QT50 carburetor issues

What is the main jet size on a stock Yamahopper QT50?

70. No doubt! Came from the factory that way. Pretty sure it was the same on the Towny as well. Update, Towny stock jet is 72.5.

I’d like to get a new carburetor for my QT50. What is your opinion of the clone carburetors found on ebay?

It’s hard to beat a new carb for $17. I’ve bought several of them for the QT50 and MJ50 and never had any issues. The main jet does not have any sizing numbers on it but that’s not the end of the world. At the very least, you can use the gaskets and other parts from the clone carb to rebuild your original Mikuni carb. Sometimes, however, your original carb is just too far gone to revive. In that case, you may have no other choice but to buy a clone carb. Many people knock the clone carbs but I’ve never had any issues and have probably purchased half a dozen or so.

My QT50 is leaking gas out of the drain hose which attaches to the bottom of the carburetor. What is wrong and how can I fix it?

QT50 carburetors have a drain hole at the bottom of the carburetor (see diagram). Attached to the drain is usually a length of hose which leads out and under the bike. I’ve found that weed-eater fuel line is a great replacement for the old hose if it is hard and brittle or missing (you should have enough hose so that you can direct fuel below the center stand). The drain allows you to drain the gas out of the carburetor if you need to remove the carburetor and take it apart (after shutting the fuel off at the petcock, just loosen the drain screw on the bottom of the carburetor and gas will flow out the drain hose).

If gas is leaking out the drain hose, you may just need to tighten the drain screw at the base of the carburetor. The drain screw might have a tiny o-ring or gasket on it that needs replaced. An old gasket could also cause it to leak.

The drain hose also evacuates fuel from the carburetor when gas overflows the level of the float. This can happen when the float needle is not sealing the seat because of dirt or other crud that is preventing the needle from seating. If you try to start your QT50 in this condition, it will flood and not start. Or it may start and then die and then refuse to start again. You will need to take apart your carb and clean the seat and needle to remove the dirt or other source of blockage.

You probably want to verify that the source of the leak is in fact coming from the drain hole. I’ve experienced similar situations where I initially thought the source of the leak was the drain hole but it ended up being the float bowl gasket. Gas was actually leaking down the side of the carb and then running down the hose. At first glance, it appeared to be leaking out the hose but gas was just taking that path after it had leaked out of the top of the float bowl. In that situation, you’ll want to replace the float bowl gasket to stop the leak (if you’re lucky, maybe the float bowl screws are just loose and need tightened). Or you can purchase a new carburetor entirely for less than $20.

Another source of fuel leakage is where the fuel line connects to the carburetor. I’ve generally found that 1/4″ fuel line is to big. I prefer 3/8″ fuel line for a tighter fit around the fuel inlet. I often use zip ties where the fuel line goes over the fuel inlet on the carb to snug it up.

I’ve even had fuel leak out the side of the carburetor where the air filter attaches. You may also want to check the petcock and make sure fuel is not leaking from it, down the fuel line and then down the carburetor. In that case, you’ll want to replace the petcock. If you go that route, drain the gas tank first either by running it dry, draining gas through the drain hose at the base of the carb (this will take a while), removing fuel line from carb and draining from that point, or you could drain gas from the petcock (this might be the easiest – just remove fuel line and turn petcock on).

Fuel leaks can be a huge headache. Hopefully, this eases your pain.

My Yamaha QT50 runs fine on the center stand but when I go down the street, it bogs down and won’t accelerate or it dies or stalls out. What’s wrong?

More than likely, your carburetor needs cleaning. If your QT50 or other moped dies, stalls, or bogs under load then it is probably not getting enough gas. This is the same issue if it struggles going up hills but to a lessor degree. If you’ve already cleaned your carburetor then you probably need to clean it again. The idle just often gets plugged up with dirt and crud and that is usually the problem. It will be hard to start as well if this is the case. If you take the top (cap covering the choke and slide chambers) off the carburetor and also remove the float bowl, a flashlight shining through the slide and choke chambers will reveal the extent of the cleaning needed.

If your carburetor is clean, you should see a tiny speck of light through the idle jet and a little larger speck of light through the main jet. If you fail to see the light in either or both, your carb needs cleaned again.

You can try to remove the idle jet with a small flathead screwdriver but if your carburetor is as old as the bike then you’ll probably strip the jet. You can either soak the carburetor in stronger cleaning solution (after removing all the gaskets and other soft materials) or you can take a small micro drill bit and run it through the idle jet. Take a small enough bit so that you don’t enlarge the jet. You just want to clean it. 0.30 mm should be small enough.

If everything is fine with your carburetor but your QT50 or other moped dies, stalls or bogs under load then you most likely have an air leak. An air leak has the same effect as not enough gas but now you are just getting too much air. Common culprits for air leaks are the head gasket, base gasket, where the carburetor connects to the intake, the mixing chamber cap on top of the carburetor and perhaps even the exhaust gasket. Your crank seals may also be an issue but generally require a leak down test to evaluate. The other areas can be tested for an air leak by spraying carburetor cleaner on them while your bike is running. If your bike starts to stall out then you’ve found the air leak. Try a new gasket or tighten the trouble spot as necessary but don’t over tighten.