2015 yz 250 odd problem

Have you checked the exhaust power valve operation? Have you cleaned the power valve? Depending on what 2 stroke oil and mixture you need to clean it regularly. If you think it might be electrical 1st thing to do is disconnect the kill button..

A small amount of water in the fuel tank can cause that kind of issue. The water beads up into droplets and rolls around on the bottom of the tank. When a droplet rolls into the fuel pickup your bike will run like crap or not at all.

The YZ250 is so simple, not much can cause that kind of thing. It sure sounds like you have an electrical component heating up and failing intermittently.

Electrical issues can be very odd and very expensive if you just throw parts at it. You really need to have someone troubleshoot the issue at the track when the bike is warm. Intermittent electrical issues are extremely tough to find.


A few years ago I unloaded my '08 YZ125 at the track, started it and let it warm up while I put on my gear. Shut the engine off with the kill button after a few revs to clear it out. Same routine I do every practice day. Poured in some fuel, put on my helmet and goggles and the bike just wouldn't start. I didn't get to ride that day. When I got home I checked resistance through the ignition coil windings, the crank-trigger and the stator coil. Turned out the winding on the stator was bad.

More odd electrical stuff... That same 125 was spec'd with a #9 plug. I was running a #8 plug and not changing it out very often. Two winters in a row I showed up at a winter race (cold and dry air) and the bike started sputtering and missing bad. The extra heat finally killed those #8 plugs. Needless to say I run the #9 in my current YZ125.

There was a giant 5 pages post with someone who had similiar problems. He didn't figure it out so far, changed a lot of sensors, the whole electrical harness and so on. Nothing worked. Maybe you should search for that topic and contact the guy. (It was the same bike as yours)

I think you'll find the thread you're referring to was concerning a YZ four stroke. Much more complicated machine then the bike referred to here, which is a YZ two stroke machine.

You will need to do a process of elimination, start with fuel, new batch in fresh container, vent lines carb and tank, look closely at the upper vent hoses there has been a lot of issue with dirt entering this route, most over look item,
most electrical issues I see now a days are pickup coils and the way you explain this popping miss firing sound like timing. I also agree with earlier reply small amount of water laying in fuel tank. look real close in the carb for small pieces of debris, pull all the jets and needle valve don't cut corners, look for water in the harnest from power washing causing a ground short, spray with WD40 a blow out with comp. air. lastly and a far shot but look at the rotor is it touching the stator, usually a bad crank bearing. disconnect kill button till diagnosed

After he put new stator in, ran fine for about 5 hours and now acting up again with new stator. The fact that it can run so crisp and then under load during race just start acting up is totally frustrating. Yamaha has suggested inspecting/replacing rotor which is next on the list. Thanks for the input guys, really want to get this kid ripping again.

Fly wheel? in the old day the magnets would break free from epoxy, different setup on a YZ, was the fly wheel struck by a hammer at all to remove? this can affect the magnets, put a screw drive to the inner rotor should be pretty good resistance sliding it around the rotor, what the relucktor looking like,( little tab welded to outside of rotor) they allow electrical field to flow to the SCR in the black box.

Did you say the wiring harnest was replaced? I dough the rotor will fix this, brand new stator out of the box?

If your fuel quality is good engine is in proper spec. including reeds, no fancy vent hose on the fuel cap. I'm sticking with pickup coil or pulled wire in the harnest creating resistance and reducing a/c voltage output to box.

I just experienced this same issue on my 2015 YZ250. Exact same thing you are describing. It turns out my stator plug was shorted out due to the seal on the wiring harness side of the stator connector leaking water into the connection. I had to replace the connector on the wiring harness and stator side as a whole because mine was burnt/shorted beyond repair. if you have a new stator I would just look into repairing the wiring harness. I think I melted mine because I kept riding it in the bogging state for one hour to finish a two hour harescramble race. The reason the connector was leaking water was because the rubber seal was pushed in too far from the factory, also the connector is vertical, allowing water direct gravity feed on top of the connection . I wrapped my new connector in electrical tape to prevent further issues, I have experienced this same issue on another YZ250 once before and I fixed and taped it up, with my customer experiencing no further issues to date. I would also recommend taping up the connector on all YZ two strokes that haven't experienced this before it be comes an issue.

I have a 2015 yz250 and had the same issues as you. Most frustrated thing of my life! I had about 20 hours on the bike and it started acting up. I tested everything and i narrowed it down to electrical. I couldn't really figure out what it was and i had a feeling it was the stator so I ordered one. It ended up being the stator. That fixed the problem.

But then about 15 hours later the same thing happened again. So this time I brought the stator to my local dealer and they tested it and they said it was fine. I didn't believe them so I ordered another stator, and I also ordered a wiring harness.

I then put dialectical grease on all the connectors. I never had a problem since then. I think it's a water issue that is happening.

Not really sure thought because I've had a 2005, 2009, and 2011 yz250 and never had an issue.

This 2015 has been a nightmare though. I came to conclusion my bike was made on a Friday night haha and is a lemon.

I would order a new stator and harness and grease everything up

Sounds like all the bikes in this thread have been narrowed down to electrical and connector problems but here is another thing to check for others having these type of symptoms.

On my 2006 YZ250, the bike would rip if you ran it through the gears on flat ground. As soon as you went on the track and tried to accelerate over stutter or gassing it as the bike does a G-out into the face of a jump it would start burbling/bogging. Really annoying, dangerous, and frustrating as hell to troubleshoot.

This was all when I first bought the bike in 2011. The previous owner had allowed it to sit for 2 or 3 years with pump gas and Castor-927 in the carb. The carb had obvious varnish and corrosion on the brass parts. I replaced everything that could be replaced and cleaned everything that could be cleaned. There are 2 parts in particular that are a PITA. First, the special O-ring under the jet block. This part is unobtainable and if it leaks will allow air/gas leakage past the pilot and main jets. Second, the small rubber tip on the power-jet solenoid plunger. It's a piece of rubber about the size of a grain of rice and it is not sold separately. If the bike sits, it can get stuck in it's orifice or gummed up. When you try to pull it out to clean it it rips off. Now the power jet will not properly seal and will allow gas to leak intermittently. Your choice is to spend a bunch of money on a replacement solenoid assembly. Also, there are some extremely small passages in the carb that are near impossible to clean out, like the one that angles up into the throat in front of the slide. I believe this is a "starter jet" or bleed for initial starting.

Research indicates that when castor oil breaks down it turns into stearic acid and stearic acid is particularly corrosive to brass. Not a good thing to have sitting in your carb.

Ultimately, I ended up buying a new carb. Retail is like $650 and they go online for about $450. The dirty pieces of junk on ebay go for about $175 but then you might be getting something worse than you already have. I was lucky to score a NITB one for $170.

Just some more info on a source of intermittent poor running that is difficult to troubleshoot.

FML I also have a 2015 and have been experiencing the same problems since about 20hrs. Replaced the stator and it happened again. Turned out just as the other guy described with water getting into the stator plug. Went almost a year without any problems and now it’s back.

Are you guys using pressure washers or hose to clean the bikes?

I don't have a Yamaha, but majority of the time I've switched to use hose and I gently spray near any electrical. I think all these modern bikes are too sensitive for high power pressure washers.

Unfortunately I’m using a regular garden hose.. I’ve been washing motorcycles this way for about 30yrs now and this is the first dirt bike that has ever been a problem.. :,(