needing said:
Surely the altitude correction method would be to alter the fuel level in the bowl to correct air:fuel ratio across all throttle openings and not just Wide-Open-Throttle on main jet.
What isn't so obvious is that the float level directly affects how the different circuits in the carb interact. For instance, if too low, you'll have a bog coming off a dead idle into the realm that's controlled by the transfer ports, slide cutaway, and needle taper. Conversely, too high will cause an overly rich mixture during a cruise, assuming you've set the idle mixture correctly with the carbs fully closed at idle.
Scrutinizing the emulsifier tube in the Amal (threaded into the top of the main jet holder, check out the top-right picture at
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html), you can envision that the area around the brass tube is filled with fuel through the main jet to equal the level in the float bowl. As you snap open the throttle, this well of fuel is allowed to be drawn past the needle without any hindrance from the main jet, in effect acting like a poor man's accelerator pump. Once emptied, you go onto main jet/needle to control fuel flow. Now, if the float level was low, this well wouldn't contain enough fuel to perform this job. Even if the carb was jetted to properly meter in a steady state, this simple operation would be compromised. This is just one example of what can change with float level deviation.
Some time ago, some locals bought aftermarket float bowls for the Mikuni carbs on their snow machines that allowed external main jet adjustment with the simple twist of a screw to offset the blubbering experienced at the high altitudes they could reach around here. It worked great
while they were at that altitude. Here's the problem; they'd come back down into camp, do their thing, then fire up and head back up the hill, forgetting that they'd leaned the carbs. One good pull out of camp, and it's all over. Into my shop for a top-end rebuild. We did a number of those over the course of about two years.
Like Greg said earlier, changing jetting for different altitudes is hardly worth the effort, unless you expect to be there a long time and/or are racing there. And yet, I won't argue with Glen about dropping the needles. You'd be reminded by the poor running conditions before doing any damage (I hope) once you come back down. Your call...
Nathan
