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Old 05-31-2010, 04:49 AM   #16
Beemerboff
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Speed , and suction , is most everything.

These pictures are OK, but they just dont mention speed, and therefore suction and flow.

And understanding that is the key to understanding and tuning Constant Velocity carbs.

If you read most tests of FI bikes the tester notes problems with fueling at low revs, but if it was simple mixture problem it would be easily sorted.

The problem is maintaining air flow speed without a CV device somewhere in the system - Hondas new VFR 1200 has one for that very reason..

When I fitted SUs to a Z I was building a torquer 2850 cc P90 motor , and as the 1 3/4" Hitachis only flow enough for 175/180 hp it was hitting the wall before 5000 revs.

Triple 38mm Webers are the track set up, but I wanted something more streetable, and I already had the 2" SUs, which are good for around 250 hp aqnd would therefore do the job with a bit to spare.

Starting from the Healy 3000 settings it wasnt a big job the get them close in warm weather, but fitted to a unheated manifold and with pod filters, tuning for cold running and the changing seasons was a bit harder.
As was final fine tuning.

Both problems were solved by stiffer springs , a good stiff set for the freezing winter mornings, and progressively softer as the seasons warmed up, but still quite stiff.

So I am as certain as I can be of the effect of stronger springs.

FWIW there is more than one spring available for BMW Bings - the part number for the spring fitted to the later bikes is different, and the spring is slightly stiffer.

Some guys on the English Owners Club forum did a comparison , and found the later, stiffer , spring to perform better , but I have never tried them myself as I still have a good selection of SU ones.
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Old 05-31-2010, 05:36 AM   #17
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Old 05-31-2010, 11:05 AM   #18
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Lets remember what CV stands for...

CV, as in CONSTANT VELOCITY of airflow through the slide area, is the basic goal of the Bing (and similar) carb designs.

The idea is to maintain the same velocity of air through the slide to provide the same venturi suction pressure drawing fuel up at all times.
Improves atomization, makes metering more predictable...

When the engine RPM increases faster than the slide rises, the velocity through the slide area increases, which increases venturi pressure/vacuum. That will draw more fuel up for the same size opening as a lower velocity.

A stiffer spring (or diaphram) will slow down the rate that the slide rises, causing a temporary increase in velocity through the metering section (slide) of the carburator while accelerating. That will result in more fuel (richer mix) being drawn for that timeframe at the same needle setting.
As the slide catches up as steady engine speed, the velocity in the metering section reduces, and the fuel metering (mixture) goes back to the same as with a softer (faster) spring.

What is happening at the butterfly is irrelevant to this analysis. All Beemerboff is talking about is what happens at the needle during the transition phase while the slide is rising.

There IS a temporary enrichening of the mixture due to the slower response of the slide with a stiffer spring or diaphram.
That allows you to run a leaner needle or main jet setting for improved fuel economy at steady speed riding, while still pumping more fuel when accelerating to help performance.

-----

Yes, it may seem counter-intuitive at first, but it does work that way...
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Old 05-31-2010, 07:13 PM   #19
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Quote:
Originally Posted by BMWzenrider
CV, as in CONSTANT VELOCITY of airflow through the slide area, is the basic goal of the Bing (and similar) carb designs.

The idea is to maintain the same velocity of air through the slide to provide the same venturi suction pressure drawing fuel up at all times.
Improves atomization, makes metering more predictable...
This type of carb is called, depending on your country a
CD Constant Depression, or
CV Constant Vacuum or more colloquial called a Constant Velocity carb.

The name derives from the slide lifting due to intact tract vacuum.

The more proper term is not "slide area" but rather throat, *choke or venturi. (*choke here is not in reference to the cold start device but rather the passage way where the fuel and air mix)
Air doesn't not flow through the slide. The slide lifts as air moves through the throat of the carb.

