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  • How much elevation change before jet changes?

    How much of an elevation change or climate change does it take for a properly tuned carbed car to need jet changes and other carb tuning?
    I have only traveled once with a carbed car up the east coast a couple states. Not much elevation change. No tuning needed that trip. Reading the updates on the LSX shootout trip got me wondering about that. I really like the simplicity of the carbed stuff. Don't mind doin' jet changes and stuff. Just curious.
    Bakersfield, CA.

  • #2
    Re: How much elevation change before jet changes?

    I'll be watching this tread too. Good question. Dave should field this one...

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    • #3
      Re: How much elevation change before jet changes?

      I don't know about a "properly tuned carbed car", but I never have changed jets on my travels in my junk...and they usually include going from 4500 ft to 1000 or less. Sometimes up to 9000.

      My fabulous web page

      "If it don't go, chrome it!" --Stroker McGurk

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      • #4
        Re: How much elevation change before jet changes?

        Thanks Squirrel. Maybe that has more to do with humidity then?

        I had a '66 Mustang coupe a few years ago. Bolted on an out of the box 600 cfm holley. Made my adjustments and it ran great... in California. Moved to Georgia a couple months later and towed the Mustang. Went to start it up and it ran like complete crap until I made some carb adjustments. Then I could drive it. It still ran rich, but good so I grabbed a jet kit a swapped to about 4 sizes leaner. Ran better and cleaner on the top end aside from the different everyday climate of the south.

        Any ideas why it ran that way? Is it the change in air density or humidity?

        You are running Holleys on that '55 right Squirrel?
        Bakersfield, CA.

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        • #5
          Re: How much elevation change before jet changes?

          Yeah, holleys. I've also not noticed much difference with Qjets. Although idle speed increases at lower elevations. As does pinging if compression is on the edge.


          My fabulous web page

          "If it don't go, chrome it!" --Stroker McGurk

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          • #6
            Re: How much elevation change before jet changes?

            There's more then 1 thing that effects the carb. We did a TransAm race in Denver one week and a race in Houston the next weekend and didn't change jetting.
            Cognizant Dissident

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            • #7
              Re: How much elevation change before jet changes?

              So what is it that will make a car run badly from one kind of terrain to another? For instance, do any of you Power Tour guys have any issues with tuning along the way?

              Here is what I am trying to settle in my own mind: I like old, simple, carbed vehicles. I enjoy the simplicity and underhood cleanliness that can be achieved by just bolting on a single mixer to a simple engine. I enjoy the simple cause and effect of a carbed vehicle diagnosis.
              The only thing I don't like about the old school way is that I thought it was not as versitile as EFI for long drives through varied weather and terrain. As I get older, it seems I am spending more time on long trips in dull, boring EFI'ed car with unwavering drivability (so long as everything works as designed). If I knew that a carb could yield even most of the drivability of a late model EFI vehicle, this concern would pretty much go away. Can anyone help me with this thought bias I am trying to reason through?

              I don't mind having to get out there and make adjustments on a daily driver even once a week, but on a road trip the last thing you want to think about is the reliablity of the vehicle you are driving.
              Bakersfield, CA.

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              • #8
                Re: How much elevation change before jet changes?

                Usually carbs don't work exactly right all the time...they are lean sometimes, rich other times, stumble occasionally, have trouble starting cold or hot. I'm used to it...it's just a part of life.

                I have driven my 55 on the power tour twice all the way, one time was from home in the AZ mountains, out to LA, then thru Colorado mountains, and the midwest, up to Detroit, then back thru the rain for a couple days. I did see a few guys screwing with their carbs in the Colorado mountains, but I never had any reason to mess with mine.

                EFI is usually pretty boring. Although in a good way.
                My fabulous web page

                "If it don't go, chrome it!" --Stroker McGurk

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                • #9
                  Re: How much elevation change before jet changes?

