Carburetors and Altitude

[Greg Hermann]

At 2:09 PM 12/9/01, efi_student wrote:
>Never been through the Eisenhower tunnel in Colorado on I-70 I see.  Nor
>Wolf Creek Pass.  Both are well over 6,000' above MSL.  My 79 AMX with
>its marvelous Stromberg 2 barrel on an I-6 crawled through the
>Eisenhower tunnel at 35 mph even with 5 degrees timing advance.  Jetted
>down to mains, and flew through at normal driving speeds.  IME
>Carburetors have a dismal time compensating for altitude.  I have tuned
>both cars and bikes with carburetors for living in Denver after living
>in Sacramento.  Sac is about 12' above MSL, Denver is well known as the
>mile high city.  Oddly, I needed bigger pilots and smaller mains to work
>properly at altitude.  But then again, I was dealing with carburetors of
>a lesser design than the Webers.  Mikuni flatslides, Keihin CRs, and the
>venerable Stromberg 2bbl are far less advanced.
>
>Lance
>
Lance--you are quite correct. See my earlier post. The main jet diameter
needs to vary with the fourth root of the air density ratio in order to
maintain a constant A/F ratio from any carby.

Your need for a slightly richer idle jet is related to having less vacuum
at idle at altitude--since the idle circuit uses manifold vacuum (as
opposed to the choke, or venturi) to meter the fuel. However--this isn't
usually a problem IF you get the relationship between the transition
slot/holes and the edge of the butterflies correct at idle.

EVEN with the correct A/F ratio and timing at altitude, you are going to be
down on power slightly less than would be indicated by the air density
ratio at altitude. (Slightly less because the less dense air does not lose
as much static pressure flowing through the ports at altitude!)

Greg (home elevation, Steamboat Springs, =7250' MSL)

(Loveland Pass, Trail Ridge road, Mt. Evans road, and Independence Pass are
even more fun that way than the Ike tunnel is, too!) :-)


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