Carb Tuning for Elevation Changes?
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From: Carson Valley
Carb Tuning for Elevation Changes?
How much elevation change can a Q-jet carb handle before it needs to be retuned?
I live at 4500' elevation, the mountain passes between home and CA are higher still.
If I find a good running Corvette with a Q-jet in CA, will it make the trip through the mountains to get me home, or should I plan on towing it and possibly retuning it when I get home?
I hope to make the occasional trip through the mountains and to sea level in CA.
If the car is running good at my elevation can I plan on it making such a trip, or shall I plan on a needle and jet change?
I live at 4500' elevation, the mountain passes between home and CA are higher still.
If I find a good running Corvette with a Q-jet in CA, will it make the trip through the mountains to get me home, or should I plan on towing it and possibly retuning it when I get home?
I hope to make the occasional trip through the mountains and to sea level in CA.
If the car is running good at my elevation can I plan on it making such a trip, or shall I plan on a needle and jet change?
Last edited by AKjeff; Jan 23, 2022 at 11:03 PM.
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Something is seriously out of whack and not merely elevation maladjustment. I lived in Flagstaff (7,000 feet) with both my '67 (Holley) and '72 (Q-Jet) and would take both cars up to Snowbowl on the winding road for fun. Top of the parking lot is something like 9,000 feet. I also drove both cars to Phoenix (1,000' elevation) and neither one performed poorly enough that it was a concern – even autocrossed both down there and didn't even bother adjusting the mix screws. While you likely will feel a difference going from sea-level to 10,000 feet, it certainly won't be so severe as to require a tow truck.
Are you sure it's not running too rich at sea-level?
Are you sure it's not running too rich at sea-level?
Last edited by barkingrats; Jan 23, 2022 at 10:52 PM.
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From: Carson Valley
Something is seriously out of whack and not merely elevation maladjustment. I lived in Flagstaff (7,000 feet) with both my '67 (Holley) and '72 (Q-Jet) and would take both cars up to Snowbowl on the winding road for fun. Top of the parking lot is something like 9,000 feet. I also drove both cars to Phoenix (1,000' elevation) and neither one performed poorly enough that it was a concern – even autocrossed both down there and didn't even bother adjusting the mix screws. While you likely will feel a difference going from sea-level to 10,000 feet, it certainly won't be so severe as to require a tow truck.
Are you sure it's not running too rich at sea-level?
Are you sure it's not running too rich at sea-level?
Thanks for the info.
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Carburetors are actually pretty magical and take care of about 99% of any needs automatically.
I've "fixed" I don't know how many carbs when I lived in Denver that folks just felt they needed to modify drastically for altitude.
It will be fine..and if there are any tweaks needed eventually..it won't be the needle and seat.
JIM
I've "fixed" I don't know how many carbs when I lived in Denver that folks just felt they needed to modify drastically for altitude.
It will be fine..and if there are any tweaks needed eventually..it won't be the needle and seat.
JIM
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Oxygen thins out as you go higher. So it stands to reason that any given amount of air will have fewer Oxygen molecules in it than at lets say sea level. So yes you will run richer going over the continental divide. But if you don't live their. Your not going to get out and rejet. Colder air also causes the mixture to lean out. However, one trip over and down. You should be fine.
Dr. Detroit
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Q-Jets and Holley's will adjust well to altitude as stated by Jim........the Q-Jet's primary metering rods do an excellent job of metering as does the Holley power valve......but this is assuming that the "window" of tune in the carb is satisfactory or "in the middle".......if it is at the edge of the tune......it may run poorly and come back to life at sea level......
Jebby
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Oxygen thins out as you go higher. So it stands to reason that any given amount of air will have fewer Oxygen molecules in it than at lets say sea level. So yes you will run richer going over the continental divide. But if you don't live their. Your not going to get out and rejet. Colder air also causes the mixture to lean out. However, one trip over and down. You should be fine.
Well, the air thins out, not just the amount of oxygen in the air. So, fewer of all the various molecules that comprise air go through the carburetor into the engine. The carburetor venturi effect just happens to add fuel based on the air flowing through it, so less air flowing through it causes it to add less fuel to that reduced amount of air.
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From: Hermosa
And there is less pressure on the fuel in the bowl. Less vacuum and less atmospheric pressure. Yes the carb is self adjusting to a degree, but has it limits.
If it didn't there would not be a reason for various jet and needle sizes. Nor would aircraft have a variable mixture control. At some point the carb cannot adjust appropriately and mixture has to be delt with. It will continue to run, but not well, and it may not idle at all.
At any point it's not in the conditions it was jetted/tuned for it's not optimum.
If it didn't there would not be a reason for various jet and needle sizes. Nor would aircraft have a variable mixture control. At some point the carb cannot adjust appropriately and mixture has to be delt with. It will continue to run, but not well, and it may not idle at all.
At any point it's not in the conditions it was jetted/tuned for it's not optimum.