I googled "Backfire"
..."Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Backfiring can occur in carbureted engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced. As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion. The condition that causes this is a misfire. The result of a misfire or incomplete combustion is that unburned fuel or flammable hydrocarbons are delivered to the exhaust manifold where it may ignite unpredictably. Another backfire situation occurs when the engine is running rich (with excess fuel) and there is incomplete combustion during the Otto cycle, with similar results.
Popularly the term is used to describe a sharp report produced by almost any type of engine. However, among engine professionals, "afterfire" is the term used to describe ignition of fuel within the engine exhaust system and "backfire" is the term used to describe this same process taking place in the induction system, primarily in internal combustion engines. The separate terms are useful when troubleshooting running problems."...
Sounds like backfire. If you put your bike in a warm place it may start. The warmer air is less dense so hopefully the bike will start because it will not have to breathe cold dense air. I think the cold air is what is keeping the bike from starting. If it starts you can be reasonably sure it is a fuel-air problem due to the cold air. But check your spark plug and compare it to a chart that will give you an idea of how your cylinder is burning fuel. You probably just need to enrichen the mixture with your pilot screw and maybe adjust the valves. Good Luck!
