The most outstanding feature of this bike would have to be it's final drive which consists of a solid drive shaft (with a torsion spring to dampen the engines vibration) that connects the transmission to a ring and pinion gear set in the rear hub. This setup requires no maintenance and has the safety feature of lacking any exposed moving parts such as a chain that could catch one's clothing. The bikes all came with turn signals and oil injection making them very modern compared to many of their European counterparts. All but the very early years of production had CDI for ignition.
These bikes like most Japanese bikes of the time came with excellent finishes in both the paint and chrome, though the plastic parts on many of these have deteriorated over the years. The engine is smooth running, reliable, and long lived when proper maintenance is observed. The wheels are sturdy for being only 14" but abuse should be avoided.
Typical problem areas that may arise as a result of the passage of three or so decades since their introduction in the late seventies and early eighties include the flasher relay, crankshaft bearing seals, gas tank, carburetor, and petcock. Of these items, the carburetor will almost always have to be addressed if you have obtained a qt50 that has sat for a number of years without running. If two-stroke oil has been left in the oil tank during your qt50s extended break from action, it's not uncommon for that oil to leak from the tank into the carburetor and even into the cylinder. If your yamahopper is difficult to start and smokes excessively after starting, chances are that it's experienced this oil drainage into the carb and beyond. A carb cleaning will be necessary.
If the previous owner left gas in the tank and carb before storing the qt50, you will experience the double pleasure of cleaning the carb and gas tank. When cleaning the carb, focus on the idle jet and the seat where the needle sits. A small diameter wire is an effective cleaning tool when attempting to clear the idle jet of whatever crud that has narrowed its passageway. Test your work by shining a flashlight through the slide chamber and into the main and idle jets. You should see a large circle of light through the main jet and a much smaller circle of light through the idle jet. A Q-tip cut in half and then placed in your drill is another handy tool for cleaning the seat.
Yamaha chose to sit a metal fuel tank atop the qt50 but switched course (perhaps realizing their folly) with the Towny and gave it a plastic gas tank. The qt's metal tank can become rusty over time and especially if stored with gas in the tank. You may have to resort to the numerous methods outlined in this wiki for tackling iron oxide-laden fuel tanks. South of the gas tank is the resting place of the qt's petcock. Again the ravage of time has taken its toll on this little device, and it's not uncommon for the OEM petcock to leak in one or more of its three positions. Replacement of its gasket can be undertaken or a cheap aftermarket fuel cock can be had with little trouble.
If the turn signals fail to blink, the flasher relay has seen better days. As above, a cheap aftermarket 6 volt flasher is easily attained.
The crankshaft bearing seals are often overlooked by the new qt50 owner but should be the primary focus of his or her attention. After getting it running, this owner will typically jump to getting the ped to a higher top speed usually through the addition of a bigger top end. Overheating and piston seizure are traps for the unwary. 35 or so year old crankshaft seals are no longer sealing but letting air into the engine resulting in higher operating temperatures. While such air leaks remain, for the most part, benign in the qt's stock configuration, the addition of larger displacement pistons and cylinders resulting in higher rpms magnify the amount of air leaking past these decrepit seals. Engine failure is imminent. Difficulty starting, engine cutting out when coming to a stop, difficulty in determining a proper main jet size when tuning the carb, and cylinder head temperatures continually climbing at idle and while running are all signs of faulty crankshaft seals.
Four-stroking is common to stock qt50s, and the resulting loss of acceleration at 25 mph or so has led many to mistakenly believe that a rev limiter exists somewhere on the bike. Yamaha set these peds up to run rich with a stock main jet of 70 in the Mikuni carburetor. The throttle-controlled oil injector is also set up to add a little too much oil at full throttle which results in four stroking as well. Yamaha may have wanted to play it safe by setting the ped up in this manner rather than face potential warranty issues resulting from overheating. A 67.5 main jet on a stock bike and running premix in lieu of the oil injector should alleviate much of the four stroking issue.
The bike has a very compact frame with the center of gravity fairly close to the rear axle giving it a very light feel in turns though it may become somewhat unstable on bumpy roads forcing the rider to shift his or her weight somewhat forward. The rear brakes tend to be weak on this bike. Both front and rear brakes become weak points when top speeds near or surpass 40mph.
The QT50 drive train was shared by the Tri-Zinger and PW50. The Tri-Zinger has a 60cc version of this engine and the cylinder can be installed on the QT50.
