What is the reason for this kind of sound in the exhaust?

[YamoRide]

Hi everyone,

I was just wondering, just to learn a little more,
why when I press the kick to start the engine, It can happen sometimes a kind of "fart" in the exhaust pipe ?
I'm not talking about the famous "pop and bang", I'm talking about a noise that sounds like "psch"... lol
I made a quick google search and it seems like it was unburnt petrol remaining in the exhaust pipe that would cause this noise when the engine is ignited?
Is that true, please ?

Bonus question :
I see a lot of kits on the internet to increase the engine capacity to 240cc, I wouldn't want to start a debate about whether this really works and if it is really useful in practice, my question would be just to know in THEORY, what do these kits promise to achieve in terms of performance? An increase in top speed? What is really supposed to change?

Thank you in advance for your answers,
see you soon

 

[Awk08]

I made a quick google search and it seems like it was unburnt petrol remaining in the exhaust pipe that would cause this noise when the engine is ignited?
Is that true, please ?

unburnt vaporized fuel is in the pipe, and the rebound waves of the expansion chamber forces those hot unburnt gasses back into the cylinder as part of the 2 stroke cycle, an air leak in your exhaust will allow fresh air into this process and ignite those gases in the pipe.

Is your pipe sealed to the cylinder good ? and a good coupling to seal between the pipe and silencer ?

In racing there is a saying ... " there is no replacement for displacement" meaning 240cc will produce more power than 200cc if all engine work and modifications are equal.

 

[Larry's Shee]

BTW, An engine bored .040" IS NOT a 240 !! To achieve that you need a cylinder with 72mm piston and a stroker crank