There are two things at play. As you go higher, the air pressure becomes less so the mixture becomes richer, but the air temperature drops, so the mixture becomes leaner as the air density increases. The leaning-off effect is very pronounced. If the motor is already carburetted very lean, the combustion can become detonation. The simple fact is that the change in the cooling of the barrels is slow when the ambient temperature drops or rises, but the combustion temperature rise is almost instantaneous when the ambient temperature drops. So you get the seizure when the pistons expand too much. It is very noticeable at race meetings. Many guys jet their bikes too rich and on very cold mornings, their bikes become faster. Anyone whose bike is jetted lean has the problem that the bike becomes a pig to ride. With an air-cooled two-stroke race bike, really cold weather can spell disaster.
Sorry for being late to this thread. I just can't let the misinformation in Al's 2nd sentence pass. Al, I suspect that you actually understand how all this works. However you mis stated how. Air temperature drops approximately 2 degrees C [or 3.5 degrees F] for every 1,000 feet higher you go up [above sea level] in the atmosphere. This rate varies, due to variations in humidity & temperature. See
Standard Lapse Rate
The "mixture" does NOT get leaner with altitude, the air gets less dense. That means less oxygen, so the mixture gets richer at higher altitudes, on an engine with a fixed jet carburetor. Cooler temperatures decrease, not increase the density altitude. These factors are why piston powered gas aircraft all have a mixture control, as well as a throttle control. Pilots have to lean the mixture manually, as their aircraft gain altitude. If Al's statement above was correct, then every aircraft engine operating manual printed in the past 100 years is wrong! General rule of thumb is that you will lose about 3% power for every 1,000 feet you climb above sea level, due to thinner [less dense] air.
The Fuel / Air Mixture
You are correct that temperature change also affects fuel mixture. Colder air is more dense, hence more oxygen in the same volume of air. Carburetors with fixed jets are only optimized for one set of atmospheric conditions. As Al states above, an engine jetted on the ragged edge of being to lean, will be critically lean if the temperature drops significantly. Modern electronically fuel injected engines compensate for changes of air temperature, via the Inlet air temperature sensor aka IAT sensor. Changes in altitude & humidity are compensated for via the mass air pressure aka MAP] sensor , which reads barometric pressure. This is referred to as a speed/density system. Another fuel injection method is The Mass Air Flow [aka MAF] system. MAF systems directly measure the density of the air entering the engine's throttle body via the mass air flow aka MAF sensor. MAF systems have the advantage of being able to compensate for engine modifications, such as changes in exhaust system or camshaft.
Because of these issues, Amal and other manufacturers jet their products a tad rich, to compensate for these variations. Comnoz [Jim C] lives at 4,700 feet elevation. He lives about 90 miles south east of my sister. So I know that his motorcycle travels can take him from 4,000 to 14, 000 feet just in the local area. I suspect that is why his Norton has been converted to electronic fuel injection. Those 920cc also help compensate for the loss of power at those altitudes!
Last time I visited my sister, she complained to me how the local John Deere dealer could not fix her Gator [4 wheel ATV]. The machine would often stall at idle, once warmed up. I fixed it in 3 minutes by adjusting the carburetor's fuel mixture screw. The problem is that my sister's ranch is at 9,500 feet. The dealer is 2,500 feet lower. He adjusted it to run properly at the altitude of his shop. To run properly, the mixture had to be adjusted leaner at it's altitude of operation.
Back to the actual subject of this thread, driving in hilly or mountainous terrain increases the load on the engine. This creates more heat and expansion of the piston skirts. So, .003" piston clearance may work well on an engine that is carefully broken in and only operated in "the flat lands" like me in Florida. It might not work out for someone operating in hills or mountains. Personally, I'll err on the side of caution and use more clearance. Never know where I might travel with it in the future.