- The stock cylinder can be raised 4mm with a spacer, or 'rev plate' as seen on Ebay. Top of the cylinder must be decked by the same thickness. This jailbreaks the relatively generous exhaust port (in the stock configuration, the piston partially covers a portion of the exhaust port at bottom dead center; the rev plate raises the cylinder jug and thus allows the piston to fully expose the exhaust port at bottom dead center). pwonly.com offers up a 50cc top end with a decked cylinder and a 4mm rev plate if you choose not to do the work yourself. Expect a top speed gain of at least 10% over stock with this modification. In addition 60cc top ends are prevalent on ebay and amazon. Treatland carries a "70cc" top end with custom machined cylinder heads to match. Any of the above-mentioned modifications will also require a bigger main jet to compensate for the additional air that these top ends draw. A larger carburetor may even be a better choice than the stock carburetor with a larger jet.
- Carburetor options include the Dellorto (or really Dell'Orto) SHA 15.15 and its army of clones (direct fit on stock intake but add an o-ring; read the wiki on this carb); Mikuni VM18 or VM20 or VM24 with the aid of an MLM intake. PW80 carb and Amal 15mm carburetor moped carb are also rumored to work. Pwonly.com offers 19mm and 21mm carburetors with intake that will likely work on the qt50 as well.
- Any talk of bigger carburetors or larger top ends has to include air filters. Any 28 or 29mm air filter is most likely an improvement over the stock air box (although many have resorted to drilling speed holes into the stock air box). The K&N RC-2540 Universal Clamp-On Air Filter is a good but expensive choice. Do not use the entire oil package that comes with it to oil the filter. Half the package or less is plenty. Cone mesh air filters of the 28-29mm variety are plentiful and cheap on ebay or amazon and equally effective.
- Exhaust options include the MLM People's Sidebleed pipe; the tomos A35 biturbo pipe (will require slight modification to flange and additional bracket for mounting); and, if you can weld or have access to someone who can, anything you can weld on to the stock header. Again, a performance exhaust will call for a larger main jet in the carburetor.
- The addition of a performance exhaust, bigger top end or both has the detrimental effect of killing acceleration from a stop or slow speed. Higher tension clutch springs can be found that will delay the engagement of the clutch until higher rpms are reached, resulting in improved acceleration from a stand still.
- An oil injector block off plug can be found if you choose to go to premixed fuel. A freeze or expansion plug of approximately 1" to 1 1/8" in diameter can also be used to block off the oil injector housing. Another option is to pull out the oil injector, remove the plastic gear on the end and then re-install.
- The ignition system is another weak point on these bikes. Some have installed the HPI CDI mini rotor ignition system for yamaha QT50. Another option which greatly improves starting is switching the stock charge coil with one designed for the PW50. A complete PW50 stator can be had for less than $20. Remove the charge coil and install on the QT50 stator. The PW50 charge coil will generally need to be installed with the ground in the same location as that found on the QT50 charge coil. It also seems to help if you reconnect the common ground shared between the charge coil and the light coil.
- The Yamaha Jog CDI can be used in lieu of the stock QT50 CDI. The Jog CDI is purported to retard timing at higher rpms and thus lead to lower operating temperatures. The Jog CDI has five wires as opposed to the QT50's four. Wiring explanation follows:
White/Red from Jog CDI to White/Red on QT50; Red/Black from Jog CDI to Black/Red; Black to Black; Yellow to Orange coil wire and Black/White on Jog CDI to nothing.
- A taller and wider tire will improve handling, ride, braking and, potentially, top speed. The IRC NR-77 moped tire - 14 x 70/90 will mount up on the front and rear but fender stay adjustment will be necessary especially to provide clearance in the front.
- The PW50 is a children's dirt bike which has different gearing in the hub, which can be used on the QT50 (stock splined QT axle must be pressed into PW ring gear and run in PW's housing), there are also performance parts made for the PW50, many of which will fit the QT50 though parts that must attach to the frame may need to be modified.
Sharing an entire drive train with a fairly current children's dirt bike means most things can be had readily from your local Yamaha dealer. Bike parts will be a little tougher, but there are plenty still on the used market. Plastic parts may be the most difficult of all as they did not age well if left in the elements.
You can wire in a different CDI following the diagrams from here: Hobbit CDI Wiring The wires from your QT motor are:
- Red/black- cdi power in
- Red/white- cdi pulse
- The other 2 are power for lights or battery