Carburetor Tuning For Verner Engines

Carburetor Tuning for Verner Engines

Bing and Delorto Side Draft Slide Type Carburetors

Bing Altitude Compensating "Constant Compression" Carburetors

Tuning the Bing 64 altitude compensating carburetor

This procedure was taken from Ultralight News. Its author is unknown. FlyDivers has made a few small changes for clarity and to make the procedures generic, regardless of engine.
Required tools:
       A feeler gauge.
       A double vacuum gauge.
       Two 9 mm open end wrenches to adjust the M5 nuts on the throttle cables.
The following are the recommended steps to adjust the carburetors.

IMPORTANT: make sure cables are not effected by ANY engine movement, and that they move FREELY back and forth with no stickiness. It is also advisable to make all of the cables and housings that lead from a cable splitter to the carburetors the same length. This eliminates the possibility of differing bend radii tensioning at different rates as tension is applied to the cables. If one intake manifold supplies more fuel mixture, and as a direct result more power output than another, moderate to severe vibration will be induced. It is therefore critical that carburetors on multi carburetor engines be carefully balanced.

CAUTION: Ensure that your aircraft is immobilized by tying it to an immovable object. Wheel chocks and similar barriers are NOT sufficient.

First step is to
Make sure that the engine has been loaded with a properly adjusted propeller that limits the rpm tied down on the ground to the maximum static rpm recommended by the engine manufacturer.
Make sure that both throttle cables are allowing full opening of the throttle valve. This can be done visually with the engine turned off and full power applied. Also ensure that the cables are long enough to ensure that the throttle actuators will go all the way closed. This should be done with the idle screws all the way out counter clockwise.
With the engine turned off bring the throttle back to an idle. Adjust the idle screws counter clockwise until the throttle actuating arm is against the carburetor body.
Adjust the idle screws in until they just touch the actuating arms. Check to ensure the throttle cables are long enough to allow full closure of the throttle actuator. Now adjust the idle screws clockwise 1 1/2 turns.
Gently adjust the air mixture screws (air bleed screws) in all the way and then back them out 1 1/2 turns.
Have someone start applying throttle very slowly, Pre-adjust the cables at the fittings so both cables begin opening the throttle arms at the same time.
Start the engine and allow it to come up to the recommended EGT and oil or water temperature for your engine.
Adjust the idle screws equally until you obtain the engine manufacturer's recommended idle speed.
This procedure will normally synchronize the carburetors at an idle.
Adjust the idle mixture using the air mixture screws (air bleed) for best idle. Move each screw no more than 1/4 turn and wait for the engine to stabilize at the new mixture setting.
When best idle is achieved on both carburetors, readjust both idle speed screws equally to reset idle speed to recommended rpm.
Once you have done this bring the throttle to half throttle and measure the distance between the throttle activation arm and the carburetor body, they should be the same. This measurement should be done throughout midrange to ensure that the measurement is the same for both carburetors at all settings of the throttle. Note: Different engines will show vibration at somewhat different rpm, however, if the vibration is due to unbalanced throttle actuator cables, the vibration will normally show up early in the midrange as the throttle actuator just begins to move into midrange. Some mechanics advise leaving a small bit of slack (1/16") at idle so that changes in temperature and adjustments at midrange settings cannot affect cable tension at idle. Others prefer the cables to be adjusted to remove the slack at idle. FlyDiver prefers a small bit of slack because we will be fine tuning the cable adjustments in midrange and that may affect idle.
Advance to full throttle, both throttle arms should be in the full open position and the engine should achieve full power. Be certain to set the pitch on your propeller to limit the maximum rpm to that recommended by the engine manufacturer.

If there is a problem with the synchronization of the cables it can usually be Bing 64 Carburetor, Bing altitude compensating carburetors, Rotax 912 carburetors, Rotax 912 carburetors traced to movement in the outer throttle casing. That is when you are applying throttle instead of it moving the cable the outer casing is flexing or bending.

If this is the case you MUST secure, or route the cable so that all of the throttle movement is directed to the throttle arm. Again, making the cables and housings are the same length greatly reduces differences in throttle movement caused by the cables.

In the standard configuration you would use #60 and & #58 to adjust the throttle and choke set up with Bing carburetors.

Using a vacuum gauge

First perform a manual synchronization adjusting the Bowden cables so there is full travel from full open to full closed of the throttle valve arm, as described above.
Then remove the compensator tubes and install the gauges. Look for a small plug screw, left of the idle mixture control screw on the carburetor. Ensure that the plug screw is securely fastened
After you adjust the carburetors, the vacuum gauges should read the same at all engine settings.
Mercury column gauges are also made that will do the same job as the vacuum gauges.

