During flight testing of a carburetted Cessna 185 decades ago, I had to clear it for high altitude vapour lock (fuel pumps removed). I took the oxygen, and climbed. I was able to sustain 20,800 feet, with the stall horn screaming at 72MPH indicated, and 12" manifold pressure at full power. I had made it to 21,000, but it would not stay up there.