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Side draft carb/s tuning tips


dapbmw

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Covers Weber Carbs but the theory is applicable to other makes too from Dell'Orto, Mikuni, Solex, SK. Hope this helps a lot of you guys. Maybe this topic should be in the faq tips!

Dave "dapbmw"

Ideally you want the richness level of each circuit to be the same. The ideal intersection of the curves is where the fuel delivery of the circuit going out plus the fuel delivery of the circuit coming on added together equals the total amount of fuel delivered when either circuit is in the middle of its range.

The idle and progression circuits are cast and drilled into the carburetor body making the location of their curves at fixed engine RPMs. The idle jet provides fuel to both the idle and progression circuits. This jet determines the richness of the progression circuit and the idle mixture adjustment screw sets the richness of the idle circuit curve. The main jet stack sets the richness of the main circuit and the size of the auxiliary venturi determines the RPM at which the main circuit curve intersects with the progression circuit curve. The main cruise circuit usually comes into play around 2800 to 3000 RPM. Below that, the engine is operating off the idle jets. This should help you trouble shooting any problem that occurs only above or below around 3000 RPM.

Here is how fuel is delivered to the cylinder. At idle the throttle plates (butterfly valves) are closed and fuel flows into the cylinder from an idle hole behind the throttle plates.

As the throttle plates start to open, the top edge of the plates move towards the mouth of the carb and encounters a number of progression circuit holes. These holes provide additional fuel into the increasing air flow. As the edge of the throttle plate passes a progression hole, the vacuum behind the plate draws fuel out of that progression hole. The additional fuel added by each progression hole keeps the cylinder from burning too lean in the RPMs above idle and before the cruise circuit kicks in.

If the initial adjustment of the throttle butterflies is open enough to uncover a progression hole, the engine will suck the fuel from the progression circuit during idle, resulting in a lean off-idle flat spot and poor fuel mileage. If the idle position of the butterflies is not set right you will never obtain a good consistent idle and smooth off idle transition. This is where most people go wrong setting up DCOEs.

By the time the engine has passed about 2800 RPM, the top edge of the throttle plate has passed all the progression holes and is opening wide enough to cause the vacuum to drop to where it is drawing progressively less fuel out of the progression holes. This is when the main cruise circuit needs to start kicking in so that the increasing level of fuel delivered by the main jets supplement the decreasing level of fuel supplied by the progression holes. The size of the auxiliary choke determines when the main jets will start delivering fuel.

If it kicks in too early (too small of a auxiliary choke) you get an over rich condition and the engine bogs or stumbles in the progression (or just wastes fuel with no noticeable symptoms). If the auxiliary choke is too large there can be a lean area where the progression openings are not delivering enough fuel and the main cruise circuit has not yet kicked in.

This leanness is masked during acceleration by the accelerator pump. It would be seen as a leanness in a narrow RPM band while in a constant low RPM cruise (this is where an onboard CO monitor that can be read during driving would come in handy).

The goal here is to fit the smallest auxiliary choke that will not cause an over rich bog or stumbling during a slow opening of the throttle plates. This will assure the absence of a lean RPM band that might damage the engine over time.

A Note About Jets

Most DCOE jets have tapered ends. The tapered end of these jets sit snugly against seats in the carb body to create seals between different areas of the carburetor. If the seal is not made or is broken the carburetor will not function properly.

These jets are mounted onto holders with a friction fit. As the holder is threaded in, the taper at the end of the jet comes into contact with the passage seat and is pushed back into the holder maintaining a contact seal. If the jet is initially pushed all the way into the holder it may not reach all the way to the passage seat. The fit between the jet and holder should always be tight. A loose fit can allow a jet to back away from the seat over time.

The proper method of installing a jet is to fit it only about one eighth an inch into the holder then allow the passage seat to push the jet in the correct distance as you screw in the holder. Be careful not over tighten the jet assemblies. Once the jet is seated it doesn't take much torque to hold everything in place.

Most jet sizes are in numbers that give their actual diameter in hundredths of a millimetre. Idle jets can also have F numbers that indicate their ability to emulsify fuel. The number behind the F has nothing to do with the hole flow rate. Emulsion tube designation is by the numerical order in which they were designed and has nothing to do with their flow characteristics. There is NO flow relationship between different number designations of emulsion tubes.

