Adjustments to Your Car for Altitude

by Ed Barnard

The problem: Degradation of your cars performance when traveling from an elevation of approximately 630' above sea level to 9,600' above sea level (Breckenridge) with climbs to 11,000 + above MSL. If the carburetors are set for sea level performance, the proper fuel/air mixture (stoimetric ratio) has been established. As you increase altitude, the density of the air decreases. There is less oxygen available and the fuel/air ratio becomes richer; too much fuel for the decreased amount of air. The needles/jets need to be adjusted to bring back the proper ratio.

  • The average loss is approximately 10-12% power for each 5000 foot increase of elevation. Will it really be a 20-25% loss in power? You may not feel it unless the car is under heavy load conditions.
  • We need a temporary fix for a temporary problem. You may choose to do nothing since you will need to "undo" whatever you do once you come back down in altitude.
  • Be prepared to find only lower octane fuels available. Problems will be compounded by lower octane fuels (85, 87, and 91). Fill your tank before leaving home and only get required amount of fuel while there. Consider octane boosters.
  • Be prepared for your car to run hotter. Same problem.. .the air is less dense, so has decreased ability to reduce heat through convection. What will help is that air temperature drops 3 degrees for each 1000' gained.

SU carbs: In the SU carbs normally found in Triumphs you adjust the jet, not the needle. The adjusting nut is 5/16 W, but a 14 mm wrench works well. Turning the nut clockwise (CW) moves the nut up and the jet deeper into the carb body, leaning out the mixture. Counter-clockwise (CCW) moves the nut down and pulls the jet out, richening the mixture. We measure the amount of movement in "flats"; a nut having six flats around it. You can also count "peaks". One flat or peak equals 1/6 of a turn. Keep track of the number of flats you move the adjusting nut. When you run the flats up on an SU, you lean it out for the entire rpm range, not just idle. If you lived "at altitude" you might want to get the needles listed for "high altitude" rather than lean it out.

One method of checking the mixture on an SU is by slowly pushing the lifting pin up, which raises the piston. If the rpm's increase considerably the mixture is rich. If the rpm's increase very slightly the mixture is correct. If on the other hand the rpm's immediately drop the mixture is lean.

Early Stromberg carbs: Early Stromberg carbs are like SU's; the jet is adjusted through a knurled knob on the bottom. Screw in to lean; out to richen. Same principle as above, but, you have no flats to count. Move in lA turn increments. Many early Stromberg carbs used a similar lifting pin as SU's.

Later Stromberg carbs: These later carbs have non-adjustable jets and adjustable metering needles. In a perfect world special tool B20379Z (Moss P/N 386-310) is used to do this. The Allen wrench turns the needle adjusting screw while the outer sleeve prevents damage to the diaphragm. If you are sure your adjusting screw will rotate freely a 1/8" Allen wrench can be used. Turning the wrench CW raises the needle and richens the mixture, while turning it CCW lowers the needle and leans the mixture. Avoid over turning this screw CCW because the needle may become disengaged from the adjuster. Limit your adjustments to 1-1 Vi turns.

Unlike a Weber carb, an SU or Stromberg carb can only be "adjusted" for fuel and not air correction. If you lean it out to bring it back to stoimetric. it will loose power because it has less fuel. When you make adjustments on an SU or Stromberg. you lean it out for the entire rpm range, not just idle.

Consider advancing the timing a few degrees. The adjustment on the 25d distributor used in TR3-TR6 up to 71' makes it easy; 10 clicks on the knob makes a degree of advance. Adjust to where your idle smooths out (remember the number of clicks) then test drive. If you detect any pinging or knocking just retard the timing. Usually you will adjust the idle speed a little to compensate for the timing increase.

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