OK, so this is way too early, since I know I'll forget by next June....

What effect does altitude have on carburetion exactly? Well, OK, not exactly, but I hope you get my drift. As I understand it, there is lower air pressure, therefore the air is described as "thinner". If that is the case, I would expect less oxygen flow to a carbureted engine, therefore it would run richer. Am I correct?

I'm currently at an elevation of just under 800 feet above sea level. Colorado Springs is just over 6000, giving a difference of about 5200 feet. Now, I'm not terribly worried about my truck, which is diesel and computerized, nor am I hugely concerned about the Wildcat...it will be in the trailer behind the truck, so won't have to drive too much.

However, we are considering Suzanne driving another car...she would get the driven award with it and it would be a chance to show off another car. However, that car would need to be able to function relatively well in the Colorado elevations. If we were to go this route, what suggestions would be made to give a temporary improvement at elevation? Ideally, it would be something simple, like turning an idle screw or the like. Since it would be a driver, and would need to return to Winnipeg, I would need to be able to adjust it easily on the road or have everything set up for the entire trip. Reality is, if the car won't run well, and I can't easily deal with it, I will most likely forget about it and just bring the Wildcat in the trailer.

Then again, I could just be worrying too much.

Derek,
I wouldn't worry about it a whole lot. I spent time coming back from Seattle in altitudes to 10,000 ft while in Yellowstone and the Rockies and didn't notice any real difference in driving the 64 Riviera. It had enough power to do anything I wanted. I dont think you will notice any difference at 5-6000 feet.

Now saying that, I spent time earlier in my life in a Chevy van with a 6 banger and it choked itself when it got above 8000 feet. But there is no magic screw you can turn, you have to rejet the carb to make up for the thin air.

My 2 cents worth...

I can answer approaching from a modern car's perpective, where a computer monitors and constantly adjusts air/fuel ratios. I'm in Detroit, driving to California in a 4-valve Escort. It 'breathes' real well. Driving in the Rocky Mountains requires EXCELLENT brakes, and lots of horsepower. This is no joke. Aside from very long, steep, ascents, you get to descend for many miles, seemingly forever at times. As you ascend, temperatures drop because barometric pressure drops, and oxygen decreases.

Water boils at a much lower temperature in the Rockies. Grab any cake mix, and read the 'special instructions for high altitude.' Eggs take 1/2 hr to hard-boil because they never get hot enough; the water vaporizes first. This is a real problem for antique cars that don't have pressurized radiators. Check out this boiling point chart:
Altitude - Temperature
Sea Level - 212 degrees F
2,000 ft. - 208 degrees F
5,000 ft. - 203 degrees F -Denver, Colorado
7,500 ft. - 198 degrees F
10,000 ft. - 194 degrees F

Even at home, carbureted cars rarely have 'sweet running' days because optimum performance was setup at certain humidities, temps, and altitudes. At 6,000 feet, your car will seem a little sluggish, like you left the choke pulled out, but not too bad. Climbing will take more pedal, but the engine will seem like it went down 100 cu in. When you get to 12,000 feet, you will probably see some black smoke from a real rich mixture, as you climb. Half your HP will be gone, and you will have your pedal to the floor for many miles. This is where trucks are in low gear, engines screaming, and crawling in the right lane. There just isn't enough air, and pushing the pedal down delivers more fuel to an already choked carb.

People living in the mountains used to re-jet with smaller, leaner, jets. Their problem is worse when they come down to sea-level; their engines run lean, causing detonation, excessive heat, and pre-ignition. CLACK-CLACK-CLACK.

Running rich isn't so bad, it's just expensive to buy fuel you can't use, efficiency goes to hell. Your car will start ok, because it's already choked.

The real caution is brakes. Descending, I put my air-conditioner on 'full blast,' opened all the windows, turned on all my lights, and down shifted. The engine still wanted to 'run away.' Every so often I sparingly tapped the brakes to get back down to 80 MPH. I drove behind a new Chevy Silverado pulling a new horse trailer (loaded) and all I could smell was his brakes the whole time.

Will omit talking about the reasons for the issues, and fast forward to the solutions.

Using the Carter Carburtor Company as an example, because I have more factory documentation, and have studied Carter more than the others; but Carter is fairly representative.

Carter carburetors were calibrated as follows:

Sea level to 4000 feet - standard calibration
4000 - 6000 feet - 1 size lean
6000 - 7000 feet - 2 sizes lean
7000 - 8000 feet - 3 sizes lean

Except for a very few military applications, Carter did not specify calibrations above 8000 feet.

As "simplyconnected" mentioned in his post; the problems of trying to use a carburetor that has been calibrated for altitude at much lower altitude can cause damage to the engine.

For those who live at altitude and do all/most of their driving at altitude, it makes good sense to recalibrate the carburetor.

For those "just visiting" altitude, probably just carry a couple of spare sets of spark plugs.

For those who really want the best of everything, using Carter (metering rod) carburetors; Carter generally changed metering rods rather than jets, as this would maintain the proper calibration at all levels of vacuum. Changing rods in Carter carburetors is quite simple. In fact, while not recommended, I can change the rod(s) in most Carter carburetors while the engine is still running. It takes longer to remove/replace the air cleaner than to change the rods.

Even though Rochester used metering rods, the rods have a fixed low vacuum calibration; thus Rochester changed jets instead of rods for recalibration. Autolite also changed jets.

Holley and Stromberg changed both jets and power (economiser) valves.

Jon.

Now you have me a bit concerned about doing a PreWar Div. AfterTour.

Please correct me if I am wrong, but I think the Marvel updraft carburetor high speed jet needle valve can be adjusted to work in Colorado Springs.

Right Carb King?

Mark - the Marvel does have an adjustment; however the airvalve is controlled by a pretensioned spring.

I personally would not try it, but that is just one opinion.

Recalibration of carburetors for use with ethanol

Jon.

Actually the air valve is also adjustable, but you are right about the special spring inside the big air adjustment knob. It is a very special light weight spring. I have seen several different shapes and sizes from various Marvel carbs.

It would be good to hear from somebody at high altitude on this one.