The diaphragm works off of a pressure difference. At altitude, and lower pressure, at a given throttle position the spring will keep the slide at a lower position than it would be at say sea level. This lower position keeps the velocity through the carb
approximately the same as it is at a lower altitude. Thus the term "constant velocity". The velocity of the air creates the low pressure necessary to allow fuel to be pushed up through the main
jet. At higher altitudes with the slide in a lower position not as much fuel will be pushed through the jet so it takes a higher throttle setting than at sea level to get the same power.
As altitude increases it becomes harder for the outside pressure to compress the spring. Our bikes are supposed to be good to about 10,000 feet although I've read of those going higher. Because of its construction the stock carb can operate at high altitudes without any black smoke although the power will be reduced.
If I were in your shoes I would at least try replacing the spring with a known stock one, couldn't hurt. I have read where cutting the spring or weakening it is something people have tried to somehow get more power, doesn't make sense but it has been done. Good luck.