My QT50 won’t start or has difficulty starting, what should I do?
1. Always, always, always remove the spark plug and inspect it if you are having starting problems. I would say go ahead and just replace it with a new plug to see if that solves the starting issues or to just eliminate the spark plug as the problem. Proper plug is NGK BP4HS or BP5HS. Some even run colder plugs like the NGK BP6HS. Generally the colder the plug, the harder to start. So it should start best with the hottest plug like the NGK BP4HS but it may not run best with the plug.
Make sure the plug is correctly gapped. I believe 0.025 – 0.028 inches is the proper gap.
Two strokes are temperamental machines and even though you had a new or newer plug in it, that plug may have become fouled by too much oil/gas. A change in temperature (like from summer to fall) can result in starting problems as well.
2. Your carburetor may need adjusted. You would generally start by adjusting the pilot air screw (see this carb diagram). For most starting problems, you simply turn the screw 1/4 turn to the right so that the engine is getting more fuel and less air at start up. The stock setting is 1 1/2 turns out from lightly seated. You may have to adjust it to 1 1/4 turns out or even 1 turn out by turning it to the right 1/4 turn at a time. I’ve had this happen to me a couple times in the past. A perfectly fine and running bike refusing to start and just requiring a simple pilot air screw adjustment. (On older qt50’s the pilot air screw will be located to the left of the throttle stop screw).
It this does fix it and it won’t stay running then you may have to turn the throttle stop screw to the right a 1/4 turn or more.
Note that I’m assuming that the key is in the “on” or “start” position depending on your model; the petcock is on and gas is flowing to the carburetor; the choke is on if necessary; the kill switch is also in the run position.
3. Fuel is flowing to the carburetor. This is another major issue in starting problems. With qt50s, you are dealing with a metal gas tank that is 35 or so years old. It’s rare that you have a gas tank that is completely free of rust or other goodies besides gas. Rust and other crud can cause all kinds of fuel flow issues at various points in the fuel system (petcock, fuel line, fuel filter, carburetor). You can open up the drain valve on the bottom of the carburetor after placing a container underneath the hose and verify that gas is flowing through the system (consult carb diagram). Let gas flow out of the drain hose for a minute or so. It should flow at a steady rate and not just drip. Your petcock will need to be on for this to work properly.
You can also remove the fuel line from the fuel inlet on the carburetor to check flow rate.
Or you can remove the fuel line from the petcock and just turn the petcock on. Or you can check it at all three points to determine if one is a bottleneck.
Your fuel filter may also be the culprit. Most are transparent so you should be able to tell if it is or not although I’ve had fuel filters that look good that were partially clogged. If in doubt, put a new one on.
4. Bad gas. If gas has remained in the fuel tank or in your gas can for 45 days or more, your gas may have gone bad. You’ll know because bad gas will not cause you to immediately back away if you sniff it from the container. Good gas will back you up immediately. I once went through this whole list and then some and finally drained the fuel tank and put in some good gas and damn, if that hopper didn’t fire right up. A bottle of Tru-Fuel (ethanol) free gas is always handy to have around for these and other situations.
5. Lost spark. It’s unlikely but a possibility that you lost spark. This could be anything in that system including spark plug, ignition coil, CDI unit, magneto. Check connections between spark plug and boot; orange wire leading from ignition coil and harness; connection between magneto and harness; CDI connection. You can test spark by removing the spark plug; re-connecting it to its boot; and then holding the spark plug tip against the cylinder head or other metal (unpainted) part while kick starting the bike. You may need to do this in the dark to verify spark. I believe the key and the kill switch need to be in the “on” and/or “start” positions depending on your model.
The major weakness with the qt50 is the charge coil in the stator (which is behind the left side engine cover). 1979 qt50s had a defective charge coil but I tend to think that all the charge coils suck on these bikes. Above is a picture of a pw50 stator and I’ve kind of outlined the difference between it and a qt50 stator. You can take a charge coil off of one of these stators and put it on a qt50 stator. If the pw50 stator you buy looks like this then you will need to remove the charge coil, flip it, and then re-create the cross-over ground between the light coil and the charge coil. A new charge coil will give you a strong spark and you should experience much better starting.
As I said at the very beginning, always try a new plug when you have starting issues. It’s cheap and easy. You can get a new ignition coil for around $15 so that’s also a good place to start.
6. The battery has nothing to do with starting the qt50. If you fear it is dead or in need of a charge, it will not affect starting.
7. It’s possible that your carburetor may need cleaning. There’s a small idle jet located next to the main jet. It is at the bottom of a long tube. If you shine a flashlight through the slide chamber (with the slide out and the float bowl off), you should see two specks of light. A relatively large one will shine through the main jet and a tiny one will shine through the idle jet. Often the idle jet is clogged and you can’t see any light coming through it. You can try to take a tiny flathead screwdriver and remove the idle jet. Chances are you may strip it. If you can remove it, then you will be able to take a wire and clean out the idle jet. If you can’t remove it, you’ll need to soak the top half of your carb in some potent carb cleaner. A clogged idle jet will make your hopper difficult or impossible to start.
8. Although I’ve never experienced it (not yet anyway), your exhaust could be blocked or clogged. You can either try a different exhaust or you can remove the exhaust and try starting without it. If it still doesn’t start without the exhaust on, your problem may lie elsewhere.
9. Although rare, other culprits to the starting process could be a bad kill switch, a bad ignition switch, faulty or disconnected wiring.
These are the basics for no start troubleshooting. For a more in-depth guide, consult Fred’s guide here although not all points apply to QT50s (e.g., qt50’s don’t have points to set).
My QT50 is difficult to start and when it does start, it won’t idle. It will die unless I keep giving it throttle. How can I fix this?
Most QT50’s that are still around are about 35 years old. You may have purchased one that has not been ridden or started up in a long time. In some cases, decades have passed since the last time that it ran. Ideally, when the previous owner put it away for storage, he/she drained all the gas out of the gas tank and the carburetor; sprayed the inside of the empty gas tank with WD-40 to keep it from rusting; added a tiny bit of 2 stroke oil to the inside of the carburetor; and put a little bit of 2 stroke oil down the spark plug hole.
You rarely get this lucky. Most often, the previous owner lost interest and just left your QT in the garage to rot. Gas has a tiny bit of water in it. Over time, this tiny bit of water rusts the inside of the gas tank. Bits of rust and dirt go down the fuel line and into the carburetor. All this grime gets into all the nooks and crannies of the carburetor including your pilot jet (I like to call it the idle jet). Your pilot jet is a tiny little hole through which gas passes. Its main purpose is to deliver fuel to the engine during starting and idling. If this jet is partially clogged, starting and idling become difficult, if not impossible.
You’ll most likely need to clean the idle jet or replace it. If the jet isn’t too corroded, you can unscrew it from the carb body with a very small screwdriver and clean it outside the carb or replace it with a new one. If you can’t get it out, you can soak the carb body in carb cleaner, rinse it and then blow compressed air through the idle jet passageway. If you have micro drill bits, you can pass a small one through the idle jet (0.35mm drill bit in diameter – the idle jet is 0.40mm usually) to clean it.
Once you clean or replace the idle jet, this should solve your hard starting and inability to idle. You may need to adjust your idle air screw and your throttle stop screw afterwards (see carb diagram).
I usually have to give my qt50 throttle while I’m kick-starting it to get it to start or sometimes while riding it will die on me when I come to a stop. What’s going on?
Your crank shaft seals are bad and are letting in air. You need to replace them.