Sidedraft Central

[grannyknot]

Hey guys,

I'm trying to get access to Keith Franck's "white paper" on carbs,

Keith has accepted me into the Sidedraft Central yahoo group but for some unknown reason I can't log in and read through the archives.

Keith's advise was to try and get a new yahoo ID and I have done that but still no luck. I have tried everything....so

wondering if any of the carb guys here can advise on how I can access all the great this info, has anyone downloaded this stuff?

Thanks,

Chris

[Patcon]

I have been looking around on my laptop. I thought I had downloaded but can't find it. I will look around on another computer.

Charles

[240260280z]

Beware of the fuel depth. Keith says to set it at same level for all applications.. this resulted in a rich cruise for me that I could not jet-out. The fuel level was just too high and the passage to the aux venturi slurped it up when it should not have.... a lower fuel level would have prevented this. Since the fuel consumed at cruise was much less than at "full song" the fuel in the well stayed at the same height and was sucked up slower than the main jet regulated.

The only good thing in that white paper is the advice of disabling circuits when doing basic tuning... you have 5 circuits with a typical set up:

1. Idle

2. Cruise

3. Main

4. Choke

5. Acceleration

You can disable the mains by removing the jets.

You can disable the acceleration by replacing the bleed-backs with bolts

You can disable the choke at the lever

There is a lot of excellent info on this forum that I think exceeds Keith's with respect to its usefulness. I can send you something *very* useful if you want. I can't recall if I have your email... send me a PM and I'll blast it off.

Here are some useful docs: http://www.classiczcars.com/topic/44405-dcoe-documents/

For tuning your carbs, iteratively get the cruise where you want it by disabling the mains and accelerator circuits. You can adjust fuel level and swap idle jets to get it right.

After you get the cruise, set the idle then install the mains and get the 3500 to 6500rpm WOT where you want. Typically ~ 12.5 to 13.5.

After all of this is good, stick an O2 on the manifold to capture transients (they get subdued at the tail pipe) and install the accelerator circuit and get the gas-stomp transients right.

[grannyknot]

Patcon, thanks, I would appreciate it.

 

Blue, that's some great info right there! I'll send you my email address.

I figure if I'm going to keep these triples then I better go all the way and learn how to tune them properly

myself, really jump right into it and master the technique.

Chris

[grannyknot]

Success! The benevolent gods of yahoo finally let me through the gates.

[Patcon]

Cool glad you got it. I had thought I would try to download it here. If I could figure it out...unless someone beats me to it.

Charles

[Patcon]

I believe I found the paper but I can't figure out how to post it. It is 99.1 KB. Can I email it to someone so it could be posted for posterity?

C

[zKars]

Patcon, send to me, I'll post it for you.  z240@shaw.ca

 

You could try, click "More Reply Options", then the " Browse" button, then find the file and select it, click "Open". Then click "Attach This File", then finally (yes one more step), click the blue "Add to Post" link that shows up below the name of the file you loaded, this will add some text to your message.

 

here is how this stuff looks

 

[240260280z]

I have the file but I can't upload in mobile mode due to the format change.

Oh well... as mentioned you are not missing much.

Here is a cut and paste of the text:

An Alternative Way of Extracting the Best Tractability and

Performance from the Weber DCOE Carburetor

© 2006 Keith D. Franck

This is not intended to be a complete tuning guide. The information provided here

supplements the official Weber Tuning Manual (PN 95.0000.54PM). That manual covers

all the basic aspects of tuning that I’ ve not discussed. This is a white paper to show

professional tuners and folks with a high degree of engineering skill an alternative way to

tune these carburetors.

1. Introduction

Changing parts on Weber DCOE carburetors is easy, interpreting how these

changes affect the state of tune is more difficult. Each individual change can have such a

subtle effect that it cannot be detected by traditional seat-of-the-pants tuning methods.

Eliminating problems like the dreaded midrange flat spot (~2000-3000 rpm) requires

making a number of small changes in a particular order; only then are the carburetors

tuned to the utmost. One must have a wide-band O

2

air-fuel meter to do this type of

tuning. I use the LM-1 and LMA-2 units from Innovate Motorsports. Spark plug cuts

only tell you if something is grossly wrong- they’ re hopeless for fine tuning.

This guide assumes that all other components of the engine, other than the Weber

carburetors themselves, are in perfect working order. This includes the engine itself (i.e.

compression), the cooling system and the ignition system. Many problems that seem to

be carburetor related are actually caused by a faulty ignition system!

