Bucking / Surging / Pulsing KX500

[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.

[neverendingmods]

Gowen and Cam's explanation seems right on to me.  When I 1st got my 04 and was breaking it in, it was fouling plugs all across the throttle range.(32-1 94 oct and no surging/ran like crap).  After 12 hours I went 100oct and 40-1 Klotz synt/castor mix and started to feel it slightly.  Still fouling plugs.  Put Gnarly torque pipe on an d jetted to a 168 main(stock) and a #52 pilot still using 100 oct and 40-1 klotz mix w/ UNI dual filter.  Rarely foul now broken in and these combos.  Surging is noticable only at near peak of power band in 4th and 5th(high speed after long run).  Altitude does not affect nor does climate the surging as The last month changed by 25+ degrees cooler and altitude by as much as 1500 ft. in some cases.   Fouls easier of course at higher alts. but I have some nice straights to clean it out.  If I W.O.T it in 1st or second, and the tire is spinning crazily then lift, I feel no surging at all.  

Think of the surging as the bike telling you it is tuned and ready and wanting to run.

[jdv500]

to kxcam, surging follow-up..
 thanks for the info on my problem and suggestions with the surging. as well as my ky spelling.!!
the surging is'nt much of an issue to me now adays, my thing now is traction on slate rock.jdv

[Friar-Tuck]

 I found this and thought there were some guys who may not have been around back in '04
  &  Rather than start a new topic...here she is...
    Tuck\o/

[bigfabber]

Good info, I noticed the surging on fireroads, but the bike runs so well otherwise I thought I would just live with it. Turns out that's the correct answer, but good to know for sure.

[Polar-Bus]

I have a lot of opportunities to run my KX500 WO, as I frequent riding out on the ice, and logging roads, and bottom line from my experiences, the leaner your main jet, and the longer the WO throttle engine load, the more you will experience the imfamous KX500 "surge" when returning to an idle.  I simple deal with it, (just like many people get overly concerned when they hear a KX500 loudly backfire when trying to kickstart) It just happens....

[flyingfitz]

Mine gives a little "chug" just before the surge with throttle at idle. 

Could the ignition be dropping out a split second for a rev or 2 and then when it returns cause the extra little bit of fuel from the dead revs to ignite causing the surge?

Fitz

[ramblnman]

good info!!just wanted to put this ttt.... ran my bike for the first time this weekend runs real strong but was concerned about the surging with throttle closed coming down from a run.. now feel better knowing it is just the nature of the beast. will back off the idle screw on my next ride to see if it reduces the issue..

[J.ellwanger]

i had a surging problem...
  But when I put NEW Reeds in the bike, Surging went DOWN      a lot!
 91 Pump, 50:1 mix. Jetted For Cold weather, (40degrees) @ 5000-6000
 58 P, 168 M, 2nd clip. Think im a little Rich for now, which may as well play a roll, since the more close to optimum the more surge, maybe ill play with my jets and report back. (but i always forget everything... BAD BAD BAD) do my best...

[motopunk]

the good old detonation problem...    at the moment ihave also this problems on my86-engine. we have on our local track a long straight with sand, followed of some slow tiny turns. if i came off the straight in front of the first turn it started to surge by closing the throttle. then i found some infos about this in the web, that said this is detonating and can caused of to early ignition, to lean jetting , to high compression or just to less octane in the fuel. the cylinderhead is getting to hot by to less cooling from the incoming gas and so it ignits a splitsecond after the sparkplug by itself like an diesel-engine.  my bike was also pinging on idle with closed throttle.

so i started with a bigger pilot jet--- was better, but still there .
2. i changed a little bit the position of the statorplate into little bit later ignition ----  was better, but it was added also a pinging by soft acceleration with 1/4-1/2 throttle that was gone at full throttle... took it back to its former position.

the next is to try to make the needle richer ...

i run boesen dual stage reeds a pwk39-carb drilled up to 40mm, (like the mikuni roundslide, that was in stock on this engine) , jetted for actual cold weather 60 pilot, 170 main, n82m needle at 3.clip position...98 octane pump-fuel or the 101 octane fuel from our shell gasstation , 1:40 mix with motul 800 racing oil.

[J.ellwanger]

Lol 40 is just a base, lol ill ride into the teens.  Just not quite there. but sopposed to be ok this weekend so im gonna change jets and beg for gas money. Guess ill forget this thread.