(Transcluded and edited from another post)
Throw out the stock chain, head studs, motor mounting studs, stock head, stock acorn nuts, and stock intake if it's cast aluminum.
Purchase hardened studs from SBP, or cut your own from threaded rod. (Note, some of the motors have M8-1.0 thread pitch studs for the bottom end, with M8-1.25 pitch top ends. Whoever designed this will suffer my wrath, as 1.0mm pitch is impossible to find even on Mcmaster.com)
Use 'Heavy' nuts for the head studs, and double nut the motor mounts. Bore out the motor mounts to 8MM if you have the tap, and use the hardened rod for those. I had a rear mount stud shear clean off at the block recently, I had to sharpen a drill specifically to bore it out.
Do NOT use the stock chineese hardware for any load bearing applications. I should not have to make myself clearer.
Purchase the following:
https://www.treatland.tv/puch-moped-...-70cc-maxi.htm
https://www.treatland.tv/dellorto-SH...lever-carb.htm
Your choice of expansion chamber Exhaust. Or make up a header and use all black exhaust chamber - 28mm.
If your stock intake was cast aluminum, get Dellorto SHA 15.15 W lever Choke and 19MM bushing instead of the treatland dellorto.
https://www.treatland.tv/SHA-15mm-cl...lone-cable.htm This carb is still superior to most of the clone carbs out there, the aluminum is nicer and it has a mixture screw. I'll give a more detailed analysis shortly, my moped guru has one I'd like to buy.
Once you have all your treasures, buy mcmaster Part number 42955A22. A right angle attachment for your dremel will help tremendously. Drill out the holes on the treatland head to 21/64", and then go to town on the stock china jug. Raise the transfer port ceilings till they're flat with your carbide cutter, and aim the ceilings so they point at your spark plug. Check this with a cut off popsicle stick. You want both incoming charges to collide over the spark plug. The transfers on these kits range from poor and ambigious, to downright useless. A bigger better carb and pipe won't help you unless your porting is corrected. THAT is the secret sauce retailers will never tell you about.
Once your transfers have been corrected, clean up the intake and exhaust ports. The exhaust can be widened a mm or two, but don't get crazy, and hang a ring. And don't raise it at all unless you like having no bottom end power. Same goes for the intake: Raise that port, and bad things happen. A little width will help a bit, but not much over here due to the intakes on these motors being so small. Leave the surfaces rough on the ports, a little turbulance helps flow believe it or not.
IMPORTANT: Chamfer or radius ALL port edges. go over them with a small grinding stone or very lightly with your carbide burr and put a little fillet or chamfer on them, then scrub down the edges with some 800 grit plus sandpaper on a popsicle stick. If you have rubber polishing bits for a dremel, use those here. Remember: only you can prevent hung rings and scraped pistons.
Clean the cylinder VERY well, we don't want granules of aluminum in the motor, and slide it down over the piston. Bring the piston to top dead center, and look in the intake port. See that piston skirt overhanging there? Either cut the piston skirt to match the intake at TDC for more high end RPMs, or use JBweld to correct the intake port shape to conform to the piston skirt (Or a bit below, remember, this is all about timing something that follows sinusodial motion) to bring up low end grunt.
The piston skirt defines the opening and closing of the intake port. Cut the piston skirt to increase the open time (And make the port open sooner/close later) and you'll lose some primary compression. Lower the port ceiling, and open time decreases, the port opens later and closes earlier. Your call.
After you've ripped your motor asunder in this manner, port match your intake and exhaust of choice, make new gaskets all around (Head gasket can survive for now, but making one from copper foil has been phenominal for me.) Lap flat all the mating surfaces of the head, Cylinder, Intake, and exhaust with at Least 600 grit sandpaper, 800-1500-2000 series if you're insane like me. A mirror, windowpane, granite surface block are all good surfaces for this operation.
Balance your crank, or have someone do it for you! Seriously important, even a 'super duper pro screamer race motor' will shake your teeth out if it's not had proper balancing done. There are people who understand it better than I do on this forum, I'm a 'Keep drilling holes near the crank pin till the shaking goes away' brute force approach type of guy. Oh, and stuff the balance pads and hollow crank pin with something like ultra grey to bring your crankcase volume down, and your primary compression up. While you have the cases apart, lap the mating surface where the cylinder meets the cases, and match the cases to the cylinder. You'll thank me later. While you're there, if you have access to a sandblaster, mask off your bearings REALLY well (Or pull them out and re-assemble with press fit loctite) and texture the inside of the crank case. Every little bit helps.
Now you're ready for re-assembly! =D Buy a decent BMX chain, and make sure your front sprocket fits freely. If not, very carefully grind it with your handy dandy dremel so you have good free movement. Replace all the stock gaskets with ones you've made from Felpro (Or your material of choice) All the stock hardware with stuff that won't strip under standard amounts of torque, and install on your bicycle. Usual rules apply there.
I don't know if they choked these little motors on purpose, or just for the fun of it, but your friend is right: You can get a lot more power out of a jailbroken motor than they come stock with.