KX 500 Flat on Mid and top end

[lsampson]

Hello All,

New to the site, so be gentle I have a 1991 KX 500, all stock, that I'm putting a top end into. I have several questions.

1. When I rode it prior to tearing it down, it didn't have much spunk on mid-top end. I checked for exhaust valve operation, and they were functioning. I also cleaned the jets and they were clean. I cleaned the whole carb thorally. The bike was from my uncle, and it should be jetted right, but it is running a 160 main, 45 pilot, with the needle clip set 2nd from top. The elevation I ride is appx. 6000'. I found a jetting chart, and it looks close. The plug is a little on the lean side, but not bad. During teardown of the top end, I  inspected the power valves, and they WERE gummy, but still functioning. Anything else I can look at since I'm this far that will solve my flat mid-top end performance? It starts easily, but during deceleration it kind of "chugs" to a stop. Idle is ok.


2. I have read the outline of power valve re-installation, but am a bit confused. I have the powervalve with the notched top pin on the right side (the counter gear side), and aligned it with the rack, aligning the dots. When I fully close the rack (pushed in) the valves expose some of the exhaust valves, probably half of the outer exhaust port.. Only when the rack is pulled out some do the exhaust valves fully close. Is this to compensate for idle RPM, thus pulling the rack out and closing the valves? I have a sneaking suspicion that this is the reason for the ball bearing mod?

Thanks a bunch for the help.

[bilger69]

Maybe stator position. I found this problem when I messed with advancing the timing.

[bilger69]

Actually it is a better chance it could be  the kips arm.    Here's an explanation of a problem I had with it a while back.                                                http://www.kxriders.com/forums/index.php/topic,9425.0.html

[lsampson]

Thanks Bilger. I'll look into that. Have you torn into the Exhaust valves at all? I took them completely out and accidentally mixed up the right and left valve. There is only one way they can go in and still function correct? The Kawasaki book mentions a right and left for the 125 and 250, but not the 500. The reason I ask is the "barrel" valves seems too far closed with the rack pushed all the way in. At rest, you can see around the valve and into the cylinder exhaust port (one of them anyways) then when you pull on the rack the valve closes the ports, then full open it opens up both ports.

[motopunk]

the left and right exhaust valve of the kips-system had to be fully closed at low rpm. check out, that all valves are fully open at high rpm! if it installed in wrong position it cant work properly.

stator position is in stock the middle of the 3 marks on it.
i think that the old piston already had too low compression. check the motor performance after rebuild again. when piston and exhaust valve work in right order, you can only continue with the ignition and carburetor adjusting.

at last, go with your ignition system to a kx dealer and let it measure whether everything in it works proper.

[suicyde]

possibly a sheared stator woodruff key?

[lsampson]

I don't think its electrical, it starts great and everything, it just doesn't have the 2-stroke kick upwards of midrange, and isn't as wheelie prone as I remember when my uncle had it. The exhaust valves were pretty gummy, so I think it is related to that. They are cleaned up, and I'm hoping to have it back together Friday night.

One more question, for the left and right exhaust valve guides with the rubber o-ring, how often does the o ring need replaced? They appear ok, but I did notice some oil at the gasket when I took it apart. I'm thinking of using silicon around the guides to keep oil off the gasket. I have new o-rings coming, but they wont be here in time for the weekend. Do I need to wait for the o-rings, or can I get by with the silicon?

[motopunk]

the o-rings on the exhaust valves are really only there, that it is not falling out when you pull the cylinder. a little oil there is normal, because the exhaust valves are also lubricated with 2stroke-oil. the o-rings need to be replaced only when the valves fall out when you pull cylinder ...
i think, you should ask your uncle, which jetting and needle position he used, as he rode the bike...  this could be the keye to find back the 2stroke-punch out of the middle... 

[lsampson]

Thanks motopunk, thats EXACTLY what I wanted to hear! Now I can ride this weekend! As far as jetting, I bought several sizes up for both pilot and main jets, so I will fine tune it this weekend. The plug is a little light brown, so one size main should do it.