All air leaks in the intake and exhaust systems must be sealed. Air leaks in the

induction manifold impair the proper functioning of the idle and transition circuits. On

early carburetors, air can also leak past the threads of the idle mixture screws, causing an

excessively lean condition on overrun. Grease the threads or place an o-ring or a short

length of rubber hose underneath the screw head. Leaks in the exhaust system cause

readings from the air-fuel meter to fluctuate wildly (this may be the only symptom if the

leak is small) and also may cause popping on overrun.

2. Fuel Level

Begin by setting the fuel level. If the fuel level is not set correctly, it will have an

adverse effect on the ultimate state of tune you will be able to achieve. I’ ll explain why it

is so crucial later when discussing the main jet circuit. The standard procedure involves

gapping the floats to the top cover. Unfortunately, this method is error prone. The floats

are buoyant in a pool of fuel and should be measured as such in-situ. Construct a simple

optical gauge (Figure 1) for this wet level measurement. This tool works much like a

dipstick. The split nylon sleeve clamps gently onto the acrylic rod, this allows it to slide

freely along the rod but not slip. Rest the sleeve on the top surface of the well (Figure 2)

and slowly push the acrylic rod down. When the bottom end of the rod just touches the

fuel it will make the top end look dark (look carefully!). When it does, measure the tip to

sleeve distance. Ideally, this should be done while the engine is idling and level. If the

engine is in a poor state of tune and shuddering severely it can be done within sixty

seconds after shutting off the engine. The desired level is predetermined by the design of

the DCOE carburetor. The passageway that connects the well to the auxiliary venturi is

23mm below the mating surface of the cover plate. The fuel level must be 2mm below

that point or fuel will spill into throat when accelerating or braking. Therefore, the ideal

fuel level is 25mm +/-1mm.

Figure 1: Optical Fuel Level Gauge

Figure 2: Using the Optical Gauge to Measure the Fuel Level

3. Transition Circuit

The cornerstone of the Weber DCOE carburetor is the transition circuit. It

operates over a fixed rpm range designed into the carburetor, defined by the placement of

the progressive holes relative to the throttle plate. Essentially, this range is nonadjustablewe must tune around it. This has two practical consequences for the tuner. One, it

defines a required idling rpm so as not to create an off idle stumble. Two, it defines a

transition rpm at which the main jet system must be activated to eliminate a flat spot.

To measure the true air-fuel ratio (AFR) of the transition circuit, we must disable

the main jet circuit by temporarily removing the emulsion tubes. If the floats are set

correctly (see Section 2) the fuel level will remain 2mm below the passageway leading to

the auxiliary venturi. Consequently, no fuel should flood into the carburetor via the

auxiliary venturi, so this shouldn’ t be a safety issue. With the emulsion tubes removed,

fuel will continue to flow and the engine will run normally on the idle and transition

circuits as long as the throttle plates are not opened past the last progressive hole. This

occurs at approximately 10% of the pedal’ s full travel. Driving the car with the emulsion

tubes removed provides a baseline for how the transition circuit was designed to perform.

Be careful when doing this test because you will be lacking about 90% of normal engine

power to get out of harms’ way.

Drive the car gently at a steady speed on level ground and note the AFR measured

by the air-fuel meter. Don’ t move the throttle while taking a reading- the accelerator

pumps will shoot fuel and make the AFR reading inaccurate. Moving the throttle plates

past the last progressive hole will kill the engine so just don’ t do it. Swap the idle jets

until the AFR is approximately 12.5:1.

The next test measures the transition rpm at which the main jet system must be

activated. Shift the car into high gear and slowly increase the engine rpm until the engine

dies. The maximum rpm at which the transition circuit keeps the engine running while in

high gear is the transition rpm to which you MUST tune the main jet circuit to begin

providing fuel. Typically, this target rpm is about 1400 rpm in high gear. The main jet

circuit must be contributing fuel at this transition point without the AFR deviating from

the desired 12.5:1 value. Any gap in fuel delivery between these circuits produces the

classic and much dreaded flat spot! At part throttle cruise in the higher gears the main jet

circuit is actually providing all the fuel so concentrating on fine-tuning its low rpm

performance is crucial to achieving the best tractability possible.

4. Idle Theory

The “BEST LEAN IDLE ADJUSTMENT” method universally touted through the

years happens to be the worst tuning procedure possible, typically resulting in an AFR of

about 22:1. This extremely lean mixture produces a number of undesirable results. It

burns slowly and causes spitting and misfiring at idle. The slightest opening of the

throttle plates causes a stumble. Furthermore, when the throttles are opened abruptly the

slow burning lean charge ignites the fuel vapor in the induction manifold, causing a

backfire out of the carburetor. On overrun, the AFR will rocket to even leaner values and

the chance of a total or partial misfire is very likely; unburned fuel will collect in the

exhaust system resulting in backfiring or popping.

