Intermittent high rev on my '88 kx500

[bigbore]

hi all,

ive noticed a intermittent high rev on my kx500 when im out driving it. i recenty fully rebuilt it and have only been out a hand full of times on it.

when im driving along in any gear and pull the clutch in, it revs itself as if i was turning the throttle about a 1/4 turn. it sometimes stops and just idles normally.

also if im driving along in any gear and let the throttle off fully and let it roll in gear it does the same high rev thing and i can actually feel the engine pulling the bike with the intermittent high rev - as if its driving itself.

its wierd???    - ive cleaned the carb and air filter - no difference.

only mods are boysen power reeds, reed spacer and decompressor mod i got done by larry.

Someone sait it might be an air leak somewhere???  Maybe i niped a gasket that letting air in???

its not loosing coolant or oil, doesnt over heat and stall or cut out and still has tons of compression.

I cant be sure if its running the power it should as its been doing this since i rebuilt it - never drove before rebuild as i bought this as a project. and ive never driven any other 500's.

the bike pulls well and has lots of power, powervalve seems to come in early compared to other 2 stroke ive had. however i recently sold my Yamaha YZ250 2 stroke 2006 and that had way sharper, violent power almost felt faster than my kx500 ?

im looking for some help and advise on this matter from you experts,

cheers guys,

bigbore 

[sandblaster]

It's certainly possible that you have an air leak.
But before you get excited, have you verified your jetting (Pilot and main) as well as needle type and position?
What altitude do you ride at?

[Awtblo]

I'd start with what sandblaster said.

I'm in the same boat as you but without the high revs. I have an 09 yz250 as well as the 01 K5. To me the 250 feels faster. However I m 99% sure my k5 is set up right. It also has. Recent top end and the power valves are timed right. I think the 250 feels faster due to the to the nature of the beast. The k5 makes a lot more torque. So it feels like a 4 stroke to me.

[ZETTNORCAL]

My KX500 is doing something like that also.  When I roll off the throttle is takes a few seconds to calm back down to idle. I have swaped out complete carbs and had the same results.  Thinking that it must be an air leak.   ping...ping...ping ....ping then back to idle.  hmmmm

[Friar-Tuck]

Hey guy's,
 I think what you all are experiencing is surging.  There has been several answers to this problem if in fact this is what's going on.   I hate to just post a bunch of hit and miss suggestions, look through some of these posts and see if this sounds familiar and some of the solution
I'm sorry,  Try it now.
  http://www.kxriders.com/forums/index.php?action=search2;params=YWR2YW5jZWR8J3wxfCJ8YnJkfCd8MTMsMTIsMTQsMTksMjUsMzQsMjAsMjYsNCwyOCwyMywyNCwzMiw2LDUsNywzMywzMSwyOSwzMCwzNSwzNywyLDMsMjEsMXwifHNob3dfY29tcGxldGV8J3x8InxzdWJqZWN0X29ubHl8J3x8Inxzb3J0X2RpcnwnfGRlc2N8Inxzb3J0fCd8cmVsZXZhbmNlfCJ8c2VhcmNofCd8c3VyZ2luZw==s
 

 
Tuck\o/

[81cr450]

Hey Tuck your link didnt work.

 I'm curious about this as well. I did the carb swap thing cause I read that the slide was probably worn out. I put the carb on my cr500 though & it works like it should. The carb I put on it did the surge thing too & it was like new. I even tried jetting it out on the pilot, cause it was just right when the throttle closes, that helped a tiny amount. My guess is its just the powervalves closing a little slowly & over scavaging. If it was an air leak the surge would carry all the way through the powerband, like when you run out of fuel. Where its just overrun when you close the throttle & the powervalves being the variable between a cr5 & kx5 
Just my hypothisis, someone with more k5 knowledge might have a better/more accurate explaination. I'd like to hear a solution for sure

[Friar-Tuck]

Doh!
http://www.kxriders.com/forums/index.php?action=search2;params=YWR2YW5jZWR8J3wxfCJ8YnJkfCd8MTMsMTIsMTQsMTksMjUsMzQsMjAsMjYsNCwyOCwyMywyNCwzMiw2LDUsNywzMywzMSwyOSwzMCwzNSwzNywyLDMsMjEsMXwifHNob3dfY29tcGxldGV8J3x8InxzdWJqZWN0X29ubHl8J3x8Inxzb3J0X2RpcnwnfGRlc2N8Inxzb3J0fCd8cmVsZXZhbmNlfCJ8c2VhcmNofCd8c3VyZ2luZw==s
 
There was a thread I am looking for in particular....
  I'll B Back...
http://www.kxriders.com/forums/index.php/topic,1155.0.html

[bigbore]

It's certainly possible that you have an air leak.
But before you get excited, have you verified your jetting (Pilot and main) as well as needle type and position?
What altitude do you ride at?

i will pull the carb apart and check what jetting im running

[bigbore]

My KX500 is doing something like that also.  When I roll off the throttle is takes a few seconds to calm back down to idle. I have swaped out complete carbs and had the same results.  Thinking that it must be an air leak.   ping...ping...ping ....ping then back to idle.  hmmmm

thats exactly the same as me, after a few seconds mine will settle down most of the time.

