Green Sky Adventures, Inc.    of North Central Florida
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High Altitude Compensator Installation and Function Details
Please Note: Automatic HAC systems are no longer available. This page is for reference only. For mixture control for Bing carburetors on 2 and 4 stroke applications check out our HACman

The BING HAC can be installed on existing Bing Model 54 carburetors only if, at least one of the carburetors is equipped to accept Nipple, 15 as a pressure source. Such carburetors come standard with a tapped hole, plugged with a large slotted head screw.
A previous production series of carb bodies was cast to accept the HAC but not drilled or tapped. People with modest machinist abilities, can modify this series carb body in the field. Click HERE for images

This diagram was edited by Green Sky Adventures, Inc., 
from the Rotax Illustrated Parts manual available on line at 

  1. The HAC Chamber, 2 must be remotely mounted, and isolated from heat and vibration. This may vary depending on cowling fit and other considerations. Attachment to the Airfilters is generally a good location. On Liquid Cooled engines, we mount it vertically from the coolant line which passes just above carburetors. (plugs up installation)
  2. Remove the plug screw. Apply a single drop of Loctite 222 or 242 to Nipple, 15 to assure sealing and fit integrity, then install. Do NOT over apply the sealant. 
  3. *Float Bowl Vents 12 must be modified by cutting the center section (the part with the holes, not shown) out and fitting Tee, 11. Note: replace Vent Lines 12, if condition warrants. 
  4. An ample supply of tubing, 10 is provided. It is 3/16" ID Heavy Wall Polyurethane. Cut the tubing to necessary lengths. Attach coupling, 7 to a section of line, 10 that  is depicted at left as a curved piece.  The line then connects to the static source port on the HAC chamber
  5. Drill a 3/16" hole in the back side of the K&N Airfilter and insert the second end of Coupling, 7.
  6. The original "Factory" HAC kit did not rely on clamps for the tubing connections,  nor are they provided with our kit. 

*Plumbing Revision was implemented April of 2004. Vent Tee's are eliminated and Carb Vent lines connect directly to the HAC Chamber. See HAC Plumbing 2004 for additional details
Checking Calibration
Please note, the following information is provided as a supplement to the normal, simple installation process. Green Sky Adventures, Inc. ships HAC kits calibrated, and ready to install. Simply change the appropriate main jet, and that's it. However, if there is ever a reason to doubt the function of the HAC, or if one wishes to perform a periodic inspection, The information below is "must have"

Early HAC chambers were shipped with instruction to verify calibration upon initial installation and annually. Within a few years, field experience indicated calibration from "factory"
settings does not change (unless a failure occurs). The previously removable screw 2, above, in current production, is sealed over. Calibration can still be checked, but adjustment  cannot be made without removing the sealing material and screw. 
Variation from proper calibration  can alternatively be corrected by custom jetting change. For that reason you may see conflicting recommendations for jetting. 
Calibration check requires: 
Density Altitude information and  the corresponding HAC metering pin position information. (see table, below) 
Because the metering pin moves with
changes in Density Altitude, it's position is referenced to that and can be measured. Metering pin position measurement is taken by measuring the space between the top of the metering pin and the end of the vacuum connection pipe. 
(Dimension E) Example at right: 
After determining Density Altitude and the corresponding Dimension E, a wooden insert was cut to that length and dropped into the vacuum connection pipe. It should be flush with the top. Allowable variance is plus or minus .5 mm. 
If the metering pin is out too far,  (dimension E too small) the sealed screw must be loosened to bleed pressure from the sealed side of the chamber.

If the metering pin isn't out far enough (dimension E too large), loosen sealing screw, and apply slight suction to the vacuum connection pipe with the remaining tubes blocked off. Close the sealing screw. At this point, the metering pin should be almost to the top (Dimension E = 0) then, just as above,  loosen the (previously) sealed screw, gradually,  bleeding out pressure till the pin goes back down and dimension E is correct.

Finding Density Altitude
Density Altitude is a calculation, based on Altitude, Temperature, Atmospheric Pressure (altimeter setting), and Dew Point. You can get the 
"current weather conditions" for your area. in about 30 seconds, from the National Weather service. 
You will get data as shown at left.
Armed with that data, click on the Density Altitude Calculator and by inputting the data as in the example below, you will find the DA just as quickly. (you must also know the altitude of the reporting station)
Metering Pin Indexing Table
Dimension E (mm) / Density Altitude (ft)
 12.0 mm 
Max Diaphragm deflection
6.0 mm
12.0 mm
5.5 mm
11.5 mm
5.0 mm
11.0 mm
4.5 mm
10.5 mm
4.0 mm
10.0 mm
0 sea level
3.5 mm
9.5 mm
3.0 mm
9.0 mm
2.5 mm
8.5 mm
2.0 mm
8.0 mm
1.5 mm
7.5 mm
1.0 mm
 7.0 mm
.5 mm
6.5 mm 8,038
0 mm

Density Altitude Sample
We worked a sample on Mar 10, 2003 out of Gainesville Florida. Above is the view of the National Weather Service input, and the the DA calculator results. As luck would have it, on this morning, the DA calculated out to 0 feet msl.

Jetting changes are required. There are a variety of conflicting data in published charts. In setting up an engine for HAC operation, a technician must take into consideration the default is generally towards rich, meaning the HAC is continuously leaning the mixture. Initial jet selection therefore is larger. Below, is a recommendation which may be used as a starting point. This may not ultimately be the final combination of jets for a particular installation. As always, careful monitoring of EGT is mandatory, especially during set up process.


Measuring Deflection
Sea Level
4,700 ft msl
Mar 2003: In the past few months, Green Sky Adventures, Inc has performed numerous actual altitude tests of HAC chamber function. Testing was performed to verify HAC function against a published calibration chart. Field reports indicated HAC function at density altitude of 0 feet msl and below was incorrect. Prior to this date, the remedy has been to increase Main Jet size significantly.  Effective Mar 2003, All HAC units supplied by Green Sky Adventures, Inc. are checked for conformity to published calibration chart 2S 89-E
 Important Clarification and Jetting Note: Standard 503 Dual Carb without air intake silencer, and without HAC was changed around 1992.  Old jetting used an 8L2 Jet Needle in Position 1 and a 2.74 Needle Jet.  On that vintage, when HAC was added, the only jet change was the main jet. It went from 158 to 185.  In the early nineties, the standard mid range jetting was changed to the current 11K2 Needle in position 2 and a 2.70 Needle Jet. This setup yielded about the same mixture, but would allow for Needle Clip adjustments, as required. (notice the early version had the clip at extreme top position leaving no available adjustment) When the jetting change was made, that change was never reflected in the  503 DC HAC jetting chart. Consequently, those who changed to the HAC setup on late engines have been reverting to the older version mid range jetting for no valid reason. Initially, HAC installation should only require Main Jet changes. Close monitoring of EGTs may indicate other changes, but only on a case by case basis. If other mid range jetting is effectively in use, initial change of the Main Jet, only, is the general recommendation.

  3.  Slide or Piston
  4.  Jet Needle Clip
  5.  Jet Needle O-ring
  6.  Spring Cup
  7.  Slide Spring
  8.  Carb Top O-ring
  9.  Carb Top
10.  Carb Top Screws
11.  M6 Jam Nut
12.  Cable Housing Adjuster
13.  Cable Grommet
14.  Mixing Tube
15.  Sieve Sleeve
16.  Float Needle
17.  Float Needle Clip
18.  Floats
20.  Float Hinge
21.  Float Chamber Gasket
38.  Idler Jet
39.  Needle Jet
40.  Main Jet
      Float Bracket