Q. My son's Piaggio NRG scooter has lost power. It used to do 50, now it does 26mph at 7000rpm. It does not sound the same as his mate’s NRG and when I took the exhaust off something is rattling around in it.
It’s had a new piston, rings and barrel and I have checked the reed valve. The new spark plug is also the right colour.
A. If there is 'something' rattling in the exhaust it is a fair bet that this is what is causing your problem. Two-stroke exhausts are shaped to use the sound pressure waves from combustion to suck out exhaust gases and pull in a fresh mixture.
If there’s carbon build up, or the baffles are loose that will send it out of tune like a dented trumpet at higher revs, so its performance will fall off as you describe.
You should be able to remove the baffles and find the cause of the rattle, and whilst the baffles are out you should remove the carbon build-up by blocking the end pipe and filling it with a caustic soda solution overnight, that includes the downpipe.
If after you have done this you find you still have the problem it will probably be crankcase seal failure.