Simple backfire question

What is the root cause of backfires? If someone would be so kind as to answer this question in relation to all times they may happen I think it may help not only me but many others. IE during acceleration, at idle and decelerating.
I currently have some serious backfire going on during deceleration and mild backfire at a sustained mid range rpm.
Thanks again everyone for such a great resource.

Depends. Ignition timing can cause it by detonating the fuel while a valve is open. A fuel mix which is too lean will also cause it.

What is the root cause of backfires?

Air and fuel mixture in the exhaust igniting.

That's the sole root cause of the actual noise making event. There are various things that can cause the fuel to be in the exhaust however

A fuel mix which is too lean will also cause it.

Now, for bonus points, tell us how a too lean mixture will cause backfiring (and it is possible)

A hint (just in case you need it) it's the same reason that an AFR (Air Fuel Ratio) measurement will indicate a rich mixture under the same conditions, even though the cylinder is running lean.

As an aside, there has been a lot of research into actual combustion processes in a cylinder in the last few years, a lot of new stuff has been discovered, and a lot of old stuff discredited. For instance, fuel doesn't detonate, it just burns fairly rapidly, and what we call detonation (spark knock) really isn't.
Somewhere on the web (probably youtube), there is a video of the inside of a combustion chamber on a running engine, fascinating stuff.

Air leaks and incomplete combustion are the only two types I know. Air leaks can be on the intake side (such as poorly seated carb holders) and on the exhaust side like re-use of crush washers. These could be considered "lean" conditions as air is getting into the combustion chamber. If too much air gets in due to the pilot mixture being excessively lean, you will get decelleration backfiring. The other cause is incomplete combustion... poor spark is the usual culprit but you can also have a bike that is badly tuned... such as ignition timing off.

Well the ignition timing is right on. I have electronic ignition but have visually checked it and it is right on and the advancer is working smoothly also. I think I will check for leaks at the intake boots. If there are none, then I think I will be looking for new carbs beings my air/fuel mixture screws for the 2-3 cyls are stripped out and can not be adjusted at all. ( Previous owner assault )

Check the intake boots and wouldnt you know it that all of my hose clamps were loose. I dont know if it was from I last took hte carbs off but after a good tightening the bike seems to have lost the backfire in the mid-range!

Thanks for writing back, it's good to know about a simple fix to a problem.