I worked at a KymCo, TGB, Genuine, Adly and SYM scooter dealer from 2007 tp 2010. Part of my duties besides assembling and tuning scooters was installing aftermarket Leo-Vince and Genuine Scooter hop up kits on our bikes we sold. Most of these bike when uncorked or derestricted were close to 5 h.p. and that was with the stock exhaust.
Bikes I personally converted to performance kits were the Genuine Black-Cat, Buddy, Rattler 50 and 110 with a 125cc kit, SYM JetEuro 50 and the Adly ThunderJet 50.
The HT engines have two transfer ports and they're small. your lucky to get about 2.5 to 3 HP out of a well fettled 66cc HT engine. You can't really cut the port open much bigger without weakening the cylinder. And theres not much can be done to adding a third transfer
All of the scooters I discussed have 2 to 3 big transfer ports and case reeds. They can easily flow enough fuel-air to rev upward to 8~9k stock, even more with an aftermarket reed booster cage and bigger carb. Most run carbs in the VM18mm size stock
In every case with the scooters the adding of the expansion chamber exhaust took the wide, stock power band and moved it about 1500 ~ 3000 rpm higher, weakened the bottom end torque so you had to change the pulley weights and rear pulley spring and shoes to allow the engine to operate, (come onto the 'pipe',) at a higher rpm. And any bike with CVT's work with a pretty narrow RPM when they're tuned for a pipe, at least until the CVT is in it's tallest ratio. That's why you change the pulley weights so the engine starts to close the front sheaves at a higher rpm, and why there's a spring and flyweight kit added to the rear pulley, so the bike launches at about 4~5K instead of 2.5K.
So yeah I've got a few years professionally hopping up scooter engines, in addition to owning and running 2 cycle motorbikes from 1973 to 1990. Stock exhausts are always set up to create a smooth, wide powerband.
Mike is asking about improving torque. You can either have a wide powerband that work over a wide rpm range or you can change your exhaust to one that boosts the HP and RPM from stock but will take the wide torque range and squish it into a 3~4K range at a higher RPM with a slight boost in torque in that narrow range.
Effectively making the engine bog severely under 3500.
Several people here gave Mike decent advice cleaning up the ports, add a long intake runner, cut the internal exhaust pipe in the cap a few inches. Cannonball2 said that installing the Yamaha PW50 carb gave his bike a better transition from idle to mid throttle. It's all little changes like the PW carb which little smaller than the NT carb, but because of it's cleaner castings and better precision in metering, it's more precise in it's overall mixture, = engine runs smoother from idle on up to WOT. These are easily done minor tunes that can improve the HT engine's already wide power band.
You're not going to give the HT engine a big boost in overall torque by adding an expansion chamber only. That increase only comes by increasing the displacement, and some serious changes in the engine's intake like case reeds or a windowed piston-reed box. The scooter exhaust is made for engines running much more volume and bigger transfer ports.