Quote:
Originally Posted by BMWzenrider
When the engine RPM increases faster than the slide rises, the velocity through the slide area increases, which increases venturi pressure/vacuum. That will draw more fuel up for the same size opening as a lower velocity.
The intake tract vacuum raises the slide. if the slide doesn't lift, the RPM do not increase increase.
Except at the very bottom of the throttle movement, when there is very little intake vacuum. then the fuel is metered by the idle circuit.

Quote:
Originally Posted by BMWzenrider
A stiffer spring (or diaphram) will slow down the rate that the slide rises, causing a temporary increase in velocity through the metering section (slide) of the carburator while accelerating. That will result in more fuel (richer mix) being drawn for that timeframe at the same needle setting.
As the slide catches up as steady engine speed, the velocity in the metering section reduces, and the fuel metering (mixture) goes back to the same as with a softer (faster) spring.
The slide is the device that moves up and down, it's connected at the top to the diaphram, and at its bottom is the jet needle (or simply needle)

Here's the rub. The air moving through the venturi will pull fuel up the emulsion tube. But, sitting in the emulsion tube, hanging off the bottom of the slide is the needle. This tapered device is controlling the amount of fuel making it's way up the emulsion tube regardless of venturi velocity.
That's what it's there to do.

A heavier spring, heavier slide or stiffer diaprham will slow down the lift of the slide. this will constrict the rate of fuel that is pulled up.

Quote:
Originally Posted by BMWzenrider
What is happening at the butterfly is irrelevant to this analysis. All Beemerboff is talking about is what happens at the needle during the transition phase while the slide is rising.
The butterfly (throttle) is THE valve that controls the amount of air flowing into the system.
If it's closed there is very little air flowing, If there is no air flowing the slide is not going to lift.
The only way the slide can lift is by the vacuum created by the engine RPM increasing.

Quote:
Originally Posted by BMWzenrider
There IS a temporary enriching of the mixture due to the slower response of the slide with a stiffer spring or diaphram.
That allows you to run a leaner needle or main jet setting for improved fuel economy at steady speed riding, while still pumping more fuel when accelerating to help performance.
So let me get this straight...
The temporary "rich" situation of the mixture due to a slower slide lifting allows for a leaner needle or needle jet setting for steady speed riding...

A situation where the slide doesn't rise since it already rose to the steady engine speed position.

Quote:
Originally Posted by BMWzenrider
Yes, it may seem counter-intuitive at first, but it does work that way...
Sorry, Don't believe it for a nanosecond,
But hey it's not the first time I was wrong. So I went searching
I came up with nothing.
none of my books on CV carbs or engine tuning mention any thing of this sort.

Now I'm not saying that changing the slide weight doesn't change the character of how the engine responds. But making the slide heavier doesn't make the mixture richer.
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Old 05-31-2010, 07:26 PM   #20
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Quote:
Originally Posted by squish


The butterfly (throttle) is THE valve that controls the amount of air flowing into the system.
If it's closed there is very little air flowing, If there is no air flowing the slide is not going to lift.
The butterfly (throttle) is THE valve that controls the amount of air that can flow into the system.

An open butterfly does not cause air to flow, but a closed butterfly will restrict the airflow.
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Old 05-31-2010, 07:59 PM   #21
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Quote:
Originally Posted by Beemerboff
These pictures are OK, but they just don't mention speed, and therefore suction and flow.

And understanding that is the key to understanding and tuning Constant Velocity carbs.
Right then
So the throttle (butterfly) controls the air into the engine.
As the throttle is opened, and engine speed increases, there is a pressure differential between the throat of the carb (the bottom of the slide) and the area above the slide's diaphram.
This vacuum, (that's the V in CV carbs to some people, or the Depression in CD carbs) lifts the slide, this slide lifting, pulls the needle up, the tapered needle allows more fuel to flow up the emulsion tube and into the intake stream.

The amount of fuel flowing into the stream is controlled by this slide and needle combination. (This is assuming that we have moved off of fuel being metered by the idle circuit.)

