Here is and example. Let’s say the correct Keihin jet for your CR125 is a 168 on a 75 degree day. If the temperature climbs to 95 degrees, a 162 jet is the ticket. However if the temperature dips to 55 degrees, the correct jet may well be a 178 even though the jet proportional to the air density change would be a 172. And what is more disconcerting is the normal tuning indicators are out of whack. The plug shows rich and the EGT skyrockets. Strange, very strange.
However, not so strange when you think about what is happening to the path the fuel takes through the engine. The fuel and air mix passes through the crankcase of the engine on its way to the cylinder to be burned. The fuel is better vaporized when the crankcase is warm than when it is cold. So in those ambient temperatures of 30, 40, 50 degrees more fuel must be mixed into the air stream in the carb to maintain the correct vaporized fuel to air ratio burn in the cylinder.
Only the fuel that is sufficiently vaporized burns in the cylinder to produce power. The rest passes through the engine unburned and into the pipe where some additional burning occurs. It is all this unburned fuel passing through the engine and into the pipe that causes the plug to show rich and EGT to climb to what would normally be considered catastrophic levels.
Be prepared to jet your CR125 extra rich when operating at temperatures below 65 degrees F.