2011 EC250 chamber moving away from flange?
 

Hello, I have a 2011 EC250 Six Days that runs great with the exception that during rocky hillclimbs at high RPMS it will occassionally lose power and sound like the chamber has pulled away from the cylinder. It will correct itself and run fine again without me needing to stop but it always seems to happen again under similar circumstances and leaves me in the middle of a hill cussing.

I have tried new chamber mounts, new O-Rings, new springs (OEM), and repacked the Q-Stealth muffler. I also replaced the stock chamber with an FMF Gnarley after I crushed the stocker and it still does the same thing.

I was thinking of welding another hoop on the chamber to add a third spring or going with another muffler to reduce the backpressure.

Anyone experience this?

You can run another spring or two, they don't last forever. Mounts can get a little sloppy too. Test in the carpark revving it and see if it moves. Hold your boot against the pipe.

But when was the last time you cleaned your powervalve? Can be done from the side with the pipe off. Also check the wire rod from the governor isn't hanging up on the side of the tunnel it passes through at base of barrel area. Sometimes they need a little tweak to stop this.

I'd posit that it is the powervalve sticking open. You're cussing 'cause the valve is open, compression, and therefore power, is reduced.

+3 on the power valve being the culprit.

Thanks for the feedback.
I just did the top end over the winter and the powervalve was clean and moving free, the rod has plenty of clearance to the case. The new top end didn't help, I have the same symptoms with the fresh piston and rings.

I've been reading more on this forum about the power valve free play and adding some preload so I will check that. Also may go with some heavier springs since the stockers are a bit weak looking compared to others.

I have revved it on the stand and see no signs of movement. It seems to happen on roots and rocks and it literally sounds like the chamber has popped off. Would a sticking power valve make it sound like the chamber has fallen off?

Appreciate the suggestions!

Yes, the exhaust note changes dramatically. Had the same issue on my '05, power valve would stick open when hitting rocky terrain. You will also notice that the bike is making less power at lower RPM.

As others have stated, check the linkage. You may want to replace the PV bearings as well for good measure.

Thanks for the suggestions.
It sounds like I have been going down the wrong path with the pipe mounts and springs.
I will look at the exhaust valve and post an update if I find something wrong.

Hopefully I will get it fixed and back in the woods this weekend but will update once I have a result.

Everything looks normal
 

I checked the power valve adjustment with the recommendations on that thread and there was no play. If anything the adjustment was a little on the "too much preload" side and there was good spring pressure holding the valve closed.

One thing I did find odd was that it was very difficult to remove the right side cover even though I just had it apart over the winter and replaced the dowels and a new gasket. The case came loose easily but there was a springy resistance that kept wanting to pull it back onto the crankcase. The spring seemed like it was coming from the power valve assembly but after wiggling it for a while it came loose. I even took the water pump impeller off and it was not the cause for the resistance.

I disassembled and inspected the power valve centrifugal assembly and it seemed normal with no abnormal wear. I am thinking of polishing the ramp surfaces a bit.

Overall I think I will just reassemble and might loosen the preload up a little but didn't really feel like I found the smoking gun. I did ride it this weekend prior to inspection and it did lose power a few times which lit a fire under me. I may post a video if I got any on film.

Any new ideas?

Edit: here is a video where in the first few seconds I cross a log and you can hear the exhaust not change and "fix" itself.:: https://youtu.be/VGXA1_7T9Zo
When this happens the power is severely reduced.
BTW, following the wife on her KLX140G which isn't really made for the rocks but she is a trooper on that thing.

Aliens. .

Another Video of the power loss - a bit more action than the other
 

Here is another video of the power loss.
https://youtu.be/NGj9NQs2ol8

Happy ending but close call!

F5, I had not considered aliens but am thinking seriously about it now!