You'll definitely want to retune to sea level when you get back to NS. Sadly I don't know nearly enough to help you. Can you google general carb tuning for elevation? You should be able to find general needle setting ideas pretty easily. 

As a starting point, you can go to bingcarburetor.com, then select “Motorcycle CV - type 64 & 94”, then near the bottom of the page there is a download “Bing Jetting Chart”. That chart will give you the factory jetting by carb number, including needle jet size and needle position, and main jet. Always a good place to start. 

The needle jets are available in .002 increments like .264, .266, .268, .270. The needle jet size is the “fine adjustment”. There were also different needles used over the years, so might bear checking if you have the stock needle. The needle position 1-4 is more the “gross adjustment”. According to Snowbum, I believe, 1 increment change in needle position is equal to a 3 increment change in needle jet size. 

Bottom line, if you try the factory settings and it seems too rich at your altitude, I would step down the needle jet size to fine tune.  However, these CV carbs seem pretty good at self-compensating for altitude. Dropping the needle position might be a bit drastic, but I’m no carb tuner. 

It is hard to know what POs have tweaked and changed, so starting at factory configuration is always good, IMHO. I wouldn’t fool with the main jet (if it is correct to the chart), as that only comes into play at very high speeds / rpm’s. 

Thanks for the advice and for suggesting that chart. I should have mentioned that the needle jets were at 2.68, stock. I replaced both the needle jets and jet/slide needles with new stock ones.

The bike is running fine now with the needles at 2nd position but I'm just curious if there's a more optimal tuning, and if so how I can measure that in terms of observed power, mileage, or lean/rich condition.

When I get down to lower elevation, I will experiment with switching the slide needle back to 3rd position, and swapping in 2.66 jets.

Some thoughts...

- My take is this: People make WAY TOO much of "main jet size". The bike only runs on the Main Jet when: 1. the throttle is 7/8 to wide open, AND 2. the engine RPM reaches near max. For instance: If you are riding at 50MPH in top gear and open the twist grip all the way, then you are not on the main jet. The reason for this is that the throttle merely controls the butterfly valve, and not the slide. The slide has to be all the way at the top before the Main Jet comes into play. When that simple fact is understood, then it's easy to understand that 98.7% of your riding is done on the needle AND needle jet. Therefore, 98% of your efforts should focus on the needle and the needle jet. So unless you are tuning for the Bonneville Salt Flats, any talk of Main Jet size is usually unwarranted. 

- Due to the air pulsing in the intake tract, the needle and needle jet get beat against each other during use. On any 50 year-old motorcycle, the first step in doing ANY carb tuning is to replace these 2 items on both carbs. Period. You simply cannot "tune jetting", when the jets themselves have been enlarged due to age.

Replacing these parts also corrects any corrosion that may have altered the precise shape of the needle or jet due to the use of ethanol fuels, which have been with us for ~30 years.

- When any tuning involves manipulation or adjustment of the needle position, due to the way this is done "blind" on the Bing, it is always a good thing to compare needle extensions to insure they are the same on both carbs before returning the side to the body. This is easily done with the depth gauge of a caliper as shown below. I have had customers report all sorts of poor running only to find their needles at different heights. 

- You will typically be far better off to use the needle clip position listed in the Bing data, and change (usually increase) the needle jet size. You can consider the clip position to be a "gross" adjustment and the jet size to be the "fine" adjustment. Small adjustments are always a better first step rather than big ones. 

Continued...

- Sometime in the early 70's the US DOT and/or EPA started to establish emission output for motorcycles. They weren't as strict as the ones for automobiles, but they were there. This meant that engines started to be jetted leaner, which hurt engine life and performance. Then, within a decade, leaded fuels were discarded and performance took another hit. 

Due to all this, the first step I usually take is to increase the needle jet from 2,66 to at least 2,68. (A bigger number means more fuel.) You'll see that this is the pre-1970 size most Airheads were using, especially on European models. And too, if your jets are marked "2,66" then they have probably worn to 2,68 over the intervening 40-50 years.

On the last models with the single-sided rear shocks, you may also find restrictive Low Speed Jets in the 35 & 40 range. Those can be increased by 1 size to 40 & 45 respectively. 

See this chart...

https://bingcarburetor.com/uploads/9/8/7/9/98794296/bmw_jetting_chart_cv_carbs.pdf

- Correct jetting will usually result in the entire sparkplug center porcelain turning to a nice maple wood tan color... for each jet range (except idle). This means you'll be required to test your jetting at specific jet ranges. Which in turn means long rides at constant throttle openings. Which in turn means you'll need to mark your twist grip for half and full open.

