Kx 250 power valve

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#1
okay when I re did the topend on my 2002 kx 250 I couldn't get the indented teeth on the gears to line up with the marks on the shaft. I took it to the shop and the guy said I they where fine but I'm not sure if I can trust his opinion. What do you guys think? everything seems to be working correctly it's set so it will open all the way at full throttle and so that it is closed at low throttle. I did notice however that I am getting a bit of bogging at mid throttle right after letting it warm up for 5-10 mins but that cleared up after riding a bit. I havn't had a chance to open her up yet cause I am still breaking her in. I'm thinking the bogging is being caused cause I moved the needle valve to stock position cause it had been lowered a bit before I had it.
 
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#2
Okay I moved the needle valve down a notch and I am getting better throttle responce now. But it feels like the top speed on my bike is lower. The engine revs out okay and it sounds okay it just feels slower. Could this be caused by the power valve. And like I said it sounds okay. And I see no one responded to my power valve groove question. Theres got to be people out there that have rebuilt kx 250 power valves. Dont be shy reply. :laugh:
 
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#3
Blackcat said:
I see no one responded to my power valve groove question. Theres got to be people out there that have rebuilt kx 250 power valves. Dont be shy reply. :laugh:
Yes, the power valve linkage can be off. Unfortunately, the only way that I know of to reset it is to remove the cylinder and look in the exhaust port as you are setting it up.

The two transfer port valves should be flush with the transfer port in the open position. You should be able to see the shiny wear marks on the transfer port where the valves ride. Use those marks as a guide as to the open position of the transfer valves. If the sliding rack or the rotating valves are only one notch off, your bike will not run to it's full potential.

Also, make sure the transfer and exhaust port valves are clean. If they are coated with carbon they can stick.

If you don't have one, it would be a good idea to purchase a service manual. It will explain the proper set up for your power valves.
 
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#4
Yea I got a service manual and it's total crap in my opinion. I also don't quite understand what you are saying with the tranfer port thing. My KX has three power valves. One in the middle and two on the sides. The one in the middle I know is right. It's the outer two I'm not sure about. If I can get someone who has the same model bike to take off thier valve cover and take a pic that would really help me clear things up. I think I'll go take a pic of mine and post it on here later so everyone can see what I"m talking about. Also there is really no wear marks to go by.
 
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#5
Blackcat said:
I also don't quite understand what you are saying with the tranfer port thing. My KX has three power valves. One in the middle and two on the sides. The one in the middle I know is right. It's the outer two I'm not sure about. Also there is really no wear marks to go by.

The two on the sides that rotate are your transfer port valves. When you look in your exhaust port and physically open the powervalve, (the middle one), the two rotating valves on the side should also be in the open position. When the middle power valve is closed, the side valves will also be closed.

Unless your cylinder was bead blasted when you rebuilt the top end, there should be small shiny wear marks on the cylinder where the side valves rub the cylinder in the closed position. If those rub marks are still in there, you can use them to determine exactly where the closed position of the side valve is.

You can also look at the angle of the side valves and the port to align the valves. When you open the powervalve by hand to it's full open position, the side valves should be in the full open position. The angle cut on the open part of the side valve will align with the angle of the side of the transfer ports. If you look in the exhaust port you will be able to see this alignment. It's pretty obvious when you look in the port and open and close the powervalve by hand. If you are off by one notch either way, the side valve will either block the transfer port a little bit or open too far exposing the side of the port. If set correctly, it should line up with the edge of the port.

Good luck. :cool:
 
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#6
Ol'89r said:
It's pretty obvious when you look in the port and open and close the powervalve by hand. If you are off by one notch either way, the side valve will either block the transfer port a little bit or open too far exposing the side of the portQUOTE]
They close fully but it seems as though they don't open fully. But if they where to open fully then they wouldn't close fully. Here I took a pic of the of the power valve in the fully closed position as it can be seen from the valve cover. Can you tell if it is in the right position from the pic.

<a href="http://photobucket.com" target="_blank"><img src="http://i157.photobucket.com/albums/t72/BLACKCAT471/powervalve.jpg" border="0" alt="Photo Sharing and Video Hosting at Photobucket"></a>
 
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#7
It's hard to tell from your pic.

If the sliding rack in the bottom of the cylinder, (the one that goes from side to side) is not properly aligned with the sliding rack in the top of the cylinder, (the one that goes from front to back) the ports will not open or close all of the way. Refer to your manual for the alignment marks on those sliding racks. There are also alignment marks or teeth on the rotating valves that have to line up with the marks on the sliding rack. You will notice that one tooth on the rotating valve is shorter than the rest. The short tooth is the alignment mark on the rotating valves. Once all of these marks are lined up, you can look in the exhaust port and move the powervalve by hand to check if everything is operating correctly.

I'm doing this from memory. :yikes: It's been a while since I did one of these cylinders. All of this information is in your manual so you may want to read it again.
 
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#8
But when the short tooth and the notch line up as the manual says the one exhaust valve won't open all the way. That's why I'm so confused on this whole deal. But anyways about the picture. In the picture that little opening is my drivers side exhaust valve. When the exhaust valve opens that little hole closes. I'm thinking that little hole is there to let oil in so that gear at the top left corner of the pic gets oiled. Also the manual is a peice of crap. In one step I'll be looking at a 125 then in the next it will be talking about a 250. I think it is best used as referance only. But thanks for all your help. I'm thinking I'm going to leave it alone and try tinkering with the jetting. I know that's off a little cause all the smoke and spooge the bike spits out. See how it performs after I get that dialed in.
 
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#9
Blackcat said:
But when the short tooth and the notch line up as the manual says the one exhaust valve won't open all the way. That's why I'm so confused on this whole deal. QUOTE]

Then move that valve out one tooth or in one tooth until it does. That is what I had to do to get everything to line up. Take the time to set your powervalve system correctly. If that is not correct, all the re-jetting in the world won't help.

Where in So Cal are you located? If you want to bring the cylinder to me, I will help you set it up.
 
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#10
Yea I have it moved over one tooth. And that is the only way I could get it to work. Also I live in Hesperia not too far from you. I may take you up on that offer but I'm going to see how it runs after I re jet it. I don't have the time to take it apart right now cause it's the last two weeks of school and I am swamped with homework. But like I said it's not running too bad. It just spudders the first few mins of the ride after I warm it up. And it blows smoke real bad. So I'm going to see what the jetting does and if I don't get the throttle responce I want I'll bring it to you. Thanks for all your help.