The amount of air is controlled by the throttle (that's the butterfly)
As the throttle is opened more, the engine speed increases, as the speed increases more air is flowing through the throat of the carb, as more air moves, the slide moves up and as a result more fuel is flowing into the intake stream.
Close the throttle and air is reduced, the slide drops the needle moves deeper into the emulsion tube the amount of fuel is reduced.
A heavier slide will fall faster, shutting off the fuel flow quicker.

Quote:
Originally Posted by Beemerboff
If you read most tests of FI bikes the tester notes problems with fueling at low revs, but if it was simple mixture problem it would be easily sorted.

The problem is maintaining air flow speed without a CV device somewhere in the system - Hondas new VFR 1200 has one for that very reason..
Frankly this discussion is about CV carb'd bikes not FI but since you bring it up. I don't know about the VFR1200 but I do know that the VFR800FI
Does not have a CV device, It has four "traditional" (has FI been around that long, we can now call it traditional?) FI throttle bodies with butterfly valves and injectors.

There is a desire by manufactures to make bikes easy to ride.
By smoothing out the powerband and making the relationship between throttle inputs and reactions of the bike easy to get the hang of.

Some bikes (most notable is Suzuki's line of GSXR and DL's) employ a second throttle body, this is to change the intake velocity allowing for a better more complete intake charge at the smallest of throttle openings.
This is also done to keep intake noise down.

Many riders have reported disconnecting this device with no ill effects.

But those second throttle butterflies are not a CV or CD type device.

The CV or CD describes a Carburetor that has a slide that lifts by way of Vacuum (or Depression if you will),
This separates from a Slide Carburetor (like what's on the R90S stock) which have slides that are lifted directly by the throttle, or Fixed Venturi Carburetor (older Harleys and many lawnmowers) that don't have a venturi that changes sizes.

Quote:
Originally Posted by Beemerboff
FWIW there is more than one spring available for BMW Bings - the part number for the spring fitted to the later bikes is different, and the spring is slightly stiffer.
Not surprising since BMW made a number of different bikes for a number of different uses.

I've never said that heavier slides don't effect the way an engine responds.
Only that heavier slides don't make a bike more rich.

For instance if you want more lower end tractable power, it's easy to see that slightly slower reacting carbs would be a bonus, smoothing out abrupt throttle inputs and the like. The drawback is on closed throttle there tends to be an increase in engine braking the faster fuel is shut to the cylinders.

Where as on a sporty bike ridden on road you might want faster reaction to throttle inputs, give the bike gas it goes roll off it slows...
Again, the lighter slides allow for a slower closing of the throttle
and a little less compression braking.

Also different countries have different tastes when it comes to engine response.
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Old 05-31-2010, 08:02 PM   #22
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Quote:
Originally Posted by Rucksta
The butterfly (throttle) is THE valve that controls the amount of air that can flow into the system.

An open butterfly does not cause air to flow, but a closed butterfly will restrict the airflow.
Right, the butterfly is only the valve, the engine is what pumps the air.
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Old 05-31-2010, 09:54 PM   #23
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Yes

The engine controls how much air is pumped , the butterfly controls how much air can be pumped (up to the demand limits of the engine) the slide contols the velocity of the air through the carby and the venturi effect (modified by jets, needles, taper, emulsifiers etc) controls how much fuel can enter to engine.

Oh the fuel (when mixed with the right amount of air) controls the size and speed of the bang which is what cuases the engine to demand more air. Think diesel.

In my mind when I break it down to basics Beemerboff's "upside down" ideas of carb tuning start to gain some credibility.

I could have used backward or arse about to describe Beemerboff's ideas but that may have conveyed an opinion I wasn't expressing.
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Old 05-31-2010, 10:10 PM   #24
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Isn't it funny how logic can take us down opposite paths? I'd have thought the same thing - that stiffer diaphrams and springs would make for a leaner mixture, contrary to Beemerboff's experience.

Beemerboff - I see the logic in your explanation and it makes sense. Thanks for being willing to be scoffed at in order to express your contrary viewpoint!
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Old 05-31-2010, 10:59 PM   #25
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Quote:
Originally Posted by Beemerboff
A stiffer spring/ diaphram will richen the mixture in transition , so you could probably drop the needle a notch , and get better throttle response and gas mileage at the same time.
So I'm totally perplexed on this.

