The misunderstanding and problems of the 2004 Yamaha FZ1's 'choke' system, some of you would say that is easily answered the 2004 Yamaha FZ1 has officially no choke. The Yamaha FZ1 uses, like many other carburetor motorcycles, it instead uses a cold-start enricherner system involving separate fuel-metering circuits in each carburetor.
When you turn on the enricheners, you open a plunger valve in each carburetor that allows engine vacuum to draw a very rich fuel mixture into that carburetor's bore, just on the engine side of its butterfly valve. It has much the same effect as a choke but achieves it in a different way.
Second, if you are encounter an explosive bang out the exhaust when you open the throttle, it is called 'fterfiring', and not be mistaken by 'backfiring'. It is so named because it occurs in the exhaust, after any unburned mixture has left the combustion chamber.
It is nothing to worry about. Similarly, 'backfiring' is when the engine spits back through the intake before the mixture exits the combustion chamber.
Some 2004 Yamaha FZ1's had some small problem in which the enrichener valves did not fully closed. The problem with the enricheners is not always at the same location, therefore you should remove the fuel tank and, with the engine off, keep opening and closing the enricheners while closely watching every part of their operating system, from the cable on the handlebar to the linkage that connects all four plunger valves. Using this method, the cause of the hang-up should be easy to find and repair.