Remember, it's the length and the tightness of the cables that adjusts the carburetor synchronization in midrange. When you're adjusting them, you're simply lengthening or shortening a cable, and making each throttle valve arm move the same distance at the same time.

Note how tight the cable is & how the ends of the cables have a
stiffener tube over top of the cable to prevent it from flexing.

The following procedure applies to either Bing or Delorto "slide & needle" side draft carburetors and assumes that the carburetors are correctly jetted for the altitude and outside air temperature range the engine is operating at. These carburetors have three air fuel mixture ranges, all of which must be adjusted so that both carburetors are set identically on a dual carburetor engine. The three ranges are idle, midrange (where the slide and metering needle are just beginning to move up and out of the emulsion tube) and high range where the main jet is the primary contributor to the air fuel mixture (metering needle is mostly up with only its tip still within the emulsion tube. Failing to balance the carburetors will cause one cylinder to produce more power than the other which in turn produces unwanted vibration. In higher horsepower engines the resulting vibration can eventually damage the torsional damping system, not to mention anything on the aircraft that might work loose due to excessive vibration.

With normally aspirated spark driven aircraft engines, the finally proof of correct tune is the spark plug color. A correctly tuned carburetor will produce a tan or light chocolate color on the plugs. You will sometimes hear this described as "rusty brown". It should be noted that most engine manufacturers specify a rich idle jet so that if you leave your engine running at idle for more than the few seconds it takes to shut the engine off, then you can expect to find the plugs a darker color tending toward blackened plugs.

Preliminary Adjustments

Ensure that the throttle cables going to the carburetors are the same length. This is critical to ensure that the carburetor slides move at exactly the same rate.
Ensure that the throttle cables are slightly loose (1/16") when the idle adjustment screws are all the way out so that the slides will go all the way to the bottom of the carburetor.
Screw both of the idle thumb screws and both of the air bleed screws in fully clockwise.
Adjust the throttle cables to each carburetor so that there is 1/16th inch of slack in each cable. Be certain that both carburetor slides are as far down as they will go.
Screw both of the idle thumb screws and both of the air bleed screws out 3 full turns counter (anti) clockwise.

Starting the Engine and Tuning the Carburetors

While testing the engine, it is recommended that the aircraft or test bed be tied down in such a manner that the engine and propeller are held stationary
- Wheel chocks and other barriers are not recommended for this purpose. Physically tie the aircraft to an immovable object.
Set the enrichers (chokes) to on.
Ensure that the propeller area is clear by looking and calling, “Clear Prop!” in a loud voice.
Start the engine
- Expect that the starter may have to turn over for approximately 30 seconds to fill the carburetor float bowls before the engine starts if no priming system exists.
- Crack the throttle open slightly if the engine is having trouble starting (4 stroke engines only).
- If a new engine, expect some smoke initially from the oil that was injected into the cylinders before first start.
- Allow the engine to warm up for a minute or so, then slowly reset the chokes to off .
- Allow the engine to idle until it reaches its recommended operating temperature.
Fine tune the carburetors and throttle cables.
- CAUTION: This operation is critical to ensure minimum engine vibration which, in turn, is critical to the life of your engine.
- After the engine is warmed, use the idle thumb screws to set the idle to the smoothest rpm within the range recommended by the engine manufacturer.
- Be certain to rotate each thumb screw exactly the same amount.
- When smoothest idle is established within the range recommended by the manufacturer, turn the air bleed screws in clockwise until the engine begins to run rough.
  - Air bleed screws should be adjusted for maximum smooth idle.
  - Use small increments of 1/4 turn or less.
  - Wait between each turn of an air bleed screw for the engine rpm to change. Changes are not immediately noticed.
  - When the engine begins to run rough, back the air bleed screw back out counter clockwise until a smooth idle is again achieved.
- Reset the idle speed to the lowest rpm within the recommended range at which the engine idles smoothly.
  - If the idle cannot be gotten to a very smooth running idle speed, the timing may need to be advanced or retarded. Your Owner’s manual will show you how to set the static timing.
- With the engine immobilized so that it cannot get away from you, run the rpm up into midrange (15-2000rpm on Verner engines).
  - If the engine is running at all rough in the range of rpm between idle and 50% throttle open, adjust the throttle cable adjusters at the top of each carburetor until you can achieve the smoothest running possible. There should be very little, if any vibration in midrange if the carburetor slides are exactly the same height
  - Test the entire range of the throttle for a smooth running engine.
    - Remember that adjustments with air bleed and idle screw affect idle only, throttle cable adjustments affect mostly midrange and that high range is mostly affected by the main jet.
    - If at any rpm other than idle, you find a rough spot, readjust the throttle cables.
    - If the engine still has rough spots, particularly in mid to high range of the carburetor, you may need to change either needle settings or jets.
      - Dellorto metering needle K45 is a 7 slot dual tapered needle that can be ordered from your Verner dealer. These needles offer the widest range of mixture adjustments for mid range. Dellorto maintains an excellent carburetor tutorial on their web site at http://www.dellorto.it. Click on the “Customer Area” and select “documentation.”
  - When finished with midrange adjustments, tighten the throttle cable adjuster stop nuts (5mm).
  - Recheck full range of throttle to ensure minimum vibration. Then shut off the engine, allow it to cool until you can hold you hand on the cylinders, then pull the spark plugs and check the color to ensure the mixture is correct. Move the metering needle "C-Clip" toward the pointed end of the needle (bottom slot) to obtain a richer mixture or toward the top slot to lean the mixture. If changing the metering needle position will not produce the correct mixture, a jet change is probably necessary. When changing jets, keep in mind that idle and main jets plus the combination of emulsion tube and metering needle all affect mixture at different rpms.
General Troubleshooting Techniques
Trust the color of the spark plugs! This is an absolute must. Gages, probes, sensors and accurate measurements are all subject to both human and equipment error but the plugs never lie.

