The constant velocity carburetor

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mikenixon
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The constant velocity carburetor

#1

Post by mikenixon »

Constant velocity. Everyone knows that's what they're are called, but what does the name mean and why were these carburetors developed? What makes them so special?

The earliest motorcycle carburetors were a straight-through design much like a car carburetor. A throttle butterfly controlled airflow and thus engine rpm. This technology prevailed for a very long time but its drawback was that every time the throttle position changed, air pressure inside the carb's bore changed. Since this pressure independently determines fuel discharge into the airstream, fuel mixture was delivered erratically. When the throttle was opened the carb went lean momentarily, and when it was closed richness initially resulted. The carb "bogged" a lot. Because of this, the straight-through or "fixed venturi" carb was most successful on lower rpm engines which didn't require very much throttle and large engines whose many cylinders' vacuum made up for the oscillating pressure. But a high performance carburetor ideal for one-carb-per-cylinder applications it decidedly was not. And never could be.

At some point the throttle butterfly was removed from this carburetor and a slide substituted. In the exact center of the carb's bore, the slide actually adjusted bore size at the same time it quantified airflow. This resulted in greatly reduced pressure changes with each throttle movement because the carb bore never was bigger than needed. This "variable venturi" carburetor was extremely successful and is the basis for most high performance carburetors even today.

Eventually a further refinement was to bring the butterfly back and simply add it to the variable venturi carb. The result was a carb that was half fixed venturi and half variable venturi, literally and functionally; a fixed venturi carb for the beginning of its operation and afterward a variable venturi carb. Since vacuum was needed to raise the slide, it would rise only when there was an abundance of pressure and thus good fuel discharge was ensured. The slide not only rises in response to pressure, at the same time the slide normalizes or steadies that pressure. The constant velocity carburetor, so named because the combination of butterfly and slide together provided the least possible changes in venturi pressure, offered the most precise and satisfying throttle response of any of the three carburetor designs. Fuel was instantly there, ready to enter the airstream. No bogs. CV-equipped machines are so smooth that Honda actually used CVs when racing on rainy tracks. With a lot of development in fuel circuitry, the final early 1990s generation of CV carburetors are so precise and responsive they are nearly indistinguishable from fuel injection.

With no mechanical connection between twistgrip and slide, it is not a given that there will be identical motion between the two, and in fact anything can happen in a CV carb. And does. On the road when in top gear and with the engine working against a constant load such as a long, gradual upgrade, the slide rises the closest to linear it ever will, but not completely. It is still out of lockstep an appreciable amount. Moreover, with minimal load, whether in neutral or cruising sedately, the slide's movement is nowhere near that of the throttle. In fact, in the latter state it is typical for there to be more than a quarter throttle before the slide even starts to lift, and afterward it lifts far behind the rate of throttle opening. This is why many people find CVs hard to rejet. Percentages of throttle opening and rpm are just not relevant. A long time ago one man knew this better than virtually anyone, and as a result he became an icon in the powersports industry. Mark Dobek created Dynojet, the company whose name was a natural for a business begun with just a dyno and an exhaust gas analyzer.

Celebrate the constant velocity carburetor. Appreciate where it came from, the vehicles that have benefitted from it. Respect its engineers and developers and consider the dedication, understanding and foresight they have contributed. Disregard the occasional, unreasonable negativity and champion a job well done! For far more than just most of us, the CV is all the carb we'll ever need, and much more than we could rightly expect.
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Re: The constant velocity carburetor

#2

Post by Rat »

Good stuff as usual ....

Gord action1
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Re: The constant velocity carburetor

#3

Post by ericheath »

I had a 1.3 L Mitsubishi in the Philippines which had a single carb with a large vacuum slide. It would run well with revs up, but slow if they dropped, but very smooth. I’d never heard of a slide on a car.

We have a fair amount of folks who have switched to a single carb. When talking of air mixture speed, I can’t help but envision that intake valve closing and the charge getting halted. In our cv carbs, this has to affect what’s happening in the individual carb Venturi and slide. One would think the slide would want to close.

This happens on a single carb as well, but would there be an advantage by having the carb right in the center of all four runners? (Like the Goldwings are)There would always be a little more than one intake valve open at all times and therefore the column of air through the single carb venturi would be constant. Each reversion pulse would go to the plenum and be redirected into, or at least towards, whichever cylinder(s) is drawing it, rather than back through the carb Venturi.