1. Intake and discharge valve for accelerator circuit

2. Idle jet assembly

3. Main jet stack

4. Starter jet assembly

5. Accelerator pump jet

6. Idle jet

7. Air Correction jet

8. Emulsion tube

9. Main jet

10. Start jet (Cold start circuit)

Idle Jets

Idle jets affect the idle and progression circuits of the DCOE. They are selected primarily for proper running of the progression circuit which extends from just above idle to where the main jet assembly takes over (somewhere around 25-2800 RPM). Proper selection is critical for smooth, economical low RPM cruising.

At idle, the fuel is mixed into the airflow behind the throttle plate (butterfly valve) and the flow is regulated by the idle flow screw. There are a series of progression holes, not affected by the idle screw, that get exposed behind the throttle plates as the throttle continues to open. As the throttle plate top edge moves past each hole, the vacuum behind the plate draws fuel from the idle jet out through that progression hole. This adds progressively more and more fuel to keep the engine running smoothly off idle until the airflow is high enough to draw fuel from the main jet. Since the progression holes are not adjustable, the idle jet is chosen primarily for the progression circuit.

Idle jets have a fuel hole drilled in the bottom of the jet and an air bleed hole drilled in the side.

The fuel hole regulates the amount of fuel for idle and gradual progression from the idle circuit to the main cruise circuit. The goal is the smallest hole that will provide a good smooth progression.

The air bleed hole affects the air fuel ratio of the fuel in the idle and progression circuits. A small air bleed hole means a richer mixture ratio and enlarging the hole leans out the air fuel mixture.

Here is the complete list of idle jet air bleed holes in order from rich to lean

F6 (richest), F12, F9, F8, F11, F13, F2, F4, F5, F7, F1, F3 (leanest)

Best choices for the TR engine is 45, 50 or 55 jet with F9 or F8 air holes. 50F9 is always a good ballpark jet size for these engines with 86 or 87mm pistons. The goal is to end up with the leanest mixture that provides correct performance through the progression circuits.

Before setting up the idle

It is important to be very sure there is no throttle shaft bind or over tightened levers. Of course this can best be checked for before the carbs are mounted. Open then close the throttle slowly. Then give the lever a litle extra push in the closed direction. If the throttle shaft is binding it will not return to fully closed if you let the internal throttle springs close it gentley. An accidental drop that strikes the throttle linkage can cause the shaft to bend just a little so that it binds. Test before you buy.

After mounting, linkage going to a common throttle bar should be identical in length and you should carefully check to make sure that the throttles of all carbs are completely closed when the linkage is is the closed position. Test the complete throttle linkage for any tendency to bind and not return the throttle plates to the fully closed position. Backup external throttle springs should be considered an important safety feature. Sync the carbs before setting the final idle. When balancing multiple carbs be sure to bring the high carb(s) down to the low carb, then bring them both up to proper idle speed.

Setting up the idle

Where just about everyone goes wrong setting up DCOEs is to use the idle lever adjustment screw to adjust idle RPM. More often than not this ends up uncovering the first progression hole at idle. This will cause you to pick the wrong idle jet, or if you have the correct one there will be a lean flat spot right off idle that you will be unable to compensate for.

First you adjust the idle jets for smooth idle then you set the idle speed if needed. If the engine does not get enough air to idle on its own with the throttle plates closed, you need to adjust the idle RPM before setting up the idle jets.

Illustration key:

2. Venturi balance adjustment screw with plastic cap. Fitted to newer DCOEs

4. Idle mixture adjustment screw

6. Idle lever adjustment screw

--------------------------------------------------------------------------------

To set the butterfly valves correctly, adjust the throttle plate screw (6) to where it just touches but does not move the throttle shaft. Turn the screw in 1/2 turn then do not touch this screw again. This screw is not intended to be used to adjust idle speed. If there is enough air flow the idle RPM will be correct after properly setting the idle adjustment screw (4). If there is not enough air flow, the idle RPM needs to be adjusted after the idle mixture is set.

Next, set the idle mixture adjustment screws (4) to one turn off closed and start the engine. First, turn the mixture screw towards closed in 1/4 turn incriments until the engine dies or runs worse, then back out the screw in 1/4 turn incriments until the screw does nothing or the engine runs worse. Then turn the screw back towards closed until you reach the point where the engine just starts to run poorly. Once there, turn the mixture screw back out to the leanest point where you encounter the best, fastest and smoothest idling. Use your ear, not a scope or tuning instruments for this. You want to tune the idle mixture by sound and feel.

