|Adjustments to Your Car for Altitude
by Ed Barnard
The problem: Degradation of your
when traveling from an elevation of approximately 630' above sea level
to 9,600' above sea level (Breckenridge) with climbs to 11,000 + above
the carburetors are set for
sea level performance, the proper fuel/air mixture (stoimetric ratio)
established. As you increase altitude, the density of the air
is less oxygen available and the fuel/air
ratio becomes richer; too much fuel for the decreased amount of air.
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
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'
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
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
the mixture is correct. If on the other hand the rpm's immediately drop
mixture is lean.
Early Stromberg carbs: Early Stromberg
carbs are like SU's; the jet is adjusted through a knurled knob on the
Screw in to lean; out to richen. Same principle as above, but, you have
flats to count. Move in lA turn increments. Many early
Stromberg carbs used a similar lifting pin as SU's.
carbs: These later carbs have non-adjustable jets and adjustable
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
your adjusting screw will rotate freely a 1/8" Allen wrench can be
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
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
10 clicks on the knob makes a degree of advance. Adjust to where your
smooths out (remember the number of clicks) then test drive. If you
pinging or knocking just retard the timing. Usually you will adjust the
speed a little to compensate for the timing increase.