BBK, especially sport 70cc kits, do not need an expansion pipe to run right. Your overall MAX power will decrease by a few HP without a matched pipe, but the TORQUE will be about the same and at a more useable RPM. I plan on doing a full build on my 2001 Zuma Vertical, and use the stock pipe for torque and sleeper sound (Hard to tell cops your bike is still 49cc if it's 120db!!). Might take 5-10mph off my overall top speed but my build is for wheelies on that scooter, so I want all the low end torque I can get. If you are going for 100% top speed then a pipe will help. You will loose a little off the line, and lower RPM, and your engine will need to rev higher to "get on the pipe", anywhere from 9-14k RPM. The stock pipe allows the Sport kits to last much, much longer due to lower RPM. Gas mileage will also stay closer to stock, instead of getting cut in half with an expansion pipe. If you purchase a "race" bore, those will be utterly pointless on stock exhaust, and honestly pointless to ride on the street re-ringing every 1500mi. As far as installing the kits in the bike, no problem at all with most Kymcos. If you've done it before, and have the cylinder prepped (Ring gap, chamfer, etc), its a one or two beer type of job. The L/C engine takes slightly longer due to coolant burping. I can replace top end on the A/c Kymco engines in under 30 minutes including plastic removal. I honestly would suggest doing the BBK BEFORE a pipe, the main reason being that say you buy a pipe for the stock cylinder, that pipe will not be the right setup for any kit. Your better off with a kit, then if you need more power you can get a good matched pipe for your kit. Different kits take different pipes, you have to research what other people have tried, preferably dyno testing. I've put cheap aluminum 70cc jugs in the old People 50, had it pegging the speed-o with only a kit and lighter rollers. As far as the OP question, If all you have done is a kit, no airbox mods and stock everything else you should be under #100 main jet. I would bump up the a/f screw a bit and break the cylinder in with a half oz of 2t oil premixed in the gas. If I had to guess, which is all any of us can do, you'll end up around ~#92-95 main jet. Start at #100, go run it for a while see how it feels then check the plug. Don't go full throttle for more than a second or two until the motor is broken in. Your not going to get accurate plug readings until the motor is broken in, so just start big and go by feel. No one on the internet will ever know what jetting to use on almost any bike, it's just guessing like the Z master said. Good luck though!
