Ok I can try to explain it but please bear in mind that unlike Kev I am not that mechanical minded and I don't know how to properly explain all the terms correctly ok?

So here goes my understanding of why a single cylinder backfires:

if you imagine a twin cylinder bike has two fuel burning chambers i.e. the vessel where the pistons go up and down. When a piston reaches the top it pushes the fuel towards the spark plug and at that precise moment the spark ignites the fuel and turns it into exhaust gases and these are expelled out through the headers and the exhausts.

A twin cylinder engine will always have one piston up and one down therefore that burning process reoccurs at regular intervals efficiently burning all the fuel all the times even on closed throttle.

A 4 cylinder bike does exactly the same, at any time it will have 2 pistons down and 2 pistons up again burning its fuel efficiently.

Now a single cylinder only burns the fuel once at every cycle when the piston is up, therefore on it's decompression stage with closed throttle the amount of unburnt gases will cause that popping sound.



Here are a few facts I've gathered from various sources:

A single cylinder engine is the most basic configuration of an internal combustion engine. It is often used for motorcycles (a large four-stroke single is often colloquially known as a thumper) but has many uses in portable tools and has been used in cars and tractors.

Pros and Cons
Single cylinder engines are simple and compact and will normally deliver more power within a given envelope. They are prone to vibration, unless this is controlled with balance shafts. Rotating mass tends to be high, restricting sharp changes of speed and acceleration. Cooling is simpler than with multiple cylinders, potentially saving weight.
Compared to multi-cylinder engines, single cylinder engines have several advantages, primarily their simple and economical construction. Balance shafts and counterweights on the crankshaft must be used to balance the weight of reciprocating parts, and can be expensive and complicated due to the collective mass of multiple cylinders. The biggest downside of the single cylinder engine is that it develops considerably lower power to weight ratios than a multi-cylinder of the same type.

Ok so... I hope this help, remember I am not a mechanic and perhaps Kev can either explain better or thrash my whole theory