- Correct jetting also requires a full fuel system, ignition system, and air intake system check. Is your air filter fresh and clean? Are your plug wires "metal core"? Are your plug caps new? Are your sparkplugs NON-resistor type? (These can be hard to find.) Has your ignition timing recently been set using a strobe lamp at Full Ignition Advance? 

Carb settings are the LAST step in a long list of tuning steps, which means they are fully dependent upon all the other steps being set correctly. GIGO ! If your inputs are garbage, then what comes out of your time and effort will be garbage. The "devil is in the details" applies here.

Thanks Richard, that's all very helpful, especially regarding the main jet. I did measure my needles and they're both at 39 mm. And I've done all the prior steps for carb tuning. I'm working through the back end right now but once I get all the service done I'm going to experiment with different needle sizes. Any special techniques for checking the plugs? I assume just kill the engine on a straightaway at the target RPM but how long should run it first?

Posted by: @matthieu-aikins

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Thanks Richard, that's all very helpful, especially regarding the main jet. Any special techniques for checking the plugs? I assume just kill the engine on a straightaway at the target RPM but how long should run it first?

As in any other venture that you want to succeed, 90% of the work is in the prep.

* Most of my work has been done with NGK plugs. I find they have a wide heat range and are easy to read. Almost all Airheads do well with BP7ES plugs. (Some modern reference charts may list heat range 8, but those are far to "cold".) I can't speak for Champion or Bosch, and note that the numbering system on Champion is in reverse order to NGK. 

* As a matter of engine safety, it should be your "best practice" to lubricate the plug threads, especially on new plugs, but also everytime any plug is reinstalled. Just pull the engine dipstick and add 2 drops of motor oil if nothing else is around. Nevr-Seize is obviously best, but oil, chassis grease... anything is much, much better than nothing. And this applies to taking these plug readings on the road.

* You'll want the carbs recently "balanced" with either a manometer or gauge. (Not by eye or ear.) And you want that balance adjustment to be made at idle (using the speed screws), as well as at 1800 RPM using the throttle cable adjustments. 

* All readings are taken with the engine fully warmed and running in top gear. Getting an Airhead to initially "warm up", you may have as much as 8-10 miles of riding, but you can use that distance to get to a 4-lane highway where you can safely ride uninterrupted at a fixed throttle without traffic lights or stop signs.

* I like to take along a shop rag and whatever sparkplug wrench that's needed, and head out to the highway. After the engine fully warms, I ride another 4+ miles at a constant throttle. When I want to take a reading, I hit the kill button and at the same time pull in the clutch so that the reading I get is exactly what the engine was doing. The plug is going to be blazing hot which is where the shop rag comes in handy. 

* Reading the plug consists of close observation of the center porcelain. It should have a light tan (golden oak or maple) to light brown color. If you consult YouTube or Google this subject you'll see some people cut away the threads and destroy the sparkplug to take a one-time reading. While the best readings are deep on the porcelain, let's not get carried away. That's only needed when you're doing full-bore racing at Daytona or the IOM. As road riders what we're looking for is a consistent color that's clean, smooth and free of black color. Black is too rich, while white (no color) is too lean. Black fluffy deposits could be burned engine oil, which has to be investigated. Both plugs should generally look nearly identical if your tuning and prep is good.

(Water cooled engines may have slightly lighter or splotchy colors, like a calico cat. But a water-cooled engine can dissipate the extra heat in the liquid cooling system and is a whole separate beast.)

[Grossly mis-matched plug colors could indicate poor throttle balancing, mis-matched carb needle heights, mis-match jetting, poor condition of sparkplug wires or caps, mis-matched fuel bowl fuel heights, or simply the engine breather system dumping all the oil mist into one carb. In short... poor preparation.]

Hope this helps.

@wobbly Yes that is super helpful, I'll be following this procedure on the flats near the Great Salt Lake once I get the last phase of my overhaul completed - thanks!

Not to beat a dead horse... but the reason you can't simply pull the plug "any old time" is that the 1+ minute of idling the bike did as you pulled up to the garage door... just before you turned off the ignition... has re-colored the plugs. The color you are getting has then been influenced by the Idle Jet, and it is not a true reading. It's more of a "general reading". 

No one cares what the Idle Jet reading is because at idle all we care about is that the engine idles smoothly and doesn't conk out just as the traffic light turns Green. In other words, at Idle, performance (smooth, correct RPM idling) trumps plug color. Whereas at "road speeds" correct plug color can help us find that missing horsepower needed for safe passing, and reduced head temperatures needed to make our valve seats last longer.