservatives into anarchists
----------------------------------------------------------------------------
Paul D. Blumstein (DoD #36), Transaction Technology, Inc., Santa Monica, CA
{philabs,psivax,pyramid,rutgers}!ttidca!paulb or pa...@ttidca.TTI.COM
David Dawes
In article <27...@ttidca.TTI.COM> pa...@ttidca.TTI.COM (Paul Blumstein) writes:
>
>In article <6789919...@mindcraft.com> kar...@mindcraft.com (Chuck Karish) writes:
>+
>+The advantage of a CV carburetor is that the rider can't open
>+the throttle too quickly and bog the engine; it can mediate
>+between rider demand for power and engine demand for fuel
>+mixture in the correct amount and fuel/air proportion.
>
>instead of a choke.
So do non-CV slide carbs (Ie, the ones where the slide is controlled directly
by the throttle cable). The "Choke" lever opens an enrichening starter jet.
David
--
------------------------------------------------------------------------------
David Dawes (da...@physics.su.oz.au) DoD#210 | Phone: +61 2 692 2639
School of Physics, University of Sydney, Australia | Fax: +61 2 660 2903
------------------------------------------------------------------------------
Michael L. Sturdevant
writes:
|> In article <6789919...@mindcraft.com> kar...@mindcraft.com (Chuck
Karish) writes:
|> +
|> +The advantage of a CV carburetor is that the rider can't open
|> +the throttle too quickly and bog the engine; it can mediate
|> +between rider demand for power and engine demand for fuel
|> +mixture in the correct amount and fuel/air proportion.
|>
|> Not to mention that it makes it easier for the manufacturer to meet
|> emission requirements with a CV carb.
|>
|> The disadvantage of the CV carb is that it uses a mixture enrichener
|> instead of a choke. That requires more finesse to use so that you
|> don't foul your plugs.
|>
|>
Most slide carbs these days have a mixture enrichener instead of a
choke too. At least on bikes with Keihin or Mikuni carbs. Indeed, my '78
GS has enricheners.
Go fast. Take chances.
Mike S.
David A. Braun
>d...@vuse.vanderbilt.edu (David A. Braun) writes:
>
>>p.s. If yo' bike has CV carbs and you tour, carry a spare diaphragm. They
>>are (~) cheap, very easy to pack, ~ easy to replace, and will spoil your
>>Sunday if you get a hole in one of them.
>
>If you get a hole, the diaphram won't open, and you'll have to limp around
>on a fraction of the power your bike should be making. As to being cheap,
>the diaphram for a little 250 Rebel costs $70 from Honda! (Ouch)
>--
Diaphragms for BMW's (okay not the flat top carbs) are well under $20.
Last time I bought some, I think they were on sale for about $8.
Consider carrying a diaphragm versus carrying a "black box". No, they
are NOT related. But the point is that there are things that MAY be cost
efficient for you to carry wrt the "fuck factor." If your rubber donut
costs $70, don't carry one. Mine is cheap (and I own a BMW!) so I carry one.
A tube of tire patch glue can be bought for about $1. So, why is yours
all dried up? You never hear EXPERIENCED racers say, "I KNEW that part
was going to fail." If an experienced motorcyclist (with Zen) has an inkling
then he is prepared for the problem or else forstalls it.
Stuff I ALWAYS carry:
spare headlite bub (wrapped in foam, stored inside
headlite)
BMW (damn comprehensive) tool kit
stub of roll of 'lectric tape
~ 10' #14 automotive wire
patch kit with GOOD TUBE OF GLUE
a few spare nuts/bolts
spark plugs
Stuff I carry on weekend rides:
Subject of another posting... longer, ~ 1/2 tankbag
includes diaphragm, inner tube, plug wire... et al
Stuff I carry on tour:
Subject of yet another posting... muy largo.
Seems to me that the FAQ the new riders alway miss is: What should
I carry on my bike?
================================================================================
If I had the time to tell you, would you listen?
The Nashville Flash - d...@vuse.vanderbilt.edu - DoD # 412
Chuck Karish
>instead of a choke. That requires more finesse to use so that you
>don't foul your plugs.
The CV carbs on my bike have ordinary choke butterflies. Mikuni
carburetors of both the CV and directly-actuated-slide varieties
use an air circuit to enrich the starting mixture. This hasn't
caused plug fouling problems for the bikes on which I've used
them.
Dan Dunphy
sitting.
I've a friend who's a sociology prof, and he can't finish it
either.
---
--
__ ___
| \ / \ |\ | (My advice is probably worth what you paid
| | |___| | \ | for it!) IHMO should be assumed, lest I forget.
|___/ | | | \|
Don't forget I'm back here, folks!
Dan Dunphy
dun...@col.hp.com
DoD #0281
JAC...@yalevm.ycc.yale.edu
In article <1991Jul11.1...@icd.ab.com>, m...@icd.ab.com (Michael L.
Sturdevant) says:
>In article <27...@ttidca.TTI.COM>, pa...@ttidca.TTI.COM (Paul Blumstein)
>writes:
>|> In article <6789919...@mindcraft.com> kar...@mindcraft.com (Chuck
>Karish) writes:
>|>
>|> The disadvantage of the CV carb is that it uses a mixture enrichener
>|> instead of a choke. That requires more finesse to use so that you
>|> don't foul your plugs.
>|>
> Most slide carbs these days have a mixture enrichener instead of a
^^^^^^^^^^^
Isn't it "enricher?"
-Conrad J.
DoD #38
"Technical hack writer for the summer."
Andrew Nguyen
kar...@mindcraft.com (Chuck Karish) writes:
>A CV carb is one in which the effective throttle opening is
>controlled by a vacuum-actuated piston, rather than by a slide or
>butterfly controlled directly by the throttle cable.
To nit-pick a bit, in a CV carb, the butterfly valve _is_ directly
controlled by the twist grip. The butterfly valve varies the restriction
in the carb venturi, hence varying the pressure in the venturi, hence
varying the differential between said pressure and pressure in the
chamber above the slide/diaphram, hence varying the height of the slide.
--
Andy Nguyen / Team Paranoid ...!uunet!actnyc!aqn
(212) 696-3668 ...!uunet!panix!aqn
Andrew Nguyen
pa...@ttidca.TTI.COM (Paul Blumstein) writes:
>The disadvantage of the CV carb is that it uses a mixture enrichener
>instead of a choke.
da...@suphys.physics.su.OZ.AU (David Dawes) writes:
>So do non-CV slide carbs (Ie, the ones where the slide is controlled directly
>by the throttle cable). The "Choke" lever opens an enrichening starter jet.
Just to add to the confusion =B-), on the throttle-slide carb that I have
disassembled, the choke is a sliding plate that completely closes of the
carb's venturi, forcing the air thru the idle air circuit, thus speeding
up the airflow there, thus sucking in extra fuel thru the idle jet.