Hats off to the engineers who designed our carbs.
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Re: The constant velocity carburetor

#4

Post by 5speed »

I had a 72 volvo 145 wagon with dual su carbs that had the sliding venturi.
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Re: The constant velocity carburetor

#5

Post by Bugdaddy66 »

In some old carburetor manuals, these are referred to as Constant Depression instead of constant velocity. I always felt this was more accurate as it is the vacuum (depression) signal that is being stabilized more than the air velocity. Just random musings!
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Re: The constant velocity carburetor

#6

Post by mikenixon »

Bugdaddy66 wrote:In some old carburetor manuals, these are referred to as Constant Depression instead of constant velocity. I always felt this was more accurate as it is the vacuum (depression) signal that is being stabilized more than the air velocity. Just random musings!
Yes, agreed. The signal is more constant.
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Re: The constant velocity carburetor

#7

Post by 05c50 »

Thanks for the information, I always enjoy your articles. Your mention of "variable venturi" brings back many memories....most of them bad, of the Ford Variable Venturi Carb first sold on the 302 V8 LTDS in 1979. The carb was a very smooth operating piece and I'm sure it was developed to try to meet the oncoming emission standards. I was working for a Ford dealer at the time and remember it well. As long as it was running ok, everything was good, but when problems occured the mechanics were completely confused. First off, when the air cleaner was removed, the movable venturi valve was exposed and to many it resembled a choke that was stuck closed........and then the trouble started. ;) :lol:

.........Paul
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Re: The constant velocity carburetor

#8

Post by mikenixon »

05c50 wrote:Thanks for the information, I always enjoy your articles. Your mention of "variable venturi" brings back many memories....most of them bad, of the Ford Variable Venturi Carb first sold on the 302 V8 LTDS in 1979. The carb was a very smooth operating piece and I'm sure it was developed to try to meet the oncoming emission standards. I was working for a Ford dealer at the time and remember it well. As long as it was running ok, everything was good, but when problems occured the mechanics were completely confused. First off, when the air cleaner was removed, the movable venturi valve was exposed and to many it resembled a choke that was stuck closed........and then the trouble started. ;) :lol:

.........Paul
Wow. Another car application I was unaware of. The collective experience on this forum is amazing. :)
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Re: The constant velocity carburetor

#9

Post by Rat »

We sorta cheat .... we have a LOT of older gentlemen ....

Gord :-D
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Re: The constant velocity carburetor

#10

Post by salukispeed »

RAT wrote:We sorta cheat .... we have a LOT of older gentlemen ....

Gord :-D
Go easy, I resemble that remark. But not so sure about gentleman application.
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Re: The constant velocity carburetor

#11

Post by taunusrainer »

05c50 wrote:Thanks for the information, I always enjoy your articles. Your mention of "variable venturi" brings back many memories....most of them bad, of the Ford Variable Venturi Carb first sold on the 302 V8 LTDS in 1979. The carb was a very smooth operating piece and I'm sure it was developed to try to meet the oncoming emission standards. I was working for a Ford dealer at the time and remember it well. As long as it was running ok, everything was good, but when problems occured the mechanics were completely confused. First off, when the air cleaner was removed, the movable venturi valve was exposed and to many it resembled a choke that was stuck closed........and then the trouble started. ;) :lol:

.........Paul
1000% agreed to this! The Ford VV was a masterpiece, mainly the dual VV for the V8. Here in Europe they sold a single VV and it suffered from the same fate: The mechanics were not properly trained to it by a comprehensive Ford educational programme. After the first slight trouble, mainly with the cold start unit, people brought it to the dealer and got it back completely messed up. They ended up getting a free conversion to a Weber or Solex carb, which was less effective but easier to service. 1986 Ford switched to EFI.
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Re: The constant velocity carburetor

#12

Post by 05c50 »

RAT wrote:We sorta cheat .... we have a LOT of older gentlemen ....

