Changes

No change in size ,  11:09, 26 October 2017
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carb update
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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.  
 
*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.  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 VM24 carburetor.   
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*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.
 
*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.  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.   
 
*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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