Avoid all these usual pitfalls by setting the idling AFR to 12.5:1. This should be

done with a wide-band O

2

air-fuel meter. To start, set the idle mixture screws as per the

“BEST LEAN IDLE ADJUSTMENT” procedure to get all the same lean AFR

contributed from every screw. Reduce the AFR to 12.5:1 by opening all the idle mixture

screws by an additional equal amount of twist for each. A good indication that the AFR is

equal on every cylinder is that the shuddering motion of the engine on its compliant

mounts is at a minimum.

5. Airflow Balancing

If the amounts of air entering the combustion chambers are unequal, the

combustion strengths will also be unequal and the engine will rock back and forth on its

rubber mounts. Measurement of the airflow must be done with a high quality, calibrated

gauge. I use and recommend the synchrometer made by STE. If the airflow is balanced

correctly you should be able to place a brimming cup of water on the engine and slowly

increase the rpm via the idle speed screw to above 3000 without spilling a drop. Older

carburetors might require small holes to be drilled through some of the throttle plates to

balance the airflow to the throat with the highest rate of flow. The Weber Tuning Manual

describes how to drill these holes. Newer carburetors have an adjustable air bypass bleed

screw. The airflow synchronization between carburetors via the linkage is reserved for

the cruise throttle condition at about 15% power so the engine pulls smoothly while in a

cruise driving mode and this adjustment is done first. This cannot be done while in

motion so raise the engine revs to where the engine shudders about worst and do the

synchronization via the linkage there to smooth out the engines’ torque reaction force.

Next procedure is adjusting the airflow at idle of all the others to match that of the

cylinder with the highest flow.

One last word of advice: Do not twist the throttle spindle to balance the airflow

past the pair of throttle plates. The factory made an extremely fine adjustment of the

throttle plate edge to first progressive hole when the carburetor was first assembled.

Close inspection may show that one of the first progressive holes has had additional

metal scraped away downstream to make it even with its other paired hole. Twisting the

spindle just makes the preset idling rpm less effective and increases the likelihood of an

off-idle stumble.

6. Idling RPM Determination

The Weber manual states that the DCOE was designed to idle at 1000 rpm.

Slowly speed up the revs by a few hundred rpm via the idle speed screw and listen for a

stumble. A stumble is caused by a lean condition; you can confirm this with the wideband O

2

sensor. If there isn’ t a stumble then either it’ s okay or the fuel is being supplied

by the first progressive hole. To test for the latter, close each idle mixture screw in turn

and confirm that the cylinder cuts out and misfires from a lack of fuel. If it does not

stumble and the cylinder cuts out like it should then that idle speed setting is correct.

7. Lazy Idle Syndrome

An engine that does not quickly return to the set idle speed after a blip of the

throttle has the classic “ lazy idle” syndrome. My theory is that on overrun, the additional

air sucked past the throttle plates draws fuel from the first progressive hole when the

induction manifold vacuum is highest. Both additional air and fuel is necessary for this

fault condition to happen. This is a self-feeding situation that normally slowly decays

away after several seconds. To eliminate a lazy idle, reduce the idle speed until these

symptoms vanish thus moving the throttle plate away from the first progressive hole.

8. Main Jet Circuit Theory

Now we get to the toughest part, understanding and tuning the main jet circuit.

Since the available vacuum energy to operate this circuit is so tiny a number of factors

impact its adjustment. As described in Section 2, the preset fuel level is 2mm below the

well passageway leading to the auxiliary venturi. The force to lift the fuel up these 2mm

is derived entirely from the auxiliary venturi vacuum signal. Typically, the total amount

of vacuum of the combined auxiliary venturi and main choke assembly of a 40 DCOE at

6000-rpm with a 30mm main choke (1600cc Lotus Twincam) is only able to lift a

manometer column of fuel about 4mm. This 4mm value was measured without an air

corrector jet and the test facilities could not accommodate going to WOT at redline.

Therefore this value may change and get slightly bigger when better testing procedures

are employed. Since an air corrector jet introduces an additional air leak the total vacuum

achievable is always going to be a bit less.

At first, it seems that there is not enough force to drive the main jet circuit. True,

if the vacuum were acting on the dense liquid fuel alone the system would be inefficient

and the AFR could not be maintained at a precise value like 12.5:1 over the entire

operating range. Remember, however, that the vacuum signal does not act upon liquid

fuel at the float bowl level with most of the popular emulsion tubes installed. Instead the

fuel once sucked up to adjacent the passageway in the well stays there due to its’ surface

tension properties. Gasohol containing ethanol exhibits an even higher surface tension

tendency than neat gasoline and seems to make tuning a bit more difficult as a result.

