DIY_EFI Digest V5 #926

[Clare Snyder]

> 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!) :-)

Independence Day Pass is one of 2 places my injected Aerostar van had trouble
hauling my 17 foot trailer - it is definitely a doozey - and would be even
starting at sea level!!!

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