                  Outside a higer idle at lower elevations, I've never had a drivability issue with big altitude changes. Not saying max power isn't effected, because it certainly is (as is fuel mileage). But for my Q-jet cars and my Autolite 4300 one they accelerate with the same smoothness at 7300' elevation (my work) as they do at sea level (where I'm from). If there is a stumble at 7300' it's there at sea level and vice-a-versa. Now when I took my sea level tuned Q-jet to 11,300' the engine just didn't want to accelerate at all, not sure if fuel mixture changes would have helped or if there simply wasn't enough air for what the big block wanted pulling a 5,000lb car, but I did make it over the summit =)
                  Escaped on a technicality.

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                  • #10
                    Re: How much elevation change before jet changes?

                    I don't at all mind the little "character" flaws of carbs. Have any of you guys measured the difference in MPG? Doesn't really bother me. More just curious.

                    What carb would you say has the best balance in drivability, power, low maintenence? Which one is easiest to deal with when you do have to make changes. I have only been inside an Autolite 4100 and a Holley 600. The Holley was easy to learn because of so many people knowing about them. There didn't seem to be anything to the Autolite, though.

                    Sidenote: that Autolite was the most impressive stock carb I have ever driven. Easy to drive, not quirky and great throttle response.
                    Bakersfield, CA.

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                    • #11
                      Re: How much elevation change before jet changes?

                      I found this on the web, they are talking bike carbs but it must cover all types of applications.

                      "Altitude Compensation

                      For our first example let's say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down one main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 160 main jet at sea level you would drop down to a 140 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD! Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 140 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 160 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!

                      Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes "pinging" and running HOT. What happened?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually "leaned out" our mixture! One to two sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!"


                      I canīt imagine back when cars came equipped with carbs anyone would even think about a jet change to cross high elevations.


                      Taken from:
                      http://www.4strokes.com/tech/howtojet.asp

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                      • #12
                        Re: How much elevation change before jet changes?

                        if the air is less dense, wouldn't a carb have less of a signal, drawing less fuel, keeping the air/fuel ratio the same?
                        Originally posted by TC
                        also boost will make the cam act smaller

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                        • #13
                          Re: How much elevation change before jet changes?

                          yeah, that seems to be how it works in real life...carbs are amazing devices....

                          As far as mileage with EFI vs carbs, I don't know of any tests that made a fair comparison, ie. they spent the same amount of time trying to optimize both. I do know that of the cars we have on the road right now, 3 have EFI and the other 4 have carbs, and two of the ones with carbs get better mileage than one that has EFI. But they are quite different vehicles too. And two the EFI ones have overdrive while none of the carb'd ones do. So I'd have to conclude that EFI probably helps a little bit, but overdrive (and aerodynamics and weight) have much more effect on mileage than what type of fuel mixer you have.
                          My fabulous web page

                          "If it don't go, chrome it!" --Stroker McGurk

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                          • #14
                            Re: How much elevation change before jet changes?

                            Originally posted by A/Fuel
                            if the air is less dense, wouldn't a carb have less of a signal, drawing less fuel, keeping the air/fuel ratio the same?
                            not really, because the oxygen content also varies with pressure. I'm sure squirrel can put it in more scientific terms, surprised he didn't already.


                            As for EFI economy, I agree that the "mixer" doesn't matter too much, but there's always that deceleration fuel cut that you can do with EFI that you can't with a carb. I think that's the primary advantage when the discussion of carbs vs. EFI is focused on economy.

                            -scott
                            www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

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                            • #15
                              Re: How much elevation change before jet changes?

                              Squirrel: you are agreeing with A/Fuel. Is that right?

                              Malc: I appreciate you throwing a little contrast into the discussion. Thanks.

                              So, can we say that aside from the most extreme elevation changes (like going from the beach to the tall Rocky mountains) that the major adjustment of a carb is affected very little if calibrated for something in between those elevations?

                              In regard to optimizing a carb: once you get a carb setup optimized (float level, power valves, metering rods, other internal stuff), even if you drove from one extreme elevation/ climate to another, you should only have to play with jetting and the idle screws to readjust, right?
                              Bakersfield, CA.

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