Even if the fuel level falls in the float bowl by an additional 13mm the adhesion of the

liquid fuel to cling to surfaces of the emulsion tube and the well is enough to hold the

column of fuel up. Breaking this adhesion appears to be the dominating factor that

dictates the fuel flow behavior at low airflow rates through the auxiliary venturi. Further

study is required to understand this process well enough to devise a satisfactory solution

for all fuel types. The cohesion property of the fuels’ surface film also resists the

penetration of the small bubbles emitted from the air bleed holes of the emulsion tube.

This provokes no flow of air until a certain air pressure threshold is reached and then a

burst of bubbles will be emitted from any emulsion tube air bleed holes which are located

below the top of the fuel column. There appears to quite a bit of variation to that

threshold which makes the low airflow tuning tricky depending on the emulsion tube

chosen and many other factors.

The behavior of fuel-air emulsions under various conditions is described by “ twophase flow” - a fascinating, but extremely difficult area of fluid dynamics. It suffices to

say that the Weber DCOE utilizes air corrector jets to create a two-phase flow to a small

degree. By the time the emulsion has been sucked several millimeters into the

passageway leading to the auxiliary venturi the bubbles have burst and the fuel trickles

down the wall mostly under the influence of gravity at that point. When this system is set

up properly it is possible to maintain an AFR of 12.5:1 under ALL CONDITIONS.

9. Air Corrector

The size of the air corrector jet is crucial for two reasons. First, it determines the

onset of flow from the main jet circuit in conjunction with the chosen auxiliary venturi

and main choke. An oversized air corrector jet weakens the vacuum signal substantially,

delaying activation of main jet circuit past the transition rpm (~1400-rpm). This is the

cause of the classic mid-range flat spot.Second, it introduces an air leak and reduces by

diluting the highest attainable vacuum signal from the auxiliary venturi. By not

introducing to big an air leak at low flow the largest size of auxiliary venturi and main

choke can be selected to provide enough vacuum signal for the WOT mode without

overly restricting the air consumption needed at the redline for maximum power output.

Ultimately, the air corrector keeps the load-slaved vacuum signal proportional to

the fuel flow rate at low airflow consumption of the engine, thus keeping the AFR

constant. Once the airflow through the auxiliary venturi is fast enough to generate a

vacuum that creates sufficient liquid fuel pressure gradient across the main jet to meter

the fuel flow, at that point the primary role of the air corrector jets is completed. The air

corrector jets are no longer important or required for the emulsion tube assembly to

provide the AFR at the higher rpms and loads. Consequently, the air corrector jet is not

to be used to lean the air-fuel mixture out at high rpm. That is the function of the main jet

and the fuel level.

Choosing correctly sized auxiliary venturi, main chokes and air corrector jets is

paramount to getting the best, smooth, tractable performance from the main jet circuit.

Arriving at the best combination is a trial and error process, but can be achieved by

performing two simple tests.

First, check to see if that the main jet circuit is fully functioning just prior to

transition rpm measured in Section 3. To test if the main jet circuit is flowing at the

transition rpm, drive the car in high gear at that target rpm and suddenly go to wide open

throttle (WOT) and hold it open for about five seconds. At first, the car will lunge

forward slightly because of the accelerator pump shot, but then if the engine continues to

run (even if it obviously lugs) that indicates the main jet circuit is active. If it is not, then

the engine will die, just as if the ignition was switched off. This is the “WOT Main Jet

Flow Test” .

One determines the smallest air corrector jet by reducing the jet size until the

engine stalls momentarily when the throttles are held open, while briskly cornering at

about 15-30 mph on an uphill incline of at least a 5% grade. The cornering must be in the

direction opposite that the air trumpets point out. If the carburetors are oriented so they

point forward or aft then I don’ t know how to find the lower AC size limit.

I don’ t think this effect is caused by fuel starvation from the liquid sloshing

around in the float bowl. If the engine stalls I think the fuel droplets in the constantly

density changing emulsion having traveled to the carburetor throats are simply too large

so they weigh enough that they are flung out of the trumpets during cornering. In other

words, under that operating mode there is not enough air bubbles in the emulsion to

sufficiently reduce the droplet size and therefore the AC jet is too small. Going up in

diameter by two (100 microns) to four (200 microns) incremental sizes should cure the

problem.

10. Main Jet

The goal here is to achieve a steady AFR over the entire rpm range, particularly at

WOT at high rpm. Finding the smallest jet that maintains the correct AFR it is the key to

winning the tuning game!