If the idle jet size is close to correct, the best idle point should be when the idle mixture flow screws (4) are between 7/8 of a turn to 1-1/2 turns off closed. If only a half turn or so of the idle flow screw from closed obtains the correct idle RPM chances are the fuel opening in the idle jet is too large and you should try the next leaner jet. If the screws need to be turned out 1-1/2 turns or more chances are the the fuel hole in the idle jet is too small and you should try the next richer jet.

If you are taking a CO reading, 2.5% CO at idle is ideal for a street engine. Higher than 3.5% is unusable and just provides poor fuel consumption without gains.

If you can not obtain a correct idle by adjusting the idle mixture screw then the throttle plates are not passing enough air and you need to increase the amount of air going past the throttle plates.

On newer DCOEs with Venturi balance adjustment screw (2):

The venturi balance circuit is an air path that bypasses the throttle plate. The amount of air that bypasses the plates is controlled by the adjustment screw. What this does is allow additional air to go past the throttle plate at the idle position when the throttle plate does not pass enough air for the engine to idle properly (remember we do not adjust the idle adjust screw (6) after it is set at 1/2 turn). Adjusted properly it will let enough air past the throttle plates to allow the engine to idle properly. This adjustment is very sensitive so a little amount of turn can make a big difference in idle.

If the engine will not idle with the throttle plates closed set the idle lever adjustment screw (6) to 1/2 turn beyond initial contact, the idle adjustment screws (4) set at one turn up from fully seated. If the carbs idand both Venturi balance adjustment screws fully seated. adjust the Venturi balance adjustment screws 1/16th turn off seated then start the engine to see if it idles. If not, adjust both Venturi balance adjustment screws out in 1/16th turn increments until a ballpark idle is achieved then zero in the idle using the idle mixture adjustment screws. When you are satisfied that the Venturi balance adjustment screws are correct, tighten down on the lock nuts and put the cover over the adjustments as they should never need to be adjusted again.

On older DCOEs without Venturi balance adjustment screw (2):

The throttle valves need to be modified to allow additional air flow in the idle position. This is done by drilling holes in the throttle plate near the bottom edge. Drill a 1/2 mm dia hole in each plate on each carb, refit the carbs then try to get the engine to idle. If there is not enough air flow, enlarge the holes to 1mm and try again. If you still do not have enough air, drill a second 1/2mm hole near the first, refit the carbs and try again. Some people file the bottom of the throttle plates to get extra air flow. I do not recommend this as there is no way to assure you have filed the exact same amount off each throttle plate on each carb.

The final selection for idle jets should be based upon how the engine performs in the progression circuit over the progression band.

Before making the final testing for the idle jets make very sure the ignition is properly set up and functioning with the advance not starting until around 1200 RPM and properly advanced initial timing. With a modified engine the initial timing will probably be in the neighbourhood of 8 to 12 degrees BTDC with at total advance of 32 to 34 degrees. An ignition timing problem can be seen as a progression circuit leanness. Initial timing that is too retarded for the engine is a source of spitting out through the carburetor throats.

Slowly advance the throttle off idle and listen for any hesitation. If there is a hesitation the mixture is too weak. Adjust the idle flow screws out 1/2 turn and try again. If the hesitation is still there the jetting will need to be altered. On a TR engine if a F8 air bleed hole is too lean try an F9. If you are too lean with an F9 go up a jet size with an F8 air bleed hole. For other engines, try two steps richer on the air correction hole while leaving the fuel hole the same size. Reset the idle then retest. For non TR3-4 engines, if going to the richest level (F6) does not get rid of the hesitation, go to the next size larger fuel hole and rerun the tests with different air bleed holes.

When the no load tests are completed drive the car and retest under load conditions.

The ideal idle jet size provides an idle CO in the 2.5 to 3% range with the idle flow screws adjusted between 7/8ths of a turn and 1-1/2 turn and does not cause the engine to hesitate on the progression circuit. The next leaner air bleed size would cause a hesitation under load conditions. A richer jet will provide poorer fuel consumption. An over rich jet will not provide top performance.

