QT50 performance upgrades

How can I make my qt50 go faster?

I got the need for speed. What can I do?

Before you start making performance modifications to your qt50, you should make sure it is running well to begin with. Performance modifications will only exaggerate current issues. Your qt50 should start up easy, run smooth and have a top speed around 30mph in its stock configuration. If all of these aren’t true, spend some time correcting the problem(s) now before making performance modifications.

1. Install a new and bigger rear tire.

Chances are your tires on your 35 or so year old qt50 are a mess. Most likely they are the original tires with 4000-5000 miles on them. Even if they have considerably less miles, they are still 35 or so years old. If your bike was owned by a kid in the past, your back tire may even be bald. I’ve had at least two qt50’s with bald back tires.

The original tire size on the qt50 is 2.25″ wide by 14″ in diameter. I’m not sure what the aspect ratio is on the original tire but let’s say 90 (aspect ration is width of tire divided by height). If I bump up to an IRC NC 77 tire, I’m going to a 70/90×14 tire. So 70mm wide or 2.75 inches wide with an aspect ratio of 90 would make the height 0.81 inches higher than the stock tire. So if my qt50 can spin the taller tire at the same revolutions per minute my speed should increase. Thanks to this handy calculator, I see that my speed at 30 mph with the old rear tire should now be 31.53 mph. That’s an almost 2 mph increase by just changing the rear tire.

Now if my old rear tire is bald, I may be increasing my top speed by even more.

What’s the trade off? My initial acceleration may be a bit slower with the bigger rear tire. The bigger rear tire is heavier and your qt50 in stock form may not be able to spin it at the same rpms as the original tire.

Of course, this makes me wonder if I can use an IRC NC 77 80/90×14 tire. Tire width goes to 3.15 inches and top speed goes to 32.71 mph. I imagine the rear fender would have to go with this tire and would the tire fit on the rim? Here’s a nice picture of the stock tire vs. the IRC tire.

2. Install a mesh air filter.

Get an aftermarket mesh air filter. This filter will allow the engine to get more air. Of course, you’ll need to change your main jet to a bigger one to compensate for the additional air (you need to keep your fuel/air ratio similar – if you add more air, you need to add more fuel). The stock main jet in a qt50 is a 70. You may need to bump up to a 72.5 main jet with this filter. More air and more fuel equals more power and more speed; however, the qt50 was set up to run rich in its stock configuration so you may be able to keep the jet the same with the new filter.

3. Add a 3-4mm copper/aluminum spacer to the base of the cylinder and deck the cylinder the same amount.

I’ve added a video on this modification here at the bottom of this page.

4. Install an MLM people’s sidebleed exhaust.

I talk about this above. You’ll need to increase the size of the main jet when you do this.

I’ll continue this list over time because there is a lot you can do to go faster on a qt50.

I just bought an MLM people’s sidebleed pipe for my QT50/MJ50. How do I fix it so that my center stand/kickstand doesn’t hit it?

You need to get another Yamaha Center Stand Stopper (part no. 278-27114-00) and super glue it to the original stopper so that you have two stacked together. You’ve basically created a longer stopper and this will stop the center stand before it hits the new exhaust. Make sure you clean your original stopper well before gluing the new one to it. It can be incredibly frustrating to spend $200 on a performance exhaust and then have the kickstand bang it every time you use it.

I just bought an MLM people’s sidebleed pipe for my QT50/MJ50. Is there anything else I need to do before installing/using it?

YES! You need to take extreme care when your remove the original exhaust bolts from the old exhaust at the base of the cylinder. Chances are those old exhaust bolts are super rusty. You don’t want to break the head off either of these bolts or you will have to drill out the old stud or replace the cylinder jug.

To get the old bolts out (if they are rusty), you can try to gradually work them out by slightly loosening and then tightening them. Rinse and repeat several times. Do this little by little in combination with PB-Blaster. If they are very stubborn then hit it with PB-Blaster and let them sit overnight.

When you do successfully extract them, replace them with new bolts of the correct size. Add a new exhaust gasket and put a little anti-seize on the new bolts before installing.

You will need to replace your main jet with a bigger main jet. Try an 80 or 85 main jet. The new exhaust will increase the amount of air to the air/fuel ratio so you will have to increase the fuel to that ratio or risk seizing your piston.

Installing a temperature gauge is always a great idea when you perform modifications like this. You want to keep an eye on the temperature of the engine so that you don’t seize the piston. Trail Tech offers a 14mm gauge for $39 or so.

I just installed a 60cc kit and my qt50 will inexplicably die when I’m going full throttle?

You more than likely over-jetted. Your main jet is way too big. You’ll think your qt is not getting enough gas because it stalls. Truth is – it is getting too much gas and dying as a result. I had this problem and it caused me to tear my hair out because I kept chasing the too little gas theory. Look how wrong you can be. I solved it by going from a 102.5 jet to an 87 and finally to an 85 main jet. I also went to a BP4HS plug and ditched the colder BP6HS plug.

Or your crankshaft seals are bad and need replaced. If these seals are bad then they will allow more air to enter the engine. At full throttle, the bad seals may let in so much air that it causes your bike to die. So, in essence, it would be getting too little fuel. Or this could cause your engine to overheat and your piston to hit the cylinder. Right and left side crankshaft seals can be found on ebay or treatland.tv.


Crankshaft bearing seal replacement on mj50, qt50 and pw50

Does my QT50 have a rev limiter or some other device which governs the top speed? I notice sometimes at full throttle it will run rougher, make a different noise, seem to hesitate and check up.

The QT50 does not have a governor or a rev limiter. What you are experiencing is a phenomenon known as four-stroking. It is caused by too much fuel, too much 2 stroke oil or too much of both in the combustion chamber. 2 stroke engines are designed to four stroke in certain situations. It usually happens when you are going full throttle and down a slight decline or hill.

Yamaha has tuned the engine to do this in that situation and it represents a compromise. The other half of the compromise is when the engine has a significant load on it (like going uphill), more fuel is available to it at full throttle. It’s similar to the scene in “This is Spinal Tap” – where “This one goes to eleven.” Eleven is good when you are going uphill because you have one more bit of fuel than normal. The engine needs more fuel going uphill because it is under a bigger load.

Eleven is not so good when you don’t need it – like going slightly downhill. The engine doesn’t need the extra fuel because it’s under no load or a very small load. As a result, it can’t burn that fuel and starts four stroking.

Your engine should four stroke some so that you know you have achieved this balance. If it four strokes excessively (new riders may not be able to truly evaluate excessive four stroking), you can try several things to lessen the four stroking:

1. Install a free-flowing air filter;
2. Adjust the oil injection pump;
3. Eliminate your oil pump and just use pre-mixed gas/oil at a specific ratio (50:1 or so);
4. Try a slightly smaller main jet;
5. Try a hotter spark plug;
6. Adjust your throttle stop screw and your idle air screw on your carb;
7. A combination of the above.

Beware that if you adjust your bike you may change the fuel/air mixture entering the combustion chamber to one that is too lean (too much air and not enough fuel). A too lean mixture can cause serious engine damage. Yamaha may have intended that the bike run rich to promote engine life. Better a fouled spark plug than a seized piston, right?