Have you ever had an issue with your KTM 50sx, 50 Mini, or 50 JR not starting? If you have spent any time around KTM’s fleet of mini bikes, you know they can be a little finicky. Just go ahead and ask any parent who has been racing these little bikes for a while. One of the more common issues you will run into will be a bad stator. So much so, that many parents will keep a handful of these handy on race day just in case. This can be a frustrating experience but after reading this article you will know how to diagnose, test, and replace a faulty stator on your child’s KTM 50.
Diagnosing a Stator Issue
Stators usually fail because the coating on the windings breaks down. The KTM 50s tend to overheat if not ridden hard enough. This heat causes wear on the stator and can cause it to prematurely fail.
“Common signs of a bad stator are sputtering at high revs or a bike that starts when cold but becomes difficult to start once it has warmed up.”
Before you rush out and spend your hard-earned money, it’s a good idea to test your stator so you can be confident.
Testing a Stator on KTM 50
Stators can be checked by using an ohmmeter and measuring the resistance through the stator itself. The stator doesn’t need to be removed to perform this test. This can come in handy when you’re at the track.
“According to the service manual, stators on KTM 50 bikes should have a resistance of 500 Ohms plus or minus 50 Ohms”
To test the stator, we are first going to disconnect the wires connecting it to the ignition. You can find the stator on the left-hand side of the bike. You should see a cable running from the stator cover-up into the ignition system located under the gas tank. This wire first connects to the ignition, then continues where it connects to the kill switch. You will need to disconnect both locations. If you happen to leave the stator connected to the kill switch, it could interfere with your reading leading you to conclude the stator is bad when it is a faulty kill switch.
Protip: You may find it easier to get to your ignition cables by removing the seat and gas tank from the bike.
Now that you have the stator completely disconnected, turn your meter on and to the resistance setting. Insert one prong of your meter into the ignition connector. This will be the first connection on the stator wire. With that prong inserted, touch the other prong to the engine head. If your reading measures outside of the 450-550 Ohm range, you have a faulty stator that will need to be replaced. If your stator does fall within this range, your problem is probably not a bad stator.
Replacing a bad stator on KTM 50 Sr / KTM 50 SX / 50SX / KTM 50 Mini
If your stator is faulty it will need to be replaced. If you have not already, disconnect the wire connecting the stator to the ignition and the kill switch. There are three 8mm bolts holding the stator case in place. Remove these three bolts and slide the cover off.
Note: If you have a KTM 50 Mini, you will have an oil pump. There will be a plastic tube that connects to the stator case. Be careful not to disconnect this tube or you will spill oil all over the place.
With the cover removed, you can unclip the stator wire from the engine case. You will see two 4mm allen heads holding the stator in place. These 2 bolts will have Loctite on them and will be difficult to remove. This can cause them to strip and become rounded so it’s a good idea to replace these when you replace a stator. They are very cheap to replace and we prefer to keep a few extra sets in the toolbox.
If you have a KTM Mini, you will have an oil pump gear making the stator difficult to remove. Some people have said they were able to remove the stator without removing the oil pump gear, but this is tricky, and you risk scratching or gouging the engine case. Remove the 13 mm nut holding the drive gear in place. The nut can be difficult to remove and may require an impact wrench. With the nut removed, the drive gear can be taken off making the removal of the stator much easier.
Place the new stator in place, lining up the 2 allen head screw. Add a little Loctite 243 to the allen bolts and fasten the stator down. The service manual calls for these bolts to be tightened to 16 N-m. Use a torque wrench and tighten these to the proper specs.
If you have a KTM 50 Mini, slide the oil pump drive wheel over the shaft and replace the nut being sure to add some Loctite 243 to the nut and tighten down.
Reclip the new stator wire to the case. If working on a Mini, place a gasket over the 2 dowels on the right side. The third bolt does not have a dowel. Replace the stator cover and tighten the three 8mm bolts. Using your torque wrench, tighten these bolts down to 16 N-m. Connect the stator wire to the ignition and the kill switch. Using some cable ties will really help to keep it clean. If you removed the seat and gas tank you can now replace those as well.
Conclusion
Stator failure is a common problem for these small bikes. The bikes run hot and if your little racer isn’t moving fast enough to move enough air to keep it cool it will continue to happen. If you have a newer rider who is just learning, we recommend you use a cheaper Chinese part instead of the OEM version. These Chinese parts are much cheaper and can save you a few bucks until your child becomes more comfortable on the bike.
Do these flywheel rotors have to be timed with the piston at TDC? We rebuilt our 50 ax pro and have good ohms, but the bike will not start and with the piston at TDC the timing line on the rotor is at 3 o’clock.
It’s hard to say what is wrong without taking a look at the engine but yes, these should be aligned. You may also want to check the distance between the stator and the rotor. You should be able to find this value in your service manual, it should be around 0.35mm.