Maintenance & Technical > KX500 Original

1988 KX500 pulsing/surging/bucking

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quartertondiesel:
Hi guys,

Just picked up an '88 KX500 recently and am pretty new to the big bore stuff, they sure are different animals. I've read a lot on surging with this bike and have had this issue already (had an air leak, replaced intake boot, problem solved) but am noticing now that the surging has come down to a reasonable level. Now, I am noticing a very repetitive mid to high stroke when the bike idles almost like a bucking sound. The motor is brand new (top and bottom end) and is now broken in, the previous owner swore by running 91 octane with Castrol super 2 at 32:1, which seemed to run fine as the motor broke in but now it does this bucking... I have just mixed myself a fresh batch of 91 with Amsoil Interceptor synthetic at 40:1 and have noticed a huge difference in the bike's behavior. However, it still seems to buck somewhat and doesn't always come back down to idle after cracking the throttle when in neutral, it really is completely random.

I have been told just about everything... from running a higher octane to not bothering with synthetic, also have been told to possibly try a thicker head gasket? I have been told to run it rich and run it lean, some people run 32:1 and some run 50:1... like I said, kind of new to this bike so anything can help right now!

Also, I have added a stealthy flywheel weight to the bike so I'm guessing that might play a factor in why it won't always come down to idle from a high rpm?

Thanks for the add!

Regards

Rick

Foxx4Beaver:
some surging is normal on the 500, depending how your jetting is set-up...this can play a role in how excessive the surging can be. There's a ton of threads on here about surging issues. To be on the safe side, you should do a leak down test 1st, and be sure that you're not sucking air somewhere else and causing it to run lean.

sandblaster:
+1
And, while your poking around, what altitude are you riding?
Pilot jet size?
Main jet size?
Needle number?
Clip on the needle position?

quartertondiesel:
I had the carb apart not that long ago and didn't think to write any of the jet sizes down but from what the previous owner had told me the carb/jetting was factory. I ride about 580 ft above sea level and it's rather flat out here so not much altitude changing. Unsure what the best jetting would be for my situation? I'll have to run a leak down test soon, but thanks for the response guys.

sandblaster:
Get your air leak fixed (If you have one( then look at 55/168 3rd position on the clip.

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