[DoldGuy]

Sampson,
Check the power valve as I'm sure if it's a "91" model they have had a lot of time put on them since new. In the open position, see if the "center" valve lifts all the way up flush to the port window, even if it hangs down just 1mm or 2, it will cause a big loss in top end power. There has been a lot of info posted about the cause & fixes for such on here, give it a search & see if it answers any questions.
Hope it's as simple as this.

DoldGuy

[suicyde]

I don't think its electrical, it starts great and everything, it just doesn't have the 2-stroke kick upwards of midrange, and isn't as wheelie prone as I remember when my uncle had it. The exhaust valves were pretty gummy, so I think it is related to that. They are cleaned up, and I'm hoping to have it back together Friday night.

One more question, for the left and right exhaust valve guides with the rubber o-ring, how often does the o ring need replaced? They appear ok, but I did notice some oil at the gasket when I took it apart. I'm thinking of using silicon around the guides to keep oil off the gasket. I have new o-rings coming, but they wont be here in time for the weekend. Do I need to wait for the o-rings, or can I get by with the silicon?

I had a sheared woodruff key retard the timing approx 30 degrees, thats why I suggested checking it. The symptoms are exactly like what you are describing. Its a simple and easy check to isolate that it isn't the problem.

[Metal Mulisha]

Did you check the compression bro? 
Low comp would put a hurtin on ur top end bro.

[motopunk]

much luck isampson.  want to hear that after the weekend the bike runs well again ...

[Polar-Bus]

Hello All,

New to the site, so be gentle I have a 1991 KX 500, all stock, that I'm putting a top end into. I have several questions.

1. When I rode it prior to tearing it down, it didn't have much spunk on mid-top end. I checked for exhaust valve operation, and they were functioning. I also cleaned the jets and they were clean. I cleaned the whole carb thorally. The bike was from my uncle, and it should be jetted right, but it is running a 160 main, 45 pilot, with the needle clip set 2nd from top. The elevation I ride is appx. 6000'. I found a jetting chart, and it looks close. The plug is a little on the lean side, but not bad. During teardown of the top end, I  inspected the power valves, and they WERE gummy, but still functioning. Anything else I can look at since I'm this far that will solve my flat mid-top end performance? It starts easily, but during deceleration it kind of "chugs" to a stop. Idle is ok.


2. I have read the outline of power valve re-installation, but am a bit confused. I have the powervalve with the notched top pin on the right side (the counter gear side), and aligned it with the rack, aligning the dots. When I fully close the rack (pushed in) the valves expose some of the exhaust valves, probably half of the outer exhaust port.. Only when the rack is pulled out some do the exhaust valves fully close. Is this to compensate for idle RPM, thus pulling the rack out and closing the valves? I have a sneaking suspicion that this is the reason for the ball bearing mod?

Thanks a bunch for the help.

When I did my K5 top end a few years back, I had the same exact lazy midrange acceleration as you are describing. I ended up tearing down the top end again, and upon a closer inspection of the power valves, I accidentally had them one tooth off on the timing. The manual is a little confusing, and the pics are not very clear. But in short my valves were both incorrectly partially closed . I'll bet this is your issue.

[lsampson]

Thanks for the reply guys. I've looked over the powervalves closely now, and I have placed a longer bolt to be used as a stopper to keep them from overclosing. Now, with the valve rail pushed all the way in the drum valves close off the side ports completely and are flush with the main exhaust port, and the center valve is all the way down. When I pull it all the way open, the center valve rises to flush with the main exhaust port, and the side valves fully open the two side ports on both sides.

Its crazy how different the powervalve setups are compared to the yamahas. If I remember correctly, the YZ250 of 1994 has just one "cresent moon" valve that is located in the main exhaust port. Not necessarily better, but simpler for sure.

I'm also suspecting bad fuel may have been a culprit on my ride last week. The fuel was a mix of fresh, 6 month old, and ancient fuel. I didn't want to throw it out, but I ended up trashing all of the fuel in the tank. I don't want to run SH*t fuel and always wonder if thats the problem. I'll let you all know after this weekend!