Do all the carb'ed bikes have to rejet for this kind of trip?
Have you seen any that don't need it?
Rejetting is about maintaining the proper air to fuel ratio so that you get as close to complete combustion as possible on each combustion stroke. Too much air (lean) and you lose power. Too little air (rich) and you lose power. The engine will actually run over a fairly wide range of ratios, but not efficiently.
Carbs are averaging devices. There is really only one set of conditions where they will produce the optimum air to fuel ratio, defined as complete combustion of all air and fuel, which I think is 14.7 parts air to one part fuel. On either side of those conditions, they are either lean or rich. The idea is that your carb is jetted so that you are within an acceptable range on either side of the optimum. Temperature affects air density. Altitude affects air density. So changes in both affect the combustion.
The issue is what constitutes an acceptable loss in performance based on the changing conditions? For racing, where you are trying to squeeze out as much power as possible, you want your jetting to be as close to perfect as possible (most of the time on the rich side though). Even then, if conditions change during the race, like say you are racing up and down a mountain, you are going to have to shoot for jetting that tries to maximize performance at some specific range and sacrifice performance at other parts of the range. The difference between seal level and say 13000 feet is quite a bit. The JD jet kit in my 530 goes in steps of 3000 feet. So there is a setting for 0-3000, 3000-6000, 6000-9000, and 9000-12000. It doesn't include anything for over 12000, but the bike will still run, just not at max performance.
At high altitudes, you lose power because there is less air, which means you have to use less fuel to maintain the right ratio. So even if you have it perfect, you still lose power. Then, if you are on either end of the altitude range you jetted for, you will lose a bit more power because you are drifting from the ideal mixture. It's all about averaging. So a bike jetted for sea level may indeed run at a high altitude, like 10-12K feet, but it is going to feel fairly sluggish compared to how it feels at sea level. Some bikes might stall when you try to lug the engine at low RPM, like say on a steep hill climb where you are starting to lose momentum. That is NOT fun!
When I had my KLR out there years ago, I did not re jet or re gear it. I was able to do the rides, but I was fighting the bike the whole time. It took more of my energy and made the ride less enjoyable. So it ran, but it could have run MUCH better! I also should have dropped a tooth on the front sprocket. Live and learn. My KTM will be jetted for the 9-12K range and I will be running 13-50 gearing, which tops me out around 60mph but gives me mountain goat climbing ability.
Many carbs have either an air screw or a fuel screw. These can add a little bit of manual adjustment without having to change to main jet or needle in the carb by either controlling the amount of air or fuel coming into the system through the idle circuit. At idle, the main jet is basically closed and not contributing to the flow at all. When you get past 1/2 throttle, the main jet and needle are the main influence but the idle circuit is still contributing even though its not changing as you speed up or slow down. It is not really something you can do on the fly. You generally have to stop, adjust it, then keep going.
On my KLR, as we would get higher and higher, the bike would not idle as smoothly as it should. I turned the knob on the carb until it would smooth out. (I can't recall if it was an air or fuel screw). However, once we get up around 11500 feet, the effect was minimal and the bike just had to be revved to keep it running, which can make things harder if you are in a technical spot where you don't want to be moving real fast. Also, if the engine started bogging, I'd have to drop a gear to keep the revs up, assuming I had a lower gear to select. Since I as using stock gearing, which has a tall first gear, I was often wishing I could have dropped down another gear when I couldn't
A fuel injected bike uses a "map" to determine ideal fueling to keep the ratio as close to ideal as possible. It uses temperature sensors, throttle position sensors, oxygen sensors (in the exhaust), and some even use a barometer to sense air pressure. It has a map that tells it what fueling to use based on all those sensor inputs. The aftermarket tuning kits you see either replace the stock map with a new one or they introduce a bias signal based on their own maps that add or subtract to the signal generated by the stock map. Most maps come from the factory leaned out at 5500 RPM, which is where emissions standards are usually tested. That can create a noticeable dip in power when accelerating. A lot of the tuner kits let you get rid of that, making the power curve smoother as you run up through the RPM range. Even FI bikes may have an upper altitude limit on their maps, like my old 98 VFR 800 which topped out at 12K ft. FI bikes will still lose power just because there is less air, but they don't have to worry about shooting for an average condition when you might cover sea level to 12-13K feet in a given trip.
So, NO, you don't HAVE to re jet a carbed bike to ride it in Colorado on the mountain passes. But, if you can, it does make a noticeable difference in performance. As for whether or not that is acceptable, different strokes for different folks