Back in ~1980, if you were riding across the US on a motorcycle, you needed to swap out jets in your carburetor(s) if you went through an elevation change of more than a few thousand feet - the reason being that simple carburetors are sensitive to altitude, and the air/fuel ratio will get out of whack if you don’t do so. Potential problems include fouling the plugs from running too rich, or overheating/burning an exhaust valve from running too lean.
OTOH, I remember driving across the country on family vacations around that same time, covering elevations ranging from 1,000 feet above sea level up to 12,000 feet or more. I don’t remember my dad ever doing anything to the carburetor, nor do I remember any problems arising in that regard.
I assume this means the carburetor in his car was taking care of altitude compensation on its own. So how did it do that?
My Model T has a fuel mixture adjustment knob, so you don’t need to switch out jets.
A Model T can climb Pike’s Peak, though it is painfully slow (and painful to drive since a Model T doesn’t “shift” between high and low gears like a modern cars, so in “low” gear you basically have to hold down the high/low pedal the whole time you are driving, which gets VERY tiring after a while - braking is also a challenge on the way back down).
If you’re talking about a 1970s era car, most of those have enough power to climb up Pike’s Peak or drive at other high altitude locations. As you noted, they will tend to run rich, so if you are permanently moving from say the beaches of California to someplace in Colorado you might want to swap out the jets, which is basically the same type of procedure as you describe for a motorcycle. But if you are just doing a cross-country trip, generally you wouldn’t bother. You’d just suffer with the car running a bit rich as you go over the mountains.
Advancing the timing also helps out a bit, but again, if you’re just driving cross-country, most folks wouldn’t bother.
There were also some carburetors back in the day that had a fuel/air adjustment screw.
Just like the Piper Cherokee I took flying lessons in! In fact, IIRC, “mixture” was a major control right beside the throttle, and the correct way to shut off the engine was to lean it down to zero. I forgot to do that once and found out why you should – if you just cut the ignition there’s a very loud backfire!
I remember that from Zen. It was 50 years ago or so even though I can’t remember what I had for breakfast yesterday. I spent little time on a bike at high elevation, not much more than 2000 feet. I didn’t have more jets and even if I did I wasn’t going to try to change them on top of a mountain.
I do remember reading about it there, but I also remember hearing about it directly from old farts who did that sort of thing in their youth.
If it’s not important to compensate for altitude on carburetted cars that will be in the mountains for a short time, then why was it important enough for bikes/bikers to pay attention to this detail?
I remember the recurrent family trip from Georgia (land of the ancestors and relatives) and New Mexico (7000+ feet above sea level in Los Alamos), and stopping in west Texas to turn the carburetor settings leaner, and also advance the ignition timing on the distributor.
As I understand it, the strength of the venturi effect pulling fuel through the jets varies as air density changes, and provides a decent amount of altitude compensation. I have 3 jetting set-ups for my 2-stroke dirtbikes: desert winter, mid altitude summer, high altitude summer. 2 strokes are less forgiving of bad jetting and are known for fouling plugs if rich or seizing if lean. I took my GPZ-750 25,000 miles across country in the late 80s ranging from sea level to 14,000’ and never re-jetted it.
30 years ago or so, I moved from sea level to Colorado at 5200+ feet with a Toyota Pickup and the carbureted 22R engine. I didn’t even have it in Colorado for a year (and I still wish I had it back), but I did drive it into the mountains over 10,000 feet several times. It was fine, so I didn’t bother to do any adjustments. If it ran rich I couldn’t tell, because it always got 17 MPG no matter what I did.
I remember that the manual had some information about a high altitude adjustments, so I searched to see if I could confirm that. What I found is the factory optional High Altitude Compensation system. I can’t imagine my truck had that, because I recall being told that if it ran poorly I should take it Toyota to do something, apparently to get the HAC installed.
From the service manual, linked below:
As altitude increases, the air-fuel mixture becomes richer. This system insures proper air-fuel mixture by supplying additional air to the primary low and high speed circuits and secondary high speed circuit of the carburetor, and advances the ignition timing to improve driveability at high altitude (above 1,198 m (3,930 ft)).
So there you go. At least one manufacturer compensated for altitude by letting more air into the carburetor using a complex arrangement of vacuum hoses, bellows, and other complicated carb wizardry.
Not identical, but current gas stations at elevation sometimes sell a lower octane because you don’t need as high, 85/87/89 or 91 instead of 87/89/91. It seems to be somewhere between 4000 and 5000 feet elevation where it makes the difference.
Yep. I once rented a car in Las Vegas (2000 feet elevation) and drove it up to Bryce Canyon (about 7600 feet elevation at Ruby’s Inn). We filled up on the lowest octane crap they had there, and then I drove it back to Vegas. It was a damn hot day in Vegas, and the car was not happy at all. This was in 2005, so the car was modern enough to have a knock sensor and automatic spark retard, but it couldn’t compensate enough for the crap I had put in the tank; it was knocking terribly. I gently drove it back to the rental return place an dropped it off without mentioning anything.
Did the Stromberg carbs with those diaphragm mounted long variable diameter needles compensate for air pressure? My old MGB was a finicky tune. It had those before I went to dual Webbers.
Does this have anything to do with the choke? My family’s first car, a '49 Chevvy, had both a hand choke and a hand throttle. I never used the throttle, but when starting, especially in cold weather, you pulled out the choke, started the engine and then, as it heated up, pushed the choke back in. My '68 Volvo had a control that somehow combined the choke and throttle. Again, you pulled it when starting and then gradually pushed it back in.
The opposite, actually. The choke cuts off the air supply, creating a richer mixture to help the car to start. Once it’s started, anywhere from a few seconds to a few minutes, the choke is turned off to allow a leaner mixture.
The Toyota Truck I mentioned up thread, the choke was controlled by the gas pedal. I think the cold starting procedure was to press the gas to the floor once, release, then start it. Warm I think was press the pedal just a bit while starting. Of course it was a Toyota 22R, it could be started with a dead battery, by just wanting it badly enough.
YouTube tells me the 85 is a hold over from carbureted engines, and should not be used in modern fuel injected engines. The computer with all of its sensors can compensate for the altitude and doesn’t need the lower octane to run well. Of course that means the computer ends up compensating even more for the too low octane.
I imagine 90% of the gas cars I see are running 85 octane, and there isn’t an epidemic of destroyed spark plugs in Denver, so it probably isn’t that big of a deal.
Not very well.
In 1985 I was in Air Force Tech school in Denver.
I had a fuel injected Grand Am, friend had carbureted 280Z.
Around the city his car was a good bit faster, in the mountains it was bearly drivable.
My car varied very little.
I remember pulling the choke out on starting, on older cars. Some didn’t lock in the open position, and you had to hold it out, or use something to hold it out, such as a matchbox or a piece of garden hose slit up the side. Sometimes you would forget, and go on running on rich mixture. Eventually I changed the cable to a locking one so you didn’t need the hose anymore.
Dunno what you were driving, but what I was driving was a 3 speed car, and a 5 speed motorcycle. The narrow band tuning of the bike meant that the carburetor was out-of-tune more easily.
I’ve never hear d of this before. I remember driving over the mountains in Montana on a Honda 500 in the late 1980’s, and realizing the altitude issue when I opened the throttle going uphill and response was noticeably lackluster. it worked, but I wouldn’t call it performance. After that, I still managed to get a speeding ticket in Wyoming on the interstate.
In fact, IIRC, “mixture” was a major control right beside the throttle, and the correct way to shut off the engine was to lean it down to zero. I forgot to do that once and found out why you should – if you just cut the ignition there’s a very loud backfire!