If you have the throttle linkage connected at this time, the rods between the carburetor's throttle arm and the crank need to be identical in length. Different length arms will affect the carburetor synchronization as you move off idle.

Main jet/emulsion tube/air correction jet assembly

These three items form the main jet assembly that provides fuel to the engine once the throttle plates (butterfly valves) are open beyond the progression holes. The emulsion tube is a long brass tube with openings along the side. The main jet is a friction fit into the bottom of the emulsion tube. The air correction jet is a friction fit into the top of the emulsion tube.

There is a fuel passage that goes from the float chamber, through the main jet and into the emulsion tube. When the engine is not running the fuel level inside the emulsion tube is the height of the level in the float chamber.

There is an air passage from the small hole on the face of the DCOE into the float chamber, through the air correction jet and into the emulsion tube.

As the throttle plates move towards full open they cease to draw enough vacuum to pull fuel from the progression holes and start drawing fuel from the auxiliary chokes.

The vacuum at the auxiliary choke draws an emulsified air fuel mixture from the emulsion tube and out through the auxiliary venturis. As the emulsified air fuel mixture is drawn out of the emulsion tube, fuel flows from the float chamber through the main fuel jet into the emulsion tube to replace the emulsified air fuel mixture being drawn out of the emulsion tube.

Air is also drawn down through the air correction jet into the emulsion tube where the air and fuel are mixed.

Choosing the correct stack is essential for cruise and high RPM performance.

Main jets

The main jet controls the fuel mixture in the emulsion tube in the mid RPM range when the cruise circuit is activated. As the RPM range increases the air correction jet becomes more of a factor and becomes the dominant partner in controlling the mixture at high RPMs.

The main jets are numbered by the diameter of the jet opening and come in size steps of 5 hundredths of a mm. Too lean a jet can damage the engine through overheating. Too rich a jet washes the oil off the sides of the cylinder walls and causes rapid cylinder wall wear.

The ball park rule of thumb for picking a jet main jet size for a street engine operating at sea level is to multiply the venturi size times 4. The common main jet range for the engine operated near sea level is 135 for stock engines and normal street driving through about 150 for high performance race cars. 140 and 145 seem to be the most common sizes for high performance street and autocross.

At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down 1 main jet size for every 1750 to 2000 feet of elevation you go up. If you normally run a 140 main jet at sea level you would drop down to a 130 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted!

Air correction jets

The air correction jets only affect the top end performance of the engine. The larger the number on the jet the larger the air hole and the leaner the main jet runs at higher RPM.

If the air correction jet is too lean (too large a hole) the engine will miss near peak RPM. If the air correction jet is too rich (too small a hole), the engine will not produce optimum power. For testing purposes, find the largest diameter air correction jet that causes a high RPM misfire then fit a 10 to 20 smaller dia (richer) air correction jet.

The air correction jet number is their hole diameter in hundredths of a millimetre and range in size increments of 5. When testing, the minimum increment of changes should be at least 10 with 20 being the more common increment to notice changes.

Here is a guideline for approximate air correction jet selection for those of us without a large supply of air correction jets:

For stock to mild engines with a 5000 RPM redline where fuel economy is a strong factor, a good starting point for the air correction jet is figured by adding 50 to your main jet size.

As you progress towards a full race engine and higher RPMs the size of the air correction jets decreases (fuel mixture becomes richer), with a full race engine having an air correction jet size of about 10 or 20 hundredths of a millimetre larger than the main jet size.

For a modified street engine running 34mm chokes having a red line at or below 5000 RPM, adding 30 to 50 to the main jet size would probably be a good starting point for the air correction jet.

For a modified street engine running 34mm chokes and having a red line around 6000 RPM, adding 25 to 40 to the main jet size would probably be a good starting point for the air correction jet.

For a modified engine needing 36mm chokes to produce full power above 6000 RPM adding 10 or 20 to the main jet size will probably be a close starting point.

Emulsion tubes

As the name implies the emulsion tube is where air is mixed with fuel to form an air/fuel emulsion (fuel with lots of little air bubbles in suspension). The vacuum formed in the auxiliary choke draws this emulsion out of the emulsion tube and into the air streaming through the auxiliary choke where it is atomised into the air stream and delivered into the combustion chamber.