[bigbore]

Hey guy's,
 I think what you all are experiencing is surging.  There has been several answers to this problem if in fact this is what's going on.   I hate to just post a bunch of hit and miss suggestions, look through some of these posts and see if this sounds familiar and some of the solution
I'm sorry,  Try it now.
  http://www.kxriders.com/forums/index.php?action=search2;params=YWR2YW5jZWR8J3wxfCJ8YnJkfCd8MTMsMTIsMTQsMTksMjUsMzQsMjAsMjYsNCwyOCwyMywyNCwzMiw2LDUsNywzMywzMSwyOSwzMCwzNSwzNywyLDMsMjEsMXwifHNob3dfY29tcGxldGV8J3x8InxzdWJqZWN0X29ubHl8J3x8Inxzb3J0X2RpcnwnfGRlc2N8Inxzb3J0fCd8cmVsZXZhbmNlfCJ8c2VhcmNofCd8c3VyZ2luZw==s
 
Tuck\o/

Hey Friar Tuck, i think you have nailed the exact problem, id never heard of 'surging' before.

However im confused with what i read in your links, seems a lot of kx500's have this surging problem, is it ok to run the bike like this? i take it, its running lean when surging?
 
jetting will be the first thing i check, i have a clymer manual at home so il check im running stock jetting before i do anything else. the altitude i ride at is very low, approx 100 feet above sea level, if that makes any odds?

[Philthy Phil]

could be that your throttle cable needs lubing?

[bigbore]

could be that your throttle cable needs lubing?

no i already checked that and it has enough freeplay on it.

[Friar-Tuck]

Hey BigBore,
 Yes it seems the engine gets caught in a lean burn on decel and bucks or surges.  Usually at the least opportune moment, like wanting to slow down to make a turn.  You get a "mmpow mmpow pow" and get pushed farther than you need to go!
  I don't recall anyone claiming to have any engine damage due to the condition. 
The remedies vary from a new or different carb, to fatter jetting to a kips bolt mod.   
   I'm sorry I can't give you a better answer.    The bike seems to have a lean burn condition when the jetting is close to optimum.   
          The engine draws in just enough of an air/fuel mix through the pilot circuit when the throttle is chopped and the engine is decelerating to burn lean instead of Idle down.
    Sorry I can't be of any more help than that.
  Tuck\o/

[Polar-Bus]

hi all,

ive noticed a intermittent high rev on my kx500 when im out driving it. i recenty fully rebuilt it and have only been out a hand full of times on it.

when im driving along in any gear and pull the clutch in, it revs itself as if i was turning the throttle about a 1/4 turn. it sometimes stops and just idles normally.

also if im driving along in any gear and let the throttle off fully and let it roll in gear it does the same high rev thing and i can actually feel the engine pulling the bike with the intermittent high rev - as if its driving itself.

its wierd???    - ive cleaned the carb and air filter - no difference.

only mods are boysen power reeds, reed spacer and decompressor mod i got done by larry.

Someone sait it might be an air leak somewhere???  Maybe i niped a gasket that letting air in???

its not loosing coolant or oil, doesnt over heat and stall or cut out and still has tons of compression.

I cant be sure if its running the power it should as its been doing this since i rebuilt it - never drove before rebuild as i bought this as a project. and ive never driven any other 500's.

the bike pulls well and has lots of power, powervalve seems to come in early compared to other 2 stroke ive had. however i recently sold my Yamaha YZ250 2 stroke 2006 and that had way sharper, violent power almost felt faster than my kx500 ?

im looking for some help and advise on this matter from you experts,

cheers guys,

bigbore 

Unusual engine revs are a lean condition. The challenge is to find out where. A good place to start is the intake boot and reed cage. The other less common possibilty is some overly lean jetting. This is where a seasoned "ear" helps. Listen to the engine as it revs, then pay attention to exactly how the engine behaves thereafter. A lean air fuel screw will cause erratic low speed and idle behavior, a rich air fuel screw will cause the engine idle rpm's to slowly decrease and load up the engine.

[ZETTNORCAL]

load up. yes.    after it is shut down even for a few minutes, then restarted it feels just like I started it up cold with the choke and have to blow it out before it runs clean again ........ sorry for the lingo...