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Kx 500 jetting?

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I just bought a 94 Kx 500 it has just had the top end done and replated and is bored to a 510 and had a fmf full exhaust and I am having trouble getting it to run correctly. It's got a keihin 39mm pwk carb with a 180 main and a 58 pilot. The needle clip is in the #2 position. It runs really well until it begins to get into the power band, then it breaks up and feels as if it is leaning out. The jetting seems a little rich to be leaning out and I'm all out of ideas. I have tried the air screw from just 1 turn all the way to 5 turns out without any noticeable difference. I'm at 200 ft above sea level. Any help is much appreciated. Thank you.

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180 main is a little big. how does the plug look? 168m 55p and middle clip is a standard for tuning base.  Unless you have a crank seal leak, that changes it all.

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Man I hope it's not a crank seal leak. The plug was slightly wet. I'll go down on the jets and see if it helps. It just seems odd to me that it would be running this rich with what I have done to it. Stock is 168 on the main isn't it?

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Are the jets new? I have had old jets that have larger holes I guess due to corrosion over time.

 

Check the float height and make sure its on spec with the needle tab NOT compressed.  Is the choke seating properly? Every vent and hole clear in the carb?

 

Also if the reeds are a little out of spec the bike won't rev out, if you didn't replace them with the top end you might want to check them.

 

Here is FMF and Pro Circuit jetting tips, I don't think 10cc will throw the jetting off that much. 

 

FMF:

  • MainJet
  • 175
  • Pilot Jet
  • 58
  • Needle
  • STOCK
  • Needle Position
  • 2ND
  • Power Jet
  • N/A

 

  • Air Screw
  • 1.5
  •  

 

 

Stock: KX500 Main:168 Pilot:60 N82M Clip:3RD Air Screw:1 1/2           Pro Circuit: Main:168 Pilot:58 N82M Clip:2ND  Air Screw:2 Turns

Edited by 1000k6

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Much appreciated. The float height is set to spec and I completely cleaned the carb and the jets are new and I cleaned them as well. The reeds were cleaned when the top end was done. I doubt they are out of adjustment since it was done by Rat in texas. But you never know. I hear these kx500's are very sensitive to jetting.

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Before proceeding further I would recommend you do a leak down test.  It is a great thing to do just after purchasing a new to you bike so that you have peace of mind that the engine is sound and it gives you a good starting point for future modifications (like jetting changes).  Jetting a bike with an air leak is impossible and often times costly.  You should be able to research making your own leak down setup (on this forum or on Google) and be able to get all the parts at a hardware store.  I typically try to maintain 5psi of pressure for 5 minutes.  When you build your setup don't use an air compressor to pressurize the crankcase, you will likely blow out the seals using this method.  Instead use a bicycle pump.  

 

If you do in fact have an air leak spray soapy water around the seals and gasket surfaces to determine where the leak is coming from.  I've seen a lot of KX500s leak through the power valve seal.

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Are the jets new? I have had old jets that have larger holes I guess due to corrosion over time.

 

Check the float height and make sure its on spec with the needle tab NOT compressed.  Is the choke seating properly? Every vent and hole clear in the carb?

 

Also if the reeds are a little out of spec the bike won't rev out, if you didn't replace them with the top end you might want to check them.

 

Here is FMF and Pro Circuit jetting tips, I don't think 10cc will throw the jetting off that much. 

 

FMF:

  • MainJet
  • 175
  • Pilot Jet
  • 58
  • Needle
  • STOCK
  • Needle Position
  • 2ND
  • Power Jet
  • N/A

 

  • Air Screw
  • 1.5
  •  

 

 

Stock: KX500 Main:168 Pilot:60 N82M Clip:3RD Air Screw:1 1/2           Pro Circuit: Main:168 Pilot:58 N82M Clip:2ND  Air Screw:2 Turns

That is also running a 50/50 mix of race gas and super.  For all stock to mild motors running any of the aftermarket exhausts these are base numbers that I have seen be usually a little fat.  The pro-circuit number at the bottom is a closer base.

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I am happy to report that I solved the problem with y'all's help. Before I did an engine leak down test I decided to move the needle clip down to the middle and eventually down to the very bottom slot. Every clip position change duplicated the results I had before when I had the needle in the #2 slot except worse in every way. The farther I went down the needle the worse it got. It bogged earlier and more often, had excessive smoke out of the tailpipe, etc. This obviously tells me I have been running way to rich and that I need to lean out on the jetting. Thanks for the help!

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If you want, the optional CR500 needle from 2001 (R1370N) was a nice needle that seemed to clean up the low-mid burble I had on both my KX500's.

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I've got a 1986 kx500 do you belive that the factory says that the main jet is a 380 and the polit is a 45 but the carb that came with that motor has a 350 and I changed the polit from 35 to 45 and it run on that and it runs pretty dam good the only problem I have with is that it load up a little if I'm not on the throttle all the time like if I'm trail riding then it starts to load up on the low end but as soon as I crack the throttle it clears rite out and pulls real hard

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I need to change the main to a 340 or a 330 but I don't want to go to low and lean out the motor and melt it down

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