The main jet has two forces acting upon it. Both forces are small but combined

they exert the highest possible pressure gradient across the main jet. The vacuum force

has already been discussed in Section 8. The other force is the “ hydrostatic” force

provided by the dynamic fuel level in the float bowl; it is about half the total combined

force acting upon the main jet. Lowering the fuel level actually does two things. It

reduces the hydrostatic force acting upon the main jet and increases the distance the

vacuum signal must lift the fuel in the well to be mixed via the emulsion tube into the

emulsion. Manipulating the hydrostatic force is how the rate of fuel flow at WOT should

and must be controlled. Let me emphasize: The target AFR is adjusted via the float bowl

fuel level and the main jet hole size and not by diluting the vacuum signal by installing an

oversized air corrector jet. This is why the emulsion tube only impacts the AFR at partial

throttle and has little affect at WOT. At WOT and high air induction velocity there is

enough vacuum signal strength to pull even unmixed raw liquid fuel into the engine.

11. Emulsion Tube

Changing emulsion tubes or altering its’ air bleed holes allows you to achieve the

desired AFR at partial throttle steady cruise. So far, I’ ve only experimentally tuned with

F11 tubes. The large expense of purchasing all the available type of tubes has prevented

me from conducting more tests.

I’ ve learned that adjusting the amount of air flowing through the bleed holes

above fuel level alters the AFR quite a bit. You may have to add several additional air

bleed holes to lean out the AFR at part throttle cruise. You may also have to plug up

several of the air bleed holes to fatten up the AFR. Experimenting has shown these

modifications will shift the AFR by a little more than +/- “ one part of air” from the stock

configuration. The alteration required can only be determined by testing with the air-fuel

meter.

12. Main Jet Circuit Tuning Procedure

With all this information in hand the procedure to find the best components is

actually rather simple. First, find the diameter of air corrector jet that gives the ideal

emulsion. Second, find the diameter of main jet that provides the desired AFR at WOT

full load. DO NOT change the size of the air corrector jet to change the AFR. Last, select

and tweak the emulsion tube to optimize the cruise AFR.

13.Accelerator Pump Theory

The accelerator pump provides a specific amount of fuel for a specific time

interval with a sudden application of WOT. The pump shot needs to be adjusted such that

the desired AFR creates a flat line in the wide-band O2 data graph at the engine’ s shift

points when at racing speeds. This will provide a little too much fuel and produce an

overly fat (rich) AFR when the pump is actuated at less then a racing performance level,

but this is perfectly normal and expected. You may be able to alter the bleed back hole

size on the quick gulping, one-way check valve that supplies the pump to lessen this

effect.

When doing WOT runs to find the main jet size at a starting rpm well below the

racing shift points, the first 3 seconds of the data graph contain the contributions from the

accelerator pump; this data should be ignored. To truly dial in this parameter it must be

done on the racetrack under 10/10s operating conditions.

The Weber Tuning Manual states that, under some conditions, the pump circuit

will continue to contribute fuel after the shot is expelled. I have not seen this effect firsthand on the few stock sports car engines I’ ve tuned. This may be dependant on the air

flow restriction of the induction manifold. A race engine that breathes well and revs up

high enough just might cause this type of fuel flow.

14. Conclusion

These tuning principles should apply to all racing or road car applications and all

the different brands of carburetors that share a similar design to the Weber DCOE. Using

these tuning principles should yield excellent results in a few hours of effort. Most of this

information provided has been derived from actual testing and is easily reproduced. I

strongly suggest you do your own testing before being convinced one way or the other.

15. Disclaimer

Use this information at your own risk. If you lack expertise dealing with these

units then don’ t attempt to correct the faults based solely on this article. A full

understanding of all the issues covered in the official Weber Tuning Manual is also

required. Carburetors and the fuels are inherently dangerous. Heed the warnings and use

common sense.

I will discuss this topic for awhile at: http://autos.groups.yahoo.com/group/sidedraft_central/

[Patcon]

I thought I would add the file anyway, even though Blue added the text. It might be easier to print in this format.

Thanks, Blue

Thanks, Jim for the tutorial

Charles

Weber_DCOE_Tuning_White_Paper.pdf

[Patcon]

Got this email today

Yahoo has announced today it's abandoning supporting all Group websites and as a result it is no longer useful to me. The content has been corrupted for quite awhile and as a result trying to save photos is now no longer possible. I'll be moving to another vendor which I'll announce here soon.

 

They are only going to continue to support communicating by email only.

-Keith Franck

[Patcon]

They have moved

sidedraft@vintagetechnologygarage.groups.io