Maintenance & Technical > KX500 Original

01-500, eng surges/pulses after run.

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John:
If you think logically; the ONLY thing which can cause any engine to fire with a closed throttle is that the engine gets air/fuel mixture anyway. The engine cannot fire without the air/fuel mixture. I don?t have idle on my bike (don?t? want it) so my throttle does close shut.

When I tweak the air/fuel ratio really lean the engine is less likely to fire - but it still does. This can't be caused by any leaks; its malfunction in the carb or just poor design since we all experience it more or less (including me). If it?s a serial fault, I would like to know what it is. I am suffering from this when I have the motard kit on and riding on tarmac. It's not so bad when riding in slippery mud mainly because I don?t notice it then.

I switched carb last year to the PJ (same as orderofsion) and the problem went away. Unfortunately, the carb I bought was cr*p, so back to square one. I have recently bought two additional PWK-39's which I will tweak and see if I can fix this problem.

//John

jdv500:
well, thats another one of our many therories, and yes, we'd all like to know what it is, the problem- cause of surging. thought his site tons of owners of all yr models are having this big or small. we all have our own hunch on what it may be / can be. case seals, worn slide, kips valve worn or out of time and of course jetting. well it seems that this will continue to live with us as long as we ride this model we've chosen. unless we all join and hire a team from nasa and hook probes all over it. we've learned to live with it, cause the goods far outweigh the bads with our 500's. logically.  no worries. joseph

KXcam22:
John,
  What is the basic difference between the PJ and the PWK carbs.  In particular is the slide the same shape or lots different.  I still feel the surge is caused by the PWK carb inability to hold back the big vacuum the KX makes. A different slide shape (PJ?) may be the answer. Cam.

John:
Cam,

Check out
http://www.sudco.com/Diagrams323728/exppwk.gif
for the PWK and
http://www.sudco.com/pj_diagram.html
for the PJ.

If I understand things correctly, the PJ is biased towards ATW whereas the PWK is purely motocross.
The major differences are; on the PJ, the idle is adjusted by twisting the choke knob rather than adjusting the throttle, and the throttle is oval (not flat-sided). I am way out of my league if I could claim to understand why this would change the behaviour of the KX500. I am a computer nerd with a mechanical background.

I am sorry if I offended anyone. I do realise that there is no silver-bullet magic solution for this problem and I am not saying that MY ideas about this are the correct ones. I am merely participating in a discussion and applying my limited logical thinking capacity. No fuel = no fire. The problem might be related to the distance between the reeds and the piston? Or an inherited design flaw in the intake manifold ? really, I haven?t got a clue. My goal is to fix the symptoms and I aim to give it a go.

//John

demographic:
I seem to remember reading that one way to give more power to a CR500 (which already have a PJ carb) was to use a PWK39 carb so would have thought that using a PJ would have been a backward step :?

Not used one so I am a bit in the dark about that.

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