Spark plug insulators should be a nice even chocolate brown at midrange and high range with perhaps a small dark spot on a side facing a manifold.
Light gray to white indicates a lean mixture (too much air -or- not enough fuel)
Dark gray to black indicates a rich mixture (too much fuel -or- not enough air)
At idle, it is common for manufactures to specify an idle jet that supplies an intentionally rich mixture. In this case a dark gray plug color will be normal but excessively black or damp plugs indicate too rich a mixture.

General Assumptions

Air bleed, idle speed adjust, idle jet will ONLY affect idle rpm and mixture. Adjust at idle only.
Enricher jet and control can affect all rpm ranges because it directly injects fuel into the carburetor throat
Metering needle position, slide position, main jet, emulsion tube and the midrange mixture adjustment on carburetors such as the Bing 64 (above) only affect mid and high ranges
If you must err, err on the side of rich. Rich mixtures waste more fuel but nearly always allow the engine to run cooler.

Factors producing lean mixture conditions:

Too much air (air leak)

Leaks in fuel lines at any fuel hose connection or in the hose itself. Ordinary hose clamps are notorious for cutting plastic fuel line (there are special hose clamps that don't cut).
Leaky fuel pump
Air leak in the intake manifold. Rubber carburetor boots (grommets) are notorious for developing hidden cracks along the edges of the clamping rings and/or anywhere on or inside the surface of the rubber. These items should be considered annual replacement parts despite their outrageous cost.
Improperly positioned metering needle (C clip too far away from the pointed end of the needle)
Improperly adjusted mixture control

Air bleed idle mixture adjust screwed too far in (idle only)

Too little fuel (blockage)

Jets that are too small (the smaller the number on the jet the smaller the hole in it)
Dirty fuel filters

Note that many carburetors, Dellorto included, have a fine mesh filter at the fuel inlet. In the Dellorto case, the filter is also a gasket and cannot be removed without destroying it. A judicious application of Gumout or other carburetor cleaner will suffice to clean any accumulated contaminants.

Inadequate fuel pressure.

A leaky fuel pump can introduce air to the fuel system and as a direct result of the air leak fail to supply adequate fuel pressure compounding the problem.
A fuel pump that does not produce enough pressure (most often observed at highest rpms)

Dirt or any contaminant any where in a carburetor's jets, emulsion tubes, metering needle or float bowl that restricts the flow of fuel

Low compression

This might be counter intuitive but low compression causes more of the available energy to be sent out the exhaust rather than transmitted through the piston. If everything else in the carburetion system is nominal the net result is excessively high EGT since the bulk of the energy is heat.

Octane rating too low for the compression. This actually produces pre-ignition (pinging) but the symptom resembles excessive lean condition and the net result is excessively high EGT and CHT which can do severe and permanent damage to pistons and valves.
Timing

Spark plug gap too wide -- This actually produces a timing retard because it takes longer to build enough energy for the spark to bridge the gap. Secondarily, the spark is physically larger causing an abnormal burn.
Static timing retarded resulting in late burn.

Factors producing rich mixtures

Insufficient air flow

Dirty air filter

Excess fuel flow

Float bowl needle not stopping fuel flow when bowl is full

Dirt obstruction
Improper float setting

Leaky floats that can't rise high enough to close the fuel float valve
Incorrect jetting (jet bigger than specified for altitude, humidity and temperature) or emulsion tube too big. Remember the 3 Rs here: High, Hot, Humid. If any of the 3 increase, then mixture must be leaned to reduce the amount of fuel getting though the carburetor.
Incorrect metering needle placement
Incorrect setting of mid range mixture adjust