Maintenance & Technical > KX500 Original

Bucking / Surging / Pulsing KX500

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John:
I thought I'll open a new thread rather than hogging another subject.

It seems like there are a lot of us KX500 owners who have the same problem. To quote jdv500's description "...surge by itself every time I let the throttle go after a straight and level fast run, even a slow run."

If you take a step back and think about what's actually causing this; even though the throttle is fully closed (open a bit for the idle) the engine gets enough air/fuel mixture to ignite and buck the bike. The issue is clearly related to the carburettor area (fuel - air / intake). I switched carbs from the PWK39 to another type with an oval throttle (as in not-square) and the bike behaved fine. Unfortunately, this carburettor had other problems... so I still have my old PWK39?er.

I was under the impression that my problems (with the bike that is) are related to a worn out throttle and seat - but if new 2004?s have this problem it sounds like another issue all together? Then again, there are KX500 riders who do not have this problem?

I have been extremely busy with work and moving house lately so I have not been able to do anything with my bike. Once things starts to cool down a bit I will pick this one up and fix it. It?s personal?

Maybe everyone who has this problem can add a line in this thread and tell us something about their bike; like manufacturing year for a start?

Mine is a 1989 - surges big-time.

//John

Folke:
The first i will try is an other fuel with higher octane.
Next try a bigger Main Jet.
Maybe you have to much compression, try a thicker head gasket.We have some problems with our KX5 and we change the fuel to one with moore octane and than the problems where gone. Good Luck !!

gowen:
John, between KXCam and myself, we have done days of research on surging. Here is my conclusion, the only way to clear it is to run way rich on the pilot and needle. If somebody with a KX5 has no surging (bucking) they are running rich, which you would NEVER know without trying to run leaner. I have tried EVERYTHING!! Including the fact that I'm running VP C12 (108MON) at 50:1 on MC1. I have done extensive work on the engine, including a big bore and porting, suring is very rampid and does not bother me. It runs great and is nothing wrong with the bike. I have now turned off my idle so I don't have to worry about the bucking while slowing down in a turn. I have tried diffrent fuels, every jet, a new carb, new #6 slide, timing, and much much more.. Want 50% of the details? Read the long jetting thread.

Cam is the man in this area, it was all his suggestions and my labor that got the conclusion, it is just the way the cookie crumbles and I'm just fine with it. If your KX does not surge, please give details on what you have and symptoms I guess.

KXcam22:
John,
   Gowen and I worked on this in the jetting thread.  He did all the testing and for sure the surging only shows up when your jetting starts getting close to optimum.  Gowen sent me some cool sound bytes of his bike surging that really helped to pull it all together that we were talking about the same thing.  I had got so used to mine doing it that I didn't notice.  I have a theory that it is due to the big vacuum the KX motor can pull when you chop the throttle pulling in a bit of extra air around the slide of the PWK39 without drawing enough extra fuel.  For a split-second the mixture goes extra-lean and the idle starts to run away. Just a theory though. A different style/size of carb (slide style?) would probably cure some or all of this (as your carb test shows).  Altitude or humidity doesn't seem to make any difference.  Mine is probably the worst offender I've heard although I never notice it when riding. If you are are going to tackle this I would be happy to lend a hand/ideas.  Cam.

PS where in the UK are you.  I have some rellies in Rotherham.

dezracer:
I only notice the surging when I'm cruising down a paved road.  When I'm on the pipe in the dez I don't notice it one bit.  Great explanations.

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