Stock Configuration and Basic Performance Upgrades
In its factory form, the Yamaha QT50 should reach a top speed of around 28-30 mph. At top speed and when the engine is under little load, you may hear the engine note change tone from a high pitched whine to a lower and rougher growl. Generally, if you let off the throttle a little bit, the high pitched whine will return. Many mistake the low, rough growl as a result of a rev limiter residing in the CDI unit or perhaps elsewhere. What you are really experiencing is the engine four stroking as a result of too much fuel in the combustion chamber. Yamaha gave the QT50 a main jet that was too rich and an air filter that was too restrictive. Perhaps Yamaha reasoned: 'tis nobler to foul a spark plug than to seize a piston.
Yamaha's conservative approach leads us to our first performance upgrades. Swap in a slightly smaller main jet, a less restrictive air filter or both. Stock main jet size in the QT50 is a 70. Many have had better luck with the slightly smaller 67.5 main jet. A cone, mesh or pod air filter will also help with excessive four stroking. A 28 or 29mm air filter will fit on the stock Mikuni VM12 carburetor. Be mindful of not completely eliminating all four stroking. That extra bit of fuel that causes some four stroking is required by the engine when it is under greater loads like climbing hills, running into a strong wind, etc. If you eliminate all four stroking, you may run the risk of seizing your piston.
Another culprit in QT50 excessive four stroking is the oil pump. The oil pump is reputed to add excessive amounts of two stroke oil to the fuel. Just like too much gas, too much oil can cause the engine to four stroke. To combat this, many riders have eliminated the oil pump and replaced it with an oil plate block-off or an automotive freeze plug. Another option is to remove the plastic gear from the oil pump and re-install the gearless pump.
The QT50 has the main throttle cable leading to a throttle junction where it separates into two cables. One leads to the oil pump and one leads to the carburetor. The oil pump, slide spring and junction box all provide tension to snap the throttle back after you let go. If you lose the oil pump, you also lose some of that snap-back tension. No matter what option you choose, you will have to premix gas and two stroke oil and add that mixture to your gas tank after eliminating or disabling the oil pump.
Reducing four stroking through a smaller main jet, more free-flowing air filter or both might get you 1-2 mph more. Such puny gains are not going to satisfy your QT50 fanatic. Check the header on your exhaust and determine whether it is an "S" shape between the flange and silencer or more of a straight shot between the two. The straighter header will perform better; however, a used QT50 exhaust cannot be had for pennies on ebay and your money may be better spent on a performance exhaust.
MLM makes a performance exhaust for the QT50, the MLM Yamaha QT50 People's Sidebleed Pipe. You will also want the Malossi exhaust gasket. Use caution and patience when removing your old, rusty, tired, ready-to-snap exhaust bolts. New bolts with some anti-seize compound are recommended. The MLM exhaust will also require you to install a bigger main jet. Perhaps around 10% bigger than stock. You may also choose to richen up your jet needle by moving the clip down one or more positions. All this is necessary to avoid seizing your piston as the performance exhaust will have the effect of leaning out your fuel/air mixture. Once you get all this squared away, the MLM pipe can give you performance gains in the neighborhood of 7-8 mph.
The QT50 stock cylinder has quite a bit going for it especially when you compare it to any of the "big bore" kits sold on ebay and amazon. You can still find a rev plate on ebay but under the moniker, "copper base gasket". The seller will make you thicker base gaskets in copper or aluminum. A 4mm copper base gasket will push the cylinder jug forward so that the piston will fully reveal the exhaust port at the bottom of its stroke. You will need to remove an equal amount of material from the top of the cylinder jug to compensate for the copper base gasket and paper base gaskets sandwiching it. With a fully open exhaust port expect gains of 3-4 mph. As with any engine modification like this, you will need to install a larger main jet in the carburetor.