Carburetor Tuning for Verner Engines
 

Bing and Delorto Side Draft Slide Type Carburetors    

Bing Altitude Compensating "Constant Compression" Carburetors

General Troubleshooting

Jetting Recommendations

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 

Gages, probes, sensors and accurate measurements are all subject to both human and equipment error but the plugs never lie. Lycoming test results are published indicating that setting exhaust gas temperature (EGT) to 100 degrees Fahrenheit less than peak EGT on the rich side of peak EGT will result in good fuel economy and best longevity. There are a number of anecdotal reports indicating 100 degrees fahrenheit lean of peak EGT will give best fuel economy but FlyDiver, llc has not found any manufacturers recommending that setting. Additionally many states have mandated 10% ethanol in auto gas which eliminates the ability to use spark plug color as a test of correct mixture on carburetors that do not have leaning adjustments.

Several things appear to happen with ethanol laden fuel:

1. Ethanol can absorb water. This is a very serious issue that can cause carburetor icing and corrosion of internal engine parts, particularly in engines that sit unused for extended periods of a week or more.
2. Overall power ratings will be reduced since alcohol does not generate as much power as gasoline.
3. Alcohol has the potential to seriously deteriorate plastic, fiberglass and synthetic rubber components.
4. Alcohol in gasoline tends to make the spark plugs appear to have a very lean condition. At correct mixtures, the ceramic insulators will be nearly pure white with only a slight reddish tinge near the metal components of the spark plug. Since the plug color no longer indicates correct mixture, the only known method for determining correct mixture is to lean until peak EGT is achieved and then reduce EGT by 100 degrees in the rich direction.

Where ethanol free gas is used:

• 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 and 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 out (too much air at 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 usually 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

Jetting Suggestions

A note on alcohol laden gasoline. Since most states mandate the use of alcohol in auto gas you are probably affected by it.

1. Alcohol will absorb water from open air. In an aircraft carburetor the presence of water greatly increases the possibility of carburetor icing. Precautions include: Always using fresh gas; not leaving gas in your aircraft that is older than 1-2 weeks; If flying every few days, keep the fuel tank topped off after every flight to reduce the volume of air space inside the tank; Filter all autogas through a Mr. Funnel as it is added to the aircraft fuel tank.
2. Alcohol is corrosive. It is known to soften some plastics and resins and will eventually corrode aluminum parts if left standing long enough.
3. Adding alcohol to gasoline will probably require a jet change due to the different burn rate and burn temperature of alcohol.