Think of kickback or backfire as we erroneously call it as the result of the over lean mixture being like tissue paper in your furnace, the burn is fast and super hot and obviously occurs way before tdc on the compression stroke. Kickback would be the correct term. The muffler having fuel in it makes it possible for the engine to run backwards a few seconds when it kicks back. Pictured is the result of one such kickback. This one occurred because when I set the timing so carefully at 33 degrees btdc it was actually at 43. The kickback from over rich is because the compression is increased by the greater amount of fuel. Some will argue that but I think that is the reason. Generally the lean kickback is more violent.
The term backfire is a holdover from carburetor days when raw fuel would flow into the exhaust system during deceleration and the hot exhaust system ignited it. Gasoline Farmall tractors were famous for this. Fire would blow out the top of the muffler.
Last edited by Hobbsy; 10-08-2016 at 03:05 AM.