Help Diagnosing 2015 YZ250 Engine Knock and over rev off throttle.

[LorenUSSTR]

Hello All,

I am hoping to capitalize on the knowledge of the forum.  Last ride our YZ250 started a strange engine knock and over revving when backing off the throttle.  

We cleaned the jets, which looked fine (jetting has always been perfect on this bike). Replaced the spark plug and airfilter.  Changed the gearbox oil and it was golden clean.

My son rode around the yard and it started again after the bike was well warmed up, maybe 5-10 minutes.  

I let it cool and tested it myself and it seems to have full power on full throttle, but after a couple of laps around the yard it started agin.  

We plan on replacing the top end, but was wondering if anyone could point anything else we should look at.

Here is a short youtube video of the issue.

Thank you.

 

[RidingWithStyle]

Mine is doing the exact same thing. It just rebuilt top to bottom! PLEASE whatever you find tell me how to fix it.

[john stu]

Sounds lean, like an air leak or clogged jet in the carb

[RidingWithStyle]

Sounds lean, like an air leak or clogged jet in the carb

Could it be from running to much oil? I started my fresh rebuild running between 32-36:1. We ran 40:1 before the rebuild. Im having the exact same problem on my 10’ yz 250. It like it idle surges while riding and while idling like when waiting down the trail on someone..

[Yamadude333]

My Ktm exc 300 was doing the same thing right before I melted the rings to the piston. Had suspected an air leak. Replaced crank,crank seals and all other engine seals. Running like a champ but cost me 1000$. Definitely lean condition for one reason or another. Lean means hot, hot means death!

[Shooter88]

I'm having the same issue with mine. Yesterday it was idling high after blipping the throttle, and it would surge after you let off the gas while riding. I went from a 45 to a 48 pilot and it took the surge away, but still idling much higher than what it normally is. I'm gonna put a 50 pilot in later today and see what happens. I just put a DEP pipe on which I think is what is causing my issues. If that doesn't fix it then I'm gonna focus on a possible air leak somewhere. 

[Motox367]

Either lean conditions or running hot will do that and worse if real hot

[RidingWithStyle]

Either lean conditions or running hot will do that and worse if real hot

What could i try to fix this issue? Run 40:1? I did put new rads on my bike after the rebuild. If the bike was getting too hot, wouldnt it run wayyy worse and overheat easily?

Sorry to thread jack OP! Having same issue and asking questions! 1st person ive seen with the issue im having...

[Motox367]

9 minutes ago, RidingWithStyle said:

What could i try to fix this issue? Run 40:1? I did put new rads on my bike after the rebuild. If the bike was getting too hot, wouldnt it run wayyy worse and overheat easily?

Sorry to thread jack OP! Having same issue and asking questions! 1st person ive seen with the issue im having...

Check for air leaks, make sure coolent is circulating.  They will run good even hot to a point.  Crank bearings with little play will eat seal up or just bad seal. Tgink best thing is to cancel out air leaks.  Check everywhere air cam get in or pressure test. Go from there

[Yamadude333]

Running a richer or leaner premix won’t make your bike leaner or richer. Jetting is going to let too little fuel through or not enough. Has nothing to do with your premix. My problem happened in the season change to summer to winter. Colder air is denser (more of it) so I thought my jetting needed to be richer. Your problem is the opposite. If it was jetting due to temp change you would be running rich not lean. Hot air is less dense (less of it) so you would inherently be running rich but you are def running lean. Air leak for certain. You can go to a bigger pilot but that is going to mask the problem. Need to do a leak down test. Not entirely sure how to do it but there are many threads on that. If your bike was running good with it’s original jetting and ruling out temp change then you really shouldn’t need to change jets. Sure, wearing piston/rings/reeds could affect jetting but likely not that much. Spray carb cleaner around reed cage and see if idle picks up. That would indicate a leak there and an easy fix. If not a seal is leaking. Just my thoughts. Not a mechanic but have learned a lot maintaining/repairing my fleet of bikes.

[LorenUSSTR]

Just started tearing down and noticed what appears to be premix residue coming from gasket between cylinder and bottom end case, so likely blown gasket here.

Hopefully we caught it before more significant damage.

Will order a new top end kit and let know.

[LorenUSSTR]

Just started tearing down and noticed what appears to be premix residue coming from gasket between cylinder and bottom end case, so likely blown gasket here.

Hopefully we caught it before more significant damage.

Will order a new top end kit and let know.

Ps. Sorry, failed to wear helmet/gear. Bad example.

[rick8928]

1 hour ago, Husqydude said:

Running a richer or leaner premix won’t make your bike leaner or richer. 

Actually, it will. The premix oil displaces gasoline. So, richening up on oil (going from 50/1 to 40/1 for instance) will result in a leaner fuel/air mixture. If you were on the border of being too lean before richening up on the premix, you could push the fuel/air mixture into a lean condition.

[RidingWithStyle]

Actually, it will. The premix oil displaces gasoline. So, richening up on oil (going from 50/1 to 40/1 for instance) will result in a leaner fuel/air mixture. If you were on the border of being too lean before richening up on the premix, you could push the fuel/air mixture into a lean condition.

See thats what i thinking. Since i was running 40-45:1 before rebuild and never had an issue. I thought well.... fresh motor? Run tons of oil in the gas to give it good lubrication. Im going to try mixing a can of 40:1 again and run it and see if the issue stops. If it doesnt im taking it to the shop to have them look it over and get it right.

[mototechpr2014]

Check the condition of the stator side seal. If is bad will lean the mix also!!

[jaguar57]

notice how having to check the crank seals is the last thing people want to do.

do they think they last forever?

[Yamadude333]

He is going leaner on the fuel according to your logic to fix an already lean condition? Doesn’t add up to me. My 300 runs the same on 32:1 as if I run 50:1. Normal to run 32:1 on break in.

[hondaxr650rrrr]

Try taking off the ignition cover and spraying carb cleaner near flywheel when this problem happens. If the crank seal is leaking you will know then.

[218]

It's a lean surge. Sometimes you'll get a little of this if the engine becomes extremely hot and running on the lean side, but most of the time it's an air leak that develops when the engine warms.

Typically a leak at the intake manifold, base gasket or a left side crank seal.

As mentioned above, if it goes on it can lead to a lean melt down.

In your case you seem to have a very suspect base gasket.

[bimbamboom]

Leak down tester is a great tool for 2 strokes and 4 strokes. We spend money [emoji383] on everything else[emoji857]. Essential tool in my book. Try Motion Pro