Just to make sure we are on the same page.
The way I was taught, transition refers to roughly 1/4 throttle applied to a quarter turn throttle, or roughly the point at which the the slide begins to take over fuel metering from the idle jet.

At this point, there is still fuel flowing from the idle circuit and fuel is flowing
from the main as well.

So your premise is the slower rising throttle, reduces the diameter of the venturi which in turn then raises the air speed, which in turn pulls more fuel up from the float.
Regardless of throttle butterfly opening.

Or are you saying that the reduced venturi size effectively pulls more fuel from the idle circuit?

Is this what you are saying in a nut-shell?
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Old 06-01-2010, 02:26 AM   #26
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Quote:
Originally Posted by Wirespokes
Isn't it funny how logic can take us down opposite paths?
Human logic will do that. Luckily, CV/CD carbies use the logic provided by the laws of physics.

The transition to & from idle circuit to main circuit happens each time, every time, at the same time, for exactly the same reason. It is irrelevant how 'beefy' the carb diaphram is.

Beefier diaphrams, thicker springs, or heavier dashpot oit (for you SU guys) will slow down the time it takes for the needle to react to your throttle inputs. The only thing that changes is time.
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Old 06-01-2010, 02:38 AM   #27
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Quote:
Originally Posted by squish
Here's the rub. The air moving through the venturi will pull fuel up the emulsion tube. But, sitting in the emulsion tube, hanging off the bottom of the slide is the needle. This tapered device is controlling the amount of fuel making it's way up the emulsion tube regardless of venturi velocity.
That's what it's there to do.

A heavier spring, heavier slide or stiffer diaprham will slow down the lift of the slide. this will constrict the rate of fuel that is pulled up.
I think that I see where the confusion is coming from...
YES, the taper on the needle does allow for more fuel to get sucked out at the same venturi pressure when the slide rises. But that is not what we are talking about.

Forget about the needle for a minute.

Think for a moment about a cheap lawnmower carburator, or even an expensive Holley 4-barrel downdraft....
Both of those are "FIXED" venturi carburators with no metering needles.
Yet both designs quite effectively provide varying amounts of fuel from the fixed jets in the venturi throat.
The variable is the SPEED of the air moving through the venturi that alters the vacuum pressure in the fuel jet.
The change in velocity through the venturi in these carbs changes how much fuel flows from the main jets.

That is EXACTLY what is happening for the short time while the heavier slide has not moved to its final balanced position yet...

For a little while you are getting increased velocity through the venturi area which reduces pressure in the airstream versus what is normally present. That increases the vacuum on the main jet and draws more fuel, even though the flow area is not changing (needle has not moved significantly).
Just like in that lawnmower or Holley setup...

Does that explain it a bit more clearly?


Once the slide moves up to its normal position balanced by the pressure/vacuum differential, the metering flowrate is back to what was calculated for the given needle/jet, so fuel flow goes back to 'normal'.


Quote:
Originally Posted by squish
The butterfly (throttle) is THE valve that controls the amount of air flowing into the system.
If it's closed there is very little air flowing, If there is no air flowing the slide is not going to lift.
The only way the slide can lift is by the vacuum created by the engine RPM increasing.
I don't disagree with you.
However, the discussion has been centered around what is happening in the throat of the venturi (at the slide).
Trying to think about the butterfly position in all of this will just confuse you. It is not the issue...


Quote:
Originally Posted by squish
So let me get this straight...
The temporary "rich" situation of the mixture due to a slower slide lifting allows for a leaner needle or needle jet setting for steady speed riding...
A situation where the slide doesn't rise since it already rose to the steady engine speed position.
Exactly!
Because the stock needle was selected to prevent the mixture from going too lean while accelerating, where you need a rich mix. But you do not need to be that rich when just cruising.
So being able to tweak the slides to slow the rise is like adding an accelerator pump. The temporary increase in the venturi effect allows for richer mix when revving up the motor, but still allows for a leaner cruising mix.


