These are new 4-stroke machines, not 2-strokes. This is not a muffler problem, it's a "diagnostics" problem. I'd like to thank everybody, but guessing at solutions isn't helpful.
Firstly, you passed comments on a thread which is all about 4 stroke engines - not 2 stroke engines. I could be wrong but I it looks like you do not seem to understand that according to the mechanic this is a muffler problem on a four stroke engine ? Similar to the engines on your friend’s scooters ? This is the exact reason why I guessed your friends might have a similar problem.
In this case it looks like even the professional mechanic or mechanics were guessing by times. To my understanding the owner asked for help at this forum, therefore some of us started guessing.
I personally learned a lot about scooters just by reading comments from members who started guessing about possible solutions. Even when certain “guesses” are not applicable on a spesific problem, it always make sense to try to remember what advice were given.
By times advice that was not applicable for a specific problem, help others to solve problems. To my mind forums will serve no purpose and members will loose interest if other members are not guessing about possible solutions.
As far as I am concerned - thanks to everyone for “guessing” about possible solutions all over this forum. It surely helped me to learn a lot.