Holley adjustments for higher altitude locations

[Ron S.]

Hello all - just moved from IL to CO (Ft Collins area) and had my 66 delivered yesterday. Noticed right away problem with idle and seems to be a lack of power also. Reading thru various posts I see that altitude does indeed have an affect on carb tuning. My first thought was I was having a flooding issue, but it sounds like it's running richer due to being at 5000 ft (instead of 750 back in IL). Looking for recommendations on carb tuning/adjustments I may need to make. Should I change the jets, etc. I have an L79 with original Holley.

On another note, I used Thomas Sunday to have her transported and would high recommend them to anybody who needs to have a car moved.

[Gordon W.]

Re: Holley adjustments for higher altitude locations

I googled to see what the elevation is in Ft Collins and it's over 5,000 feet. You will definitely have a power loss no matter what you do. You simply can't go from sea level to 5000' without feeling it. I'd really be curious to see what the density altitude is in the heat at that elevation. When I raced here in Atlanta in the summer (1050' ASL) we'd see DAs in the 4000' range and you could rejet all you want and the cars still felt like they were being choked.

But to answer your question, jet it down. You'll have to do some plug reading to figure out what works best but I would start by jetting it down 2-4 jet sizes and go from there.

I'm sure there will be other opinions.

[Clem Z.]

Re: Holley adjustments for higher altitude locations

reduce the "AREA" of the jets 2% for every 1500 feet above sea level.

[Gordon W.]

Re: Holley adjustments for higher altitude locations

Ron:

This is a direct quote from Holley's website:

JET CHANGES AND ALTITUDE AND TEMPERATURE

Holley carbs are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 foot increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carb's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather varibles, temperature, barometric pressure, dewpoint and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in" (if they are bracket drag racers).

[Duke W.]

Re: Holley adjustments for higher altitude locations

Start by going through the idle speed/mixture procedure. It only takes a few minutes and will clean up the idle. See your CSM.

Carburetors go rich with altitude and power is lost due to lower air density. The above jetting recommendations are a good starting point, but depending on how much you drive the car, you can get away with the OE jets. It will just be rich.

Also, if you drive the car to more moderate altitudes with smaller jets you may end up with lean surge problems.

Duke

[Ron S.]

Re: Holley adjustments for higher altitude locations

Thanks guys for your comments. I did what Duke suggested and the car idles fine now. Will most likely reduce jet sizes since all of driving will be in Ft Collins area. Moving across country is stressful enough without having to worry about the old girl.

Ron

[Michael J.]

Re: Holley adjustments for higher altitude locations

I live at 7000 feet, all my old cars have Holleys (even the '56 T-Bird and a '66 L79) and all were bought at places less than 500 above SL and moved here. I did nothing to them except adjust the idle mixture to lean it out more for a better idle. They all run fine, and I don't notice any rich running issues at all. Maybe you should just drive it around awhile before you try fixing something that isn't broken?

[Mike Z.]

Re: Holley adjustments for higher altitude locations

You did not say what primary jets you are currently running, but I live in Albuquerque-about the same altitude (we are the other Mile High city) and have an L-79: I run #65 primaries and #68 metering plate in the secondary circuit. I had to adjust the car for PV at sea level and had to step up to 68 primaries. These adjustments coincide with my racing days, as we would race at different altitudes. You can try just adjusting the idle circuit, as some have suggested, but having lived at this altitude most of my life; my experience tells me you can not run the same primary jets effectively as you do at sea level (up to about 1500)-we just do not have the air you guys do at lower altitudes. Also, do not forget to bump your initial lead on the ignition about 2 degrees and may go 3-4 extra-depending on the fuel you run, there will be no knock or ill problems, even with the high C/R of your L-79.
Mike Zamora

[Chuck B.]

Re: Holley adjustments for higher altitude locations

I replaced the 65 jets with 63's and changed the power valve from 6.5 to 4.5 in my '66 L79 when I moved from K.C. to Winter Park, CO (9200') elevation. Still runs great when I drop down to Denver. Still has the original pistons and no problem while running 91 octane pure gas.

Chuck

[Ron S.]

Re: Holley adjustments for higher altitude locations

Not sure of size on jets, it's a stock 4160 List 3367 Holley. Don't have manuals/information handy, it's all still in moving truck making it's way west. Won't have time to mess around with until we get settled in at new place.

Thanks to all for the informative replies!

Ron