The emulsion tube affects the acceleration phase as the main jets are activated. If the emulsion tube size is incorrect the engine will not accelerate cleanly when the main cruise circuit is operating. The effect of changing emulsion tubes can be very subtle to detect. Emulsion tube operation is very sensitive to the fuel level in the float chamber. So you need the right size float valves and closely set floats for the emulsion tubes to work as intended.

Emulsion tubes differ by their internal diameters and the number, size and positions of the side holes. They are complex tubes where "just the right level of emulsification happens here". Their part number reflects the order in which they were developed and not any physical attribute.

The tube sizes are (in order of rich to lean):

F7 (rich), F8, F2, F11, F16, F15, F9 (lean). There are additional sizes.

Accelerator Pump Jet

The accelerator circuit consists of:

A fuel reservoir,

A mechanically activated, spring loaded plunger (pump) that flushes the reservoir,

A one way valve that lets fuel into the reservoir from the float chamber but not back out to the float chamber while the plunger is purging the reservoir (called an Accelerator pump intake/discharge valve),

And an accelerator pump jet that both meters the amount of fuel pumped by the acceleration pump (plunger) and delivers that fuel directly into the rear of the carburetor throats.

The metering hole in the accelerator pump jet has to be large enough to remove any hesitation or stumble caused by the lean condition created by suddenly opening the throttle plates (butterfly valves) at low RPM. Too large a jet will cause a "bogging down" of the engine from too much raw fuel.

The jets are numbered for their hole size in hundredths of a millimetre and are in five hundredths of a millimetre steps (i.e. 35, 40, 45, 50).

Accelerator pump jets commonly recommended for the four cylinder engine are 40 and 45. Use the smallest jet size that will eliminate any hesitation or stalling when the throttle is suddenly opened.

The accelerator pump intake/discharge valve can have a discharge hole that finely tunes the flow of the accelerator pump jet in-between the step increments.

Accelerator pump intake/discharge valve

This is a one way valve that allows fuel to flow into the accelerator pump reservoir and keeps the fuel from going the wrong way when the accelerator pump is activated.

It can also be used to precisely tune the amount of fuel injected into the engine by the accelerator pump jet . This is accomplished by selecting a valve with a discharge hole on the side. If there is no discharge hole, the accelerator pump intake valve acts purely as a one way valve. If there is a hole, part of the fuel is discharged out the side hole back into the float chamber when the pump is activated, bleeding off the excess fuel not required to accelerate the car cleanly.

The number on a valve with a discharge hole is the size of the hole in hundredths of a millimetre, i.e. a valve marked 50 has a 0.5mm discharge hole in the side.

Needle valve

The needle or float valve assembly regulates the amount of fuel allowed into the carburetor's float chamber and maintains the fuel level within a very limited range. The top stop of the float regulates the level of the fuel while the lower open stop regulates how far open the valve can open. The diameter of the valve regulates the rate at which fuel that can enter the chamber as the valve opens.

There are a range of needle valve sizes available for the DCOE. The needle valve needs to be large enough to allow an adequate flow of fuel into the float chamber but should not be larger than necessary. Too large a valve will let too much fuel in quickly before it closes and cause a pulsing over rich condition.

Float setting

The floats open and close the float valve. The closed setting regulates the fuel level in the float chamber and in the emulsion tubes.

Traditionally DCOEs came with soldered brass floats. Lately most if not all are being provided with plastic floats. The float settings are different between metal and plastic floats.

The float setting is dependent upon the carburetor you use. Here is a chart for brass floats (My carbs have brass floats): DCOE series

float valve closed

(mm)

float valve max open

(mm)

40DCOE, series 2, 4, 18, 22/23, 24, 27, 28, 31,32

8.5

15

40DCOE series 29/30

5.0

11.5

40DCOE series 44/45

7.0

14

40DCOE series 72/73, 76/77, 80/81

7.5

14

42 DCOE series 8

5.0

13.5

45DCOE series 14, 14/18, 17

8.5

15

45DCOE series series 9

5.0

13.5

45DCOE series 72/73, 76/77, 80/81

7.5

14

45DCOE series 38/39, 62/63, 68/69

5.0

14

Both floats need to have the identical setting. You may need to bend the arms between the two floats to get them exactly the same closed height.

The ideal tool to set the closed float position is a round rod with the precise diameter of the closed float setting. The seam of the float should not be taken into account when measuring the float level so there should be groves cut into the rod to clear the float seams. This will allow you to see both the accuracy of the setting and any variation between the two floats.

