250-500cc Losing power after getting on the pipe

I race Harescrambles with my 09 WR 250 and every time I get on the pipe no matter what gear I am in and come to a stop really fast my engine and exhaust sound different and when I go to get back on the gas and twist the throttle there is no power there at all. Like going into a corner something changes really fast and I have to twist the throttle several times for it to get power back again. I have a rekluse but its not that. The clutch engages but its like the power isn't there or something. I cannot explain it but the exhaust sounds hollow somewhat. I rebuilt the top end recently but it didn't do it for at least 2 races afterwards. Anyone help?

one more thing. as long as I am not on the top end of the bike in each gear, the bike doesn't have the problem at all.

clean and adjust your power valve

Yep, something's loose in there. My 09 did it early on. I forget what exactly it was but I think it was a loose bolt.

sounds like the little nut for the plastic pv linkage may be coming loose..... quick and easy to check, just pull the cover off the left side of engine and make sure its all connected and tight.

and while its open see if it moves freely, then start it and rev the engine up and see if the valve is moving, its rpm controlled

Checked it all and all worked fine and tight. spark plug is good too. Next step probably pulling the carb off and checking Jets but I just did that recently. I rebuilt the top end too with the B piston too but I have no clue what it could be. But should find out soon when I run it wide open in a day or 2... Sucks..

Something similar happened to my '04 wr250 a while ago. My bike would lose power when I turned the throttle. It turned out to be broken petals on my reed valve.

Still doing it and float looks good so far. It messes up when say you are on a start and you are on it hard and then come to a corner and you are slamming the breaks on to slow down fast. Thats when the sound changes in the exhaust to a hollow sound and I twist the throttle and have no power at all still sounding weird. Anyone have pictures of what the Power valve should look like on my bike to see if something may be bent or anything maybe? Float looked good from what I can tell and valve but who knows on that one too lol. Wow this is so weird.

Ok Got ya. Maybe the carb just needs to be rebuilt & new jets is all then. hmm.

take this one step at a time you let off and slam the brakes,,,,, at this point or after the rpm's drop does it change sound
all other areas it runs as it should???
no other areas does this have issues?
this is like diagnosing without seeing what you are diagnosing,,,, well actually it is diagnosing without seeing what you are diagnosing ;-)
so details are important

Haha right on! Here ya go. Hope this helps.

-as I let off the gas and hit the brakes while sliding to a stop as fast as I can doing both, The exaust changes to that sound & you can hear it. instead of the zing zing its a hollow sound as soon as the RPM's are at the lowest point they can go. Could that be some sort of compression issue? I did the top end and used the B piston at 135 hours.

-I have a rekluse too.

-Everything else I notice is fine. Kick starting is sooo much easier than before the top end change, and motor is strong feeling for the most part.

exactly what I was looking to see, sort of or maybe kind of,, well back to tech
if it was the power valve it would feel flat at all low rpm's and kind of "hit" a little as you came past 1800-2000 depending on settings
I would still pull the cover and look its only a few screws
so it pulls fine all rpm's and throttle, typically nothing till till you let off and brake hard, need to think on this

Mine had the same symptoms when the plate for the power valve opening limit was loose and it was opening too far and sticking open. Here's an old thread on adjusting the power valve.

http://www.cafehusky.com/threads/wr-250-300-power-valve-adjustment-pictures.26376/

I did pull it off and you know what? I bet you anything thats the trick! Utopia, I just have a feeling that is what the matter with it. So the next step for me is knowing what way to adjust it if its off. What bolts do I loosen up and what do I move if thats the case? The ones that face me correct? The bottom right 2 or the top ones?

The bottom ones are for the limit when the PV is closed. If the bottom ones are tight, don't loosen them unless you're going to take the head off to measure and set this limit. The top ones are for the limit of PV movement when fully open. (The lower plate rotates upward until it hits the tab that sticks out on the upper plate.) If you take your pipe off and open the PV flapper with your finger you will see exactly how it works. Also with the pipe off you can see the side ports and how they operate when you open and close the PV flapper. If the opening limit is set too far the valves in the side ports will actually rotate past full open and start closing again. Here's the photo from the other thread:

.....or the bearings on the PV shaft are bad which causes the PV to stick open....that's the hollow exhaust sound you hear.

Mine does this sometimes. I replaced by PV bearings but I did not do avery good job there is still a little bind in there.

Does anyone have a foolproof way to change these? They are so small I can get them on the shaft without damaging them.

Thanks!

Thank you very much for that info also R little. I took off that side finally and everything in there was gunked up pretty bad and did stick from time to time so I cleaned the crap out of it to make it look like that picture above. Its moving freely now and I think that did the trick actually. Now only one way to find out is by riding it. Just dont have the area to do it here where I live and have to go to some property... sucks..... Oh and one bolt was a bit loose on the bottom right so I tightened it up more also. everything else looks good now. crossing fingers again.