here's a rough draft of everything i have to offer in regards to the QT. sometime ill finish it and have it posted in moped repair section.
After having gone through a couple QT's I thought I would share what I have learned and try and compile all this information in one locaton.
The service manual for this bike is available here:
This guide will assume you have basic technical competence and will not cover such basic procedures as cleaning/tuning the carb or replacing the air filter. Personally I have stripped mine down to the frame and the engine, regreased everything, blue locktited all the nuts and bolts, and basically given it the love it needs to run virtually maintenance free for years. You should do these things of course but there is already information out there and these things should be done to any used bike. I am trying to stick to things specific to the QT.
BUYING PARTS: I see all sorts of parts for these things on ebay at up to 10 times the price you can get them for at the Yamaha dealer. Before buying any parts on ebay, check the dealer first. Almost every part is still available and at 1980's prices. If you are making a large order I have found www.motogrid.com
has the lowest prices and you can use their online fiche to check prices. For small orders i just pay an extra 10 cents on the dollar and let Yamaha pay the shipping. If you are going the 60cc route I would just make one large order from motogrid.
The dealer has another use. Most dealers will order from Tucker Rocky catalogs and pay the shipping so you can get the same prices you see online without paying shipping. There are a few items in particular. Kenda 14x2.25 tires (about 12 bucks each), EBC brakepads. (about 10 bucks a pair), NGK spark plug wire splice (6$, since the wire is attached to the coil and the wire is probably worn, this is the best solution i have found). Your dealer should be able to order cables for between 10-15$ each. Some of them are out of stock from yamaha but motion pro has some. Your dealer probably has catalogs that have these items.
AIRBOX: The stock airbox is too restrictive. The best solution I have found is to drill holes on the sides of the airbox because I have noticed after riding in the mud that it splashes up everywhere else but the sides are almost mud free. If you have the stock side covers this is less of a problem, but drilling on the sides gives you plenty of clearance between the airbox and the cover to allow good airflow. The stock QT comes with a 70 jet which is too big if you dont drill the airbox, and about right if you do.
You should also pin the side covers to the airbox so they dont fall off. I have explained this procedure here:
--MUFFLER: The muffler is probably clogged. They seem to clog easy. The easiest way to deal with this is to throw it in a fire for an hour or two and just burn it out. Then clean it off, lightly sand it, and paint it with some stove black paint. While you are at it check the exhaust port for carbon. If its bad, you should probably pull the top end, clean it, and rering/hone/bore as necessary. If you follow the 60cc path at the end of this document you will be doing this anyway.
--GAS and OIL: You should premix your gas and oil for much better performance. I use Amsoil Intercepter at 50:1 but I'm sure any of the synthetics would be fine. Since you no longer need the oil pump, you should pull it out and remove the gear from the end of it, and re-install it. This prevents any problems with the oil pump seizing or dragging from lack of oil and gives you a teeny bit more power to the wheel. In doing this you get an extra benefit.
The oil tank capacity is .2 gallons. Now that you aren't putting oil in it, why not use it for gas. .2 gallons doesnt sound like much, but the stock tank capacity is only .6 gallons. So its a 33% improvement. First disconnect the oil sensor. You need it to plug the hole in the gas tank, but I dont like the idea of power going to that sensor while its sitting in gas. If you want to chance it, go ahead. Next, head down to the auto parts store and buy a fuel filter, 2-3 feet of 1/4â€ gas line, a T, a fuel shutoff valve, and about 9 clamps for the 1/4â€ hose. About 10 bucks.
First, you want to use the T to run a line from the bottom of the oil tank into the gas line. Just rip out the existing gas line and replace it all while you are doing this. Then put the shutoff valve in the line from the oil tank before it hits the T. Put the fuel filter after the T so it filters both tanks.
When you fill the tank like this you can just fill the gas tank and it will slowly fill the oil tank as it goes through the T. But I've found it takes a while so usually i just pop both caps and fill the oil tank first, then the gas tank. Once both tanks are full, you can shut the valve off to the oil tank and you have a nice .2 gallon reserve tank. If you do this, I would open the valve once in a while and run that gas through so it doesn't get old.
I consider this a 'Must do' upgrade because the fuel line probably already needs replacing, the oil pump sucks, and although it may not seem like much, it's still 33% more capacity than stock.
FORKS: When you take the forks apart to grease them, You will see that there is a rubber 'bullet' in one of the forks. This prevents the forks from bottoming out. If you can get some bent forks somehow you can get a second bullet to put in the other fork for extra damping when you hit a nice pothole.
60CC UPGRADE: Ok this is the one you have all been waiting for. If you do this right it will cost you very little.
The cylinder off a YT60 (1984-5) or YF 60 (1986) will bolt right on your QT. You can get them used and bore them, but motogrid has them for 120$ brand spanking new. You can reuse parts, but they are all cheap so lets just do the entire top end and be done with it. You need to order all of the following parts to pull this upgrade off.
YT60 parts: Cylinder, Piston, Piston Rings, Wrist Pin, Piston Bearing, 2 piston clips, head gasket, cylinder base gasket.
QT50 parts: exhaust gasket. This may not be necessary, but its cheap and you are already paying shipping on everything else. If you have any other parts you need now is a good time to order them from motogrid instead of your local dealer.
You use the stock QT head. This will increase compression and you will want to get a BP6HS or maybe even a BP7HS plug (colder plug, stock is a BP4HS) to avoid engine damage. Plug chops will tell you what plug you will need.
Once you get all this on, the stock gearing will not be able to take advantage of all this power. What you need to do is find a PW50 ring and pinion gear for the rear end on ebay. I have seen these go for as little as 10 bucks including shipping for just the gears, and you see full rear ends for 35ish shipped all the time. This gear has a 4.82:1 gear ratio vs a 5.7:1 stock gear ratio and will give you more top end, and lower rpm's all through the range. Stock the QT somewhat struggles with this gear, but with the increased displacement and compression of the new top end, you can cruise at an honest 35 and ive hit 47 with mine. The PW gears use a longer axle than the QT so you have to take the ring gears to a machine shop with a press and have them swapped. You MUST switch the pinion gear in your rear end. I have found that the spark plug wrench in many motorcycle toolkits can be used to remove the pinion gear nut. If you mix the PW and QT gears they will not mesh properly or maybe not work at all. QT gears have 10 and 57 teeth so if you get them mixed up, start counting.
This will run you about $230 or so. Now you are saying WOAH MAN thats too much!!! but wait. I have saved the best for last. You now have a QT ring and pinion gear with a PW shaft already installed. If you sell this on ebay and market it to PW50 owners they will buy them. The PW50 is a small dirt bike that has been made for 20 years and by switching to a 5.7:1 gear they get much needed acceleration and hill climbing ability. I have sold just the gears with no PW shaft for 150$ and again for 140$. If you bought a complete PW rear end, swapped the shafts, cleaned it up and regreased the bearings, I think you might get as much as $200 for it as a ready to go bolt on gear swap package.
Now im not saying you will get this much, just that I have. But even if you only get $80 out of them that brings the total cost of this upgrade to around 150$. money well spent.
Lastly, I lied. I do not actually have a PW gear on mine. I have a gear off a chinese PW copy that is 41:8 or 5.125 to 1. If you ever get this gear do not bother trying to swap shafts. Just grind 1/4" off the end of the shaft and make a spacer. I love this gear. Even on a stock QT50 if fully tuned up it works well. On the 60 its awesome. But good luck finding one.
Now you have the QT that should have been. Much more rideable, and plenty of power.
Well, I'm tired of writing. I hope you have found this guide useful or interesting.
randy's QT guide V0.1 copyright randy