The open and closed measurements should be taken with the top gasket in place. The closed position should be measured just as the floats close the valve and not with the entire weight of the floats upon the valve. This is done with the top plate tilted a little over vertical.

Fuel pressure/ filtering

Webers need a pump that can provide a high volume of fuel at a low pressure. The fuel pressure should be regulated to between 1.5 and 2.5 pounds per square inch at high RPM and no higher than 3 PSI at low RPMs.

Luckily for us the the stock fuel pump that comes on our engines pump in this range when healthy. A fuel pressure regulator is optional with the stock fuel pump. If you fit an electric fuel pump you will need to fit a fuel pressure regulator. If you use an after-market mechanical fuel pump be sure to test the output pressure over a range of RPMs to determine if you need to fit a pressure regulator.

It is important that your pump supply at least 1.5 pounds pressure at high RPMs. If it does not it will require rebuilding or replacing.

Webers have a wire screen filter in the inlet. This filter does not have a good reputation for working well over time. Considering the size of some of the jet openings I suspect the built in screen is not really fine enough to prevent clogging. You should consider installing a high volume fuel filter between the fuel pump and the fuel regulator.

A word about fuel lines - Do not use regular worm gear hose clamps with steel braided fuel lines. The steel braided fuel lines are designed not to crush. You will end up with an iffy seal.

Mounting

DCOEs prefer to be mounted with a 5 degree upward angle and should never be mounted at a greater angle that 7 degrees above horizontal. They will not perform properly at a greater angle.

A new intake manifold should be checked for proper alignment. Preferably before you pay for it. If the studs are out of alignment the carbs will be out of alignment causing differences in the way linkage opens each carb and sync problems at some part of the throttle travel.

One thing you can check in the store is that the studs are at right angles to the carb mounting surface. Also if you have a DCOE handy you can slide it over the studs and assure yourself that the carb base will sit flatly on the manifold with no gaps.

It is harder to check four cylinder intake manifolds for stud alignment because you need two manifolds per engine. You need to test fit the manifolds to a head for alignment testing. Once fitted, lay a straight edge over each row of studs and look for an out of alignment stud. The recommended maximum of allowable misalignment is 0.25 mm (0.010 inches). If it is more than that you need to weld the hole shut, resurface the top, and align drill a new stud hole. Like I said, it is best to make this measurement before money changes hands or when you can return the manifolds for replacement or credit.

The top studs should protrude 38 mm (1.5 inches) from the manifold. The bottoms ones can be between 38 and 40 mm (1.57 inches).

DCOEs are susceptible to fuel frothing and should be installed with anti vibration mounts. There are a couple of kinds and each comes with instructions for proper tightening. If you use the kind with 'O' rings that sit in a groove be sure the 'O' ring doesn't slip when you are mounting them. That would guarantee an air leak in a venturi.

The DCOE comes with a built in return spring. For safety, it is a good idea to add another return spring for each carb what will work if the linkage slips. This will be required if you go racing.

The DCOEs full of fuel are weights sitting at the end of the intake manifold. Manifolds have been known to stress crack over time and vibration. Some racers add a plate between the two carbs and a down diagonal brace from the plate arm to an anchor point connected to an oil pan mounting bolt. The brace rod is usually connected to it's anchor point at each end with rubber gaskets to reduce vibration. This relieves the weight off the manifold head studs and keeps stress cracks from appearing.

Air horns and air filters

Air is not good at making sharp right turns and turbulence is built up at the sharp edges of a DCOE throat that does not have an air horn. This decreases the amount of air fuel mixture that can be delivered to the engine and disrupts the air flow within the throats. The air horns provide a smooth flow of air into the throats of the DCOE. The rounded edges of the air horn allow air to enter with a minimum of turbulence.

Air horns of some length should always be fitted to a DCOE if the carburetor is to perform correctly and provide the most air fuel mixture to the engine.

The distance between the intake of a cylinder and the beginning edge of the air horn is called "the run". As a rule of thumb, the shorter the run, the more top end power. The longer the run the more low end torque and low end throttle response is available. The length of the run is tunable by the length of the air horn. This is why there are a number of lengths available.

Which gets me around to air filters. While the air horns on a DCOE look very good and "racy" they should be covered up by an air filter. Your expensive engine will not last all that long without an air filter.