Quote:
Originally Posted by squish
Sorry, Don't believe it for a nanosecond,
But hey it's not the first time I was wrong. So I went searching
I came up with nothing.
none of my books on CV carbs or engine tuning mention any thing of this sort.

Now I'm not saying that changing the slide weight doesn't change the character of how the engine responds. But making the slide heavier doesn't make the mixture richer.
Not sure why you had trouble finding a reference to this trick.
Found this on the second link that I clicked on when I did an online search for SU spring rate...
http://www.jetlink.net/~okayfine/su/sumain.html
"Dome Spring Rate Calculation
Another method of fine-tuning your SU carbs, especially with a modified motor, is changing the dome spring for a stiffer or softer spring. A stiffer spring will resist the upward movement of the piston, as does higher weight oil/fluid in the dampeners. This resistance to upward movement of the piston provides a degree of fuel enrichment for acceleration.
"

I didn't bother to search for other references, but I am confident that they exist...
Haynes actually publishes a separate manual dedicated to the very similar SU carbs. It goes into operating theory, and I believe has a section on needle/spring selection.
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Old 06-01-2010, 04:15 AM   #28
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What we seem to be forgetting here is that fuel does not just flow up through the needle jet on its own accord - the float level is set well below the jet outlet.

It is sucked up , and the rate it is sucked up depends on the speed of the air flow between the botom of the slide and the top of the needle jet, or the atomimiser or whatever on top of the jet.

And that speed depends on the relationship between the butterfly position and the slide position - large butterfly opening and small slide opening equals high speed , more suction and a rich mixture, tempered somewhat by the fact that the lower needle position will slow the fuel flow down a bit.

Bings are a crude carb, and most Airheads run far to rich at most speeds - cruising the interstate on a dead throttle at the state speed limit my injected car gets better consumption than the big GS.

What I was suggesting was a way to help to clean up the carburation , providing a crisper throttle response and a bit better consumption at the same time.

I have tried it and it works for me, but there are plenty of other areas where I get different results from the mainstream, and I find that rather fun.

And fun is what biking is all about for me.
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Old 06-01-2010, 03:39 PM   #29
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Taken from Jorge's BMW web page.

Throughout the years, BMW has apparently used two different types of springs in the Bing carburetors.
The BMW spare parts catalogue tells that until end 1984, a spring with 115 mm length and about 30 turns has been throughout their model range, from the R45 through R65 up to the R80 and R100. The part number was 13 11 1 335 324. From 1985 on, the spring changed to 20 turns and an overall length of 120 mm (part number 13 11 1 338 134). Again, the same springs are used in all models, both the 40-mm Bings (Type 94) of the R100GS and R100R, and the 32-mm (Type 64) of the "late" R65, R80, and R100.
An interesting experiment is to put the "new" springs into the "old" carburetors.
Following a detailed report from Rainer Restat who tried this on his R80G/S, I took the carburetor springs from my trusty R80GS and put them in the R100TIC. The result was ... a smoother idle and a better response to the throttle. In particular in the range between 2000 and 3000/min there was a noticeable difference, which is exactly the zone that I frequently use when strolling through the countryside. At higher rpm, there is no difference.
The "new" spring is harder than the "old" one, so a probable explanation is that the new spring reduces the pressure fluctuations that occur at low rpms inside the carburetor. A possible reason for the better "response" at low throttle is that the harder spring yields a reduced efficient cross-section when you open the throttle. This yields a higher air intake speed, which in turn leads to better filling and thus more "punch".
In conclusion, changing the "old" springs against the new ones is a very simple "tuning" which brings better idle and smoother acceleration at the low end. In addition, the new springs are far from expensive (about 10 CHF or 6.50 a pair), so I really recommend this exchange.
Many thanks to Rainer Restat for this hint and his detailed report!
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Old 06-01-2010, 03:48 PM   #30
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Amen
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