Technical Do any carburetors compensate for altitude changes - sea level to Pikes Peak?

Dial a jet.....

 

Hello,

As teenagers, most everyone had their sedans and hot rods tuned for sea level running, since we were at sea level +- a few feet. So, the power was there if you did it correctly. But, during those post Christmas road trips to Big Bear Lake and City for the “gathering of the tribes,” it was like living and walking around in a hissing, giant snake community. Almost every car up there had the air filter taken off and the carbs were hissing as they should.

But, most thought the removal of the air cleaner was all that was necessary to compensate for more air in the running motor. Some of our group still did the same and still had the stumbling performance while cruising around those busy teen infused streets over the holiday week until New Year’s Day.

We were at sea level, for the most part. Big Bear was around 6700 feet with the skiing areas up to 7k where the parking starts. So, the offset is there, but for most, they always thought that taking off their air cleaners were the answer to better performance. In talking with our family mechanic in Los Angeles, he gave us the run down of what needed to be done for the best performance of our tri carb 348 motor in the 58 Impala. It wasn’t just taking off the air cleaner.

Part of the zero to 7000 ft. difference may be doing all of the changes geared specifically to run at the higher elevation. But, one cannot do that at zero feet, otherwise performance is awful going up those inclines to the highest elevations. So, it had to be done at the final place of the week long stay for best results. On the way up, we did have the air cleaner off and still had some stumbling issues. But, the incline made it slightly difficult to drive, although, speed was not necessary going up hill, but safety in the curves.
1960-62
Jnaki

When we arrived, in the next several days, we adjusted the carb, checked the plugs for signs of lack of tuning color and took several test drives down some nearby empty roads. We did not want to get caught without the best tune for our 348 Impala during our stay in the mountains. It was slightly a hassle to get the carbs to run correctly, but once done, we were good for any full throttle adventures in those back woods roads.

Others could have had the same information, but it did seem like we had the only knowledge for a good running motor in the higher altitude of 7000 feet. Like it was mentioned, it was easy to just take off the air cleaner and think it was doing the change over for some power running like at our own sea level encounters.

The sound of a bunch of snakes driving by while standing on a corner in town was somewhat interesting. But, back on those empty, clear of snow, roads, the full acceleration was hampered to those that did not adjust anything. The results were pretty obvious.

NOTE: We had to stop at the base of the mountain range and re-adjust the carburetors for smooth running at sea level. One year we were so happy with the New Year's Eve/Day that we kept going until the 58 Impala began to stutter and sound awful when trying to accelerate. Time to pull over for the adjustment back to normal sea level running.

 

Great trip, have a great time.

If your truck is running well in Michigan, when you leave, you should have no problem getting across the mountains.

 

SUs only compensate partly. Luckily the mixture adjustment is very simple to do. Also, the "choke" is literally just a mixture control, which overrides the adjustment nut, without an actual air-choke baffle. In theory it should be possible to rig an on-the-fly mixture adjustment from the dash, which would be cool with a mixture gauge running off an O2 sensor.

 

The "CV" or constant velocity Zenith Strombergs had the square tops (not square body) that would compensate for air density/altitude. The CV carbs were also side-draft carbs so probably not applicable to the OP's question.

 

Have made these cross country trips (45 + )since the Air Force days in the early 70's. Never gave it much thought back then driving the 57 and other carbed vehicles. Guess when you retire and get in your 70's the mind starts rambling. Probably will take the advice above to just go for it. Then there goes the thought process again, like maybe the metering rods could be restricted from fully pulling out of the main jets..........

 

Jon I am probably mistaken about this but it seems like there was a Carter with a variable venturi. In my mind it seems like one could compensate by being able to regulate the amount of air going into the plenum as opposed to trying to compensate by using less fuel. The carb in question is something that you would know more than I would.

As far as a carb that could be adjusted on the fly Harley used a carb in the '40s through the '50s I believe. The Linkert M74 had adjustable mid-range and high-end jets. I would be useless on a car but on a bike where you could reach down and adjust the jets they worked like a champ. Perhaps you could use a cable to adjust one on a car like say a flathead (it would take several to feed a modern mill).

 

Beaner - Off the top of my head, I do not remember a variable venturi Carter. The last carburetor Carter built for motorcycles was about 1922 or so, and it was not a variable venturi.

There were a number of variable venturi carbs made in the USA in the 1910's, and 1920's.

Seems like I do remember a non-H.A.M.B.-friendly Autolite variable venturi four-barrel.

Ned Ludd - As far as adjustable jets, several manufacturers in the 1930's, 1940's, and 1950's offered externally adjustable metering jets for SOME one-barrel and two-barrel carburetors. There have been a few professional rally folks rallying older cars that have made a modification to adjust these from the driver's seat.

I do not remember externally adjustable metering jets for any four-barrel.

Jon.

 

There is or was a WWII era carb for gas powered tanks that was variable venturi. I believe that the Preditor carb was patterned after it. Back in the '70s and '80s you could find them cheap in the surplus stores all the time. I do not think that they were a big CFM carb. I knew a guy in Lynn Creek (east of Camdenton) that modified a pair of them and put them on a small block Ford.

Here is the rub, at altitude you are not getting too much fuel, you just are not getting enough air. Too many guys think of carbs as a fuel metering device and they would do better at thinking of a carb as an air metering device. One of the reasons that blower cars are popular in the mile high city and surrounding area is that they stuff more air into the motor. I know that this thought is off topic but maybe it will help someone. Free flowing heads help too.

 

On my trips to high altitude (3000-5000 feet) I adjust (only) the idle mixture setting once I am at height. Since the idle circuit is actually a major part of the fuel metering system well into the power range, it does improve the mixture most of the time.

If idle and off-idle mixture is set correctly, it will eliminate bad running at those speeds which is more noticeable.

 

As mentioned, the late 70's Q-jet had a altitude compensating bellows. I believe only used on Cads. I never saw one, but they all had the spot for one.

 

@Helvehammer , welcome to the HAMB from Illinois.

After you have ten posts on threads on the forum, you might want to post a wanted ad here:
https://www.jalopyjournal.com/forum/forums/wanted.30/
For your carb needed.

 

Cars ordered into Denver in the 70's came with the High Altitude carburetor. When a car came into the station from somewhere else we would start by going down two jet sizes. If traveling through, I wouldnt worry about it. You are running fat.

 

When I worked at Sierra Lincoln/Mercury in Fullerton Ca. in 1978 the small block V8’s had CV carbs. They would stick on occasions as I remember probably on 77’s. I think the mechanics used some type of spray recommended by Ford to free them up.

 

Pretty much the same with Fish carbs. They compensate really good, but you always can enrich them from the dash.