Sock type air filters should be avoided because they create turbulence that keeps the air horns from working properly and lowers the overall air flow into the throats of the carburetors. The worst you can do is a right angle air horn with a sock.

In general air filters seem to fit into two types. One has a short tubular type element with solid ends and the other has elements on all sides except the back plate.

The type with the short tubular element tend to be very good but take up a lot of space. For proper air flow into the air horns there should be minimum of two inches of space between the end blanking plate and the openings of the air horns.

The type that has filter elements everywhere except the back plate can be considerably closer to the air horn because air flows in from in front of the air horn.

These filters almost all have oiled foam elements. There are some very good ones and some very cheap not good ones. Stay away from the cheap filters that only have a single layer of one size foam.

Also, foam filters rely heavily upon their sticky oil coating to filter properly. Follow the care instructions that come with your filter religiously.

There is an air hole in the intake side of the carburetor above the throats that provides air to the float chamber and the various air bleed jets. It should not be blocked by the air filter backing plate and it should have the same pressure as the air horns get if you decide to build ram flow air box of some kind.

Timing considerations

If you just add DCOEs to an engine and start it up chances are that it will idle poorly and occasionally spit out the front of the carbs. Modified engines and engines with DCOEs need more than stock advance. With a cam a good idle speed should be 800-1200 RPM depending upon the duration of the cam. If the timing is too retarded the engine will not perform well below 3000-ish RPM unless you had very big idle jets. A way too rich idle jet can mask a lack of spark advance and reduce carb spittings but will drastically decrease fuel mileage.

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Guest Anonymous

Having had Mikunis for 100's of thousands of miles as oposed to Webers I haven't had to do much adjusting but with more time (and miles ) I'm sure I will and this is great. Thanks

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  • 14 years later...
On 12/15/2005 at 4:54 PM, dapbmw said:

proper method of installing a jet is to fit it only about one eighth an inch into the holder then allow the passage seat to push the jet in the correct distance as you screw in the holder.

 

Wow - never dawned on me. This makes so much sense. Here I have been trying to bottom out the jets in the holders before inserting them. Can't wait to try out the "right" way tomorrow.

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34 minutes ago, TobyB said:

current link with pictures

 

http://www.tr3a.info/WeberDCOEinfo.htm

 

t

 

 

This is super helpful. This is the only place I have ever seen it mentioned that the Idle Adjust Screw should not be used to set idle speed, but rather the venturi balance screws should be adjusted?! Do others subscribe to this philosophy? I get that you don't want to uncover the first progression hole, but not using the Idle Adjust Screw at all??

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Terinn Wakeman is well known to the Series Land Rover crowd.  She's a wealth of knowledge, interesting, and a pretty nice person to boot.  If you want to read a little about her and her Land Rover, go here:  https://www.adventure-journal.com/2019/05/meet-the-70ish-overlander-and-her-land-rover-that-rules-all-others/

 

I know it's not 2002 related, but she really has done some cool things to her Rover.  

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On 10/24/2020 at 9:14 PM, man_mark_7 said:

 

Wow - never dawned on me. This makes so much sense. Here I have been trying to bottom out the jets in the holders before inserting them. Can't wait to try out the "right" way tomorrow.

I checked and redid my jets and all four bottomed out.  Don't know if that statement about the jets is true  ???

 

 

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1 hour ago, jp5Touring said:

I checked and redid my jets and all four bottomed out.  Don't know if that statement about the jets is true  ???

 

 

It does and doesn't make sense. I'm not sure what would cause the idle mix jets to stop threading into their bores if the jets didn't hit the bottom of their wells? So while, on paper, it sounds like a good idea I'm not sure that they don't always bottom out anyway just due to the design.

 

On another note, I opened up the block off plate on my cold start mechanism and one of the block off pistons, on one of the carbs, was about half way up the bore. I had installed the Weber block off kit so I was surprised that when I pulled the pistons out of the bores all 4 of the o-rings were sitting in the bottom of their bores. I can't tell if this matters though. If I read the attached image correctly, the fact that I installed block off plugs in the cold start jet bores should(?) keep fuel from ever getting to the pistons(?) in the larger bores which are actuated by the choke mechanism?? I can't quite tell from this image how the fuel flow is routed.

 

image.png

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