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 (I've yet to run into a QT50 with a points 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 charge coil, 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 the Towny (the qt50's big brother) 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.
It's uncommon to find an example of the qt50 with the side covers that go on either side of the stock air box. Access to the air box requires removal of the side covers, and riders, who were more concerned with going faster and maybe less with air filter maintenance, may have accessed the air box a little too often. Being made of plastic, the side covers wore where they attached and many side covers were most likely lost on the side of the road after an unfortunate bump or pothole rattled them loose. Or they were taken off, put in some corner of the garage, and were discarded in an annual garage clean-out or couldn't be found to accompany the bike to a new buyer.
Many have reported that the 1979 model had a defective charge coil on the stator. I believe that Yamaha used a lousy charge coil on all models. Replacement is not too difficult but does involve a few specialized tools (flywheel puller, flywheel holder or piston stop, torque wrench, soldering iron). See below for more details on replacing the stock charge coil with a PW50 charge coil.
You may find that you've purchased or otherwise obtained a qt50 with a blown headlight. The qt50 has a sealed beam headlight. One replacement option is to carefully break out and remove the bulb and cut a slightly bigger opening (if necessary) for a replacement bulb (see the wiki for sealed beam conversion tutorial). Another option is to obtain a headlight for a Honda C70. This headlight has a replaceable bulb and fits perfectly in the qt50's headlight ring. Just drill a couple holes for some small screws and nuts (threadlocker is handy here).
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.
Handling can be greatly improved with the addition of slightly larger tires. A popular choice is the IRC NR-77 moped tire - 14 x 70/90. Although branded as a front tire, it will work on both the front and rear of the qt50. At 70mm wide or 2.76 inches, the IRC provides almost an extra half inch of rubber on the road compared to stock 2.25" tires. Ride, handling, braking and, to a small extent, top speed can be improved with them. Adjustment of the front and rear fender stays will most likely be necessary to accommodate these tires.
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.
- QT50 rev plate - 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.
- QT50 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 TM24 with the aid of an MLM intake. Amal 15mm carburetors are rumored to work. The 15mm PW80 carb works with the stock intake and generally comes with a 120 jet which will be rich on a stock bike. A 100 to 105 main jet should get you in the ball park but every bike is different. You can also find other carbs on ebay made for the PW80 that should also work on the qt50 intake (main issue may be adequate clearance for the size of the bigger carb), many of which are much larger than the 15mm PW80 stock carb. Pwonly.com offers 19mm and 21mm carburetors with intake that will likely work on the qt50 as well. The MLM 24mm spigot style intake also allows for OKO and pwk style carbs in addition to the Mikuni TM24 carburetor.
- QT50 air filter options - 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. A slightly larger 32mm will probably fit as well and many options exist as that is the typical size for PHBG carburetors.
- QT50 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. Yamaha used two different stock exhausts on the qt50. One had an "S" shaped header and the other had a much straighter header. Although unlikely that any significant difference exists, the straighter version contributed more to performance than the "S" shaped version. In late 2017, MLM came out with the QT50 Next Level Stock Pipe. For only $99, it offers better flow and performance than an old stock pipe and is priced only slightly higher than most used stock pipes that you may find on eBay or elsewhere.
- QT50 clutch springs - 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.
- QT50 Oil injection removal - 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. Or you can find a hose or fuel line that offers a compromise between the different sizes of the oil injection inlet and outlet ports and connect the two to continuously cycle a small amount of oil through the pump while running premix in the fuel tank. Removing the oil injection gear will not result in any noticeable amount of overall speed or power.
- QT50 charge coil options - 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.
- QT50 CDI options - 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.
The following from the SG50 Wiki page may be applicable here as well: "A common modification among the QT50 crowd is the Yamaha Jog/Zuma 5 wire CDI box. It has a high rpm retarding feature that keeps the temps down by retarding the ignition timing as the rpms increase. However, this 5 wire box DOES NOT work for the Sting. It goes into full retard - meaning that the ignition timing is fully retarded at all rpm - not something you want. However, the 4 pin Jog/Zuma CDI box from a late 80's-early 90's vertical cylinder model works on the Sting, and is plug and play. It appears to have a retarding curve as well." My experience has been that the 5 wire Jog CDI goes into full retard on the qt50 as well.
- QT50 Tire options - 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. Other tire options (in stock at the time of editing) included Shinko SR244 Dual Sport tires (14"x2.75"); Shinko SR241 Dual Sport tires (14"x2.75"); vee rubber VRM 020 moped tire (14 x 2.25); irc inoue tire - (14" x 2.25") hod rodddder; Kenda K261(14" x 2.25"); Yokohoma World Tour 956 (14" x 2.25" and 2.00" - not sure if you can find this still); Guang Li Dirt Tire (2.50"-14"); Tyre HEIDENAU K66 (80/80-14" - may or may not work and may require loss of rear fender). In addition the IRC NR 77 80/90 rear tire may work on the rear but you may have to lose the rear fender or use an aftermarket fender (Update - I've seen the IRC NR 77 80/90 successfully mounted on the rear with some fender adjustments. Kenda also makes a number of 60/100 x 14" dirt tires that may work as well.
- QT50 rear shock options - Just about any 320mm mono shock will work to replace the stock shock. You need one with the eyelet at the top and the clevis end at the bottom.
- 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