Hi-altitude Carb tuning....help please....

[73c20jim]

THE OBJECTIVE: BEST FUEL ECONOMY AT THESE ALTITUDES.

THE TRUCK:

1973 C20 Regular Cab, 8200 GVW, PS, AC, TH350,

195 net/260 gross 350 CID GM Goodrich crate motor, stock exhaust manifolds, true dual exhaust, HEI dizzy

Edelbrock 1405 600 CFM manual choke, square-bore carburetor, sitting on adapter plate to stock spread-bore manifold

THE ENVIRONMENT:

Home base: 4900 ft. 75% of driving
Lowest: 3500 ft. 20% of driving
Highest: 5600 ft. 5% of driving

I would appreciate any input on carb setup, timing, vacuum advance to achieve objective at these high altitudes.

Thanks in advance.

 

[73c20jim]

oH! No smog stuff.
Will tune engine as well.
rotor, cap, wires, plugs

 

[chengny]

THE ENVIRONMENT:

Home base: 4900 ft. 75% of driving
Lowest: 3500 ft. 20% of driving
Highest: 5600 ft. 5% of driving

I would appreciate any input on carb setup, timing, vacuum advance to achieve objective at these high altitudes.

Thanks in advance.

Modifying the carburetor and timing settings - for high altitude applications - has more to do with performance than fuel economy. It is not an exact science, but the general guidelines are to lean out the carburetor and advance the ignition timing.

I couldn't find a number for the elevation that GM considers to be "high altitude". But a few spot checks on various model years indicated that for H/A applications, they specify an additional advance of 6° (before normal base timing spec). That would pretty much agree with the conventional wisdom of 1° advance for every 1000 feet of elevation above sea level.

As for vacuum advance, that is pretty much a function of the mechanical advance and tracks along with it. If your distributor has an adjustment screw in the actuating can, you can try to increase MPG a bit by using the screw to advance the timing while the vacuum system has control. Be aware that overly advanced timing can cause pinging even when in cruising (vacuum controlled) conditions.

As for leaning out the carb's mixture (to compensate for the decrease in air density/oxygen content at H/A's), that will require changing rods or rods & jets. Edelbrock suggests a 2% leaner mixture for each 1500 feet of elevation over sea level. In your case that would be about a 6% leaner mix (5000 ft /1500 X .02 = .06). When you have that percentage, refer to the manual for the recommended changes in rod/jets using the applicable chart. Here is the chart for the 1405 model;

You must be registered for see images attach

 

[73c20jim]

Thanks for the excellent reply. I think I'll go with #27 on the chart.