250-500cc Power Valve Sticking?

Before I start pulling stuff apart, I figured a little advice is needed. I have an '08 WR250 that's never been apart and is due to a top end this winter (100 hrs). This weekend, I noticed that when I hit 3rd or 4th gear wide open and then back off the throttle, the bike sounds like a 4-stroke and I have about half power. I think the Power Valve is sticking open. I can blip the throttle and bring it back to normal. It happened once or twice and only happens on the open trails at high speed. All woods riding and single track riding is fine. So, has anyone seen this issue before and what are the logical steps on taking things apart and checking? - Thank John

You can access the linkage by removing the cover on the side of the cylinder. While the cover is off you can run the bike an verify if it is sticking or not.

not sure if they are the same but on my 360 the cover on top fills with exhaust oil and it gets slow on the movement till its really warm

I pulled the PV cover off today and the paper gasket was rubbing against the arm. For some reason the gasket was way oversized at the base. I trimmed it with a razor blade and the cleaned it with some carb & choke and a small brush. Looked good when all done. Pouring rain today so I'll test ride it one night this week. Photos on Monday...

Bump for photos...

My '06 WR250 had the same issue with the sticking powervalve. When I did the top end I disassembled and cleaned all parts. You'll likely have to replace the PV "O" rings also and possibly the small bearings that the PV shaft rides on. This will fix your problem.

100 hrs? I rode mine for 500hrs and about 11,000 dirt miles before top end. Single track and dual sport with numerous bursts at WOT. Rings were just slightly out of spec at re-build. Still was running great but was worried about durability. For sure could have run it longer. My brother has same bike and rode to failure. Rings would almost fall through cylinder when piston broke. I recommend stock piston replacement. These bikes are awesomely durable.

935- I'm probably being too cautious with my top end. At 100 hours its still strong and tight. I should do a quick compression check to see what it reads. To your point, I was going to do a super clean and inspection of the PV when I did the top end. Do you remember if the PV bearing and O-rings were OE only parts or something I could buy aftermarket? Thanks!

Jhunter- I bought all parts from Halls so I assume OE. Bearings are pretty small but may be available aftermarket somewhere. Same for O-rings but I assume rings are a special high temp rubber. Get the right ones to reduce the blow-by into the side covers.

I still recommend you go longer before top end if you haven't beat on it too bad in the 100 hrs. Maybe pull jug and check ring gap. If both still well in spec then just replace if piston still looks good. Save $150+ piston cost for some other mod. Be sure to check piston bearing play also.

I had the same issue with temporary loss of power at times after hard acceleration on my '13 WR250. After reading this I pulled off my PV cover and had to trim the gasket also. Took it for a ride and runs great! Thanks for the tips.

JHunter- sorry missed saying "just replace RINGS if pistons still looks good" in my earlier post. Also, note that you can make some powervalve adjustments with settings in the side cases. Search for the thread on that or at least mark your present adjustment or take a picture of it for reference.

Thanks 935, I had a good ride over the holiday and the PV was working great with NO sticking at wide open. It appears the gasket was my problem. The motor is so tight on my bike, I think I'm going to wait on that top end for now. I price the piston, gaskets ect.. from Hall's and it was $239 for all OE parts so not a big expense. Thanks John

I paid the same for OEM piston and other top end parts. If you have good compression then hold on the top end. Brother's bike that failed had really bad compression (below 90 psi) but still ran ok but without the low end punch that mine has. Could turn over by hand with that low compression. He ran that for many years before the piston finally let go. And again, the rings were so shot they would nearly drop through the cylinder when removed.

Bottom line is, "run longer, rebuild less, save money and effort and ride more. Do top end earlier if you think you need the extra kick"'