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== Tuning Options ==
 
== Tuning Options ==
 
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 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.  
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*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.  
*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.   
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*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.   
*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.
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*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. 
*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.   
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*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.   
*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.   
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*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.   
*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.  
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*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. 
*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.   
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*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.   
* 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:
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*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.
 
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."
 
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.
 
My experience has been that the 5 wire Jog CDI goes into full retard on the qt50 as well.
*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.  Kenda also makes a number of 60/100 x 14" dirt tires that may work as well.
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*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.
 
*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.
 
*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.
  
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