Gord :-D
I'm not really that old, I was only 7 when I worked at the Ford dealer. ;)

...........Paul
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Re: The constant velocity carburetor

#13

Post by 05c50 »

taunusrainer wrote:
05c50 wrote:Thanks for the information, I always enjoy your articles. Your mention of "variable venturi" brings back many memories....most of them bad, of the Ford Variable Venturi Carb first sold on the 302 V8 LTDS in 1979. The carb was a very smooth operating piece and I'm sure it was developed to try to meet the oncoming emission standards. I was working for a Ford dealer at the time and remember it well. As long as it was running ok, everything was good, but when problems occured the mechanics were completely confused. First off, when the air cleaner was removed, the movable venturi valve was exposed and to many it resembled a choke that was stuck closed........and then the trouble started. ;) :lol:

.........Paul
1000% agreed to this! The Ford VV was a masterpiece, mainly the dual VV for the V8. Here in Europe they sold a single VV and it suffered from the same fate: The mechanics were not properly trained to it by a comprehensive Ford educational programme. After the first slight trouble, mainly with the cold start unit, people brought it to the dealer and got it back completely messed up. They ended up getting a free conversion to a Weber or Solex carb, which was less effective but easier to service. 1986 Ford switched to EFI.
As luck would have it, I eventually became an instructor for the Ford VV carb repair. After a few years experience with it, I came to understand it and tried to convince my "students" that it was truly a fine piece of engineering and would operate as smooth as any carb out there and was the closest thing you could get to fuel injection. The second generation VV had a feed back actuator installed and when adjusted properly, the air/fuel mixture responded directly to the engine load instead of just throttle plate movement. Probably the reason most mechanics disliked (hated) it was other than idle speed, all adjustments required removal from the engine and the use of a dial indicator to properly set the CER (cold enrichment), CVR (control vacuum regulator) and accelerator pump. It probably would have been used longer, but electronic fuel injection was just around the corner and soon took the place of almost all the carburetors.

........Paul
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Re: The constant velocity carburetor

#14

Post by mikenixon »

05c50 wrote:
taunusrainer wrote:
05c50 wrote:Thanks for the information, I always enjoy your articles. Your mention of "variable venturi" brings back many memories....most of them bad, of the Ford Variable Venturi Carb first sold on the 302 V8 LTDS in 1979. The carb was a very smooth operating piece and I'm sure it was developed to try to meet the oncoming emission standards. I was working for a Ford dealer at the time and remember it well. As long as it was running ok, everything was good, but when problems occured the mechanics were completely confused. First off, when the air cleaner was removed, the movable venturi valve was exposed and to many it resembled a choke that was stuck closed........and then the trouble started. ;) :lol:

.........Paul
1000% agreed to this! The Ford VV was a masterpiece, mainly the dual VV for the V8. Here in Europe they sold a single VV and it suffered from the same fate: The mechanics were not properly trained to it by a comprehensive Ford educational programme. After the first slight trouble, mainly with the cold start unit, people brought it to the dealer and got it back completely messed up. They ended up getting a free conversion to a Weber or Solex carb, which was less effective but easier to service. 1986 Ford switched to EFI.
As luck would have it, I eventually became an instructor for the Ford VV carb repair. After a few years experience with it, I came to understand it and tried to convince my "students" that it was truly a fine piece of engineering and would operate as smooth as any carb out there and was the closest thing you could get to fuel injection. The second generation VV had a feed back actuator installed and when adjusted properly, the air/fuel mixture responded directly to the engine load instead of just throttle plate movement. Probably the reason most mechanics disliked (hated) it was other than idle speed, all adjustments required removal from the engine and the use of a dial indicator to properly set the CER (cold enrichment), CVR (control vacuum regulator) and accelerator pump. It probably would have been used longer, but electronic fuel injection was just around the corner and soon took the place of almost all the carburetors.

........Paul
Wow! Another example of earned wisdom that was likely lost when you quit that field, eh? Cool.
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Re: The constant velocity carburetor

#15

Post by JSBail »

Anyone know of the Predator VV carb? I think they first appeared in the early 80's and were race only carbs capable of 930 cfm, they did have a "street" version which was actually the same carb except it had an idle circuit while the race version didn't. I had one for a bit, even tried it (briefly) on a bone stock 78 Pontiac T/A I had at the time, I say briefly because being mine was the race carb with no idle circuit it had a very rich A/F ratio and fouled my plugs out after only a few miles down the road although it screamed at WOT. I think the biggest reason they never really became popular was because it was a tall carb and you may have hood clearance issues because of it. I now when I had it on my T/A I couldn't put the factory air cleaner on it let alone the shaker hood scoop. EFI was just around the corner as well and I'm not sure they're still available now though parts to rebuild one are still available.
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