Ktm 250exc knock or normal?

I have a 99 ktm 250exc, and it runs great, starts first or second kick when cold, and runs smooth for the most part. While riding the bike it runs nice, crisp, and smooth through the mid range and some of the bottom. The problem is, if I lug the bike really low, it makes a knocking type noise, but not really at all when it isn't lugging quite so low..I already removed the right side engine cover, and felt for any play in the main bearings and there was absolutely no play in the mains at all. Could this possibly be a big end rod knock? Or is it just a normal sound from lugging a ktm 2 stroke, as it's my first ktm 2 stroke and it's done it since I got it.


And the thing is, it doesn't do it when de-accelerating like I rod knock would in most instances, so I'm not sure if maybe it's just normal from lugging the bike so low..? Could it be causes by a lean pilot jet? I'm running a 42 pilot jet with the air screw 2.5 turns out running 50:1..

When I get that down low sound from lugging it's always been my chain slapping...sounds like something is gonna break.

I think that is spark knock.  And my 2011 did it brand new.  What I think causes it is a function of very high load and low rpm on the engine, which is what you would be doing when lugging it.


Fixes would be higher octane fuel.  YOu might notice it completely going away with 100 octane.  You could also somehow reduce the timing at that point - but I don't even know how you would go about it.  As the cylinder wears and your compression naturally decreases, you might find that it goes away a little bit on it's own.


As long as you are using a good premium 93 octane fuel, it should just be an annoyance.  If you aren't...you could potentially damage the piston (bust a chunk of the crown off), but unless you've cut the cylinder or head for higher compression, I think that is very unlikely.  Might be worth it to do a compression test for your own piece of mind.  I personally wouldn't feel comfortable hearing those noises if I was making 200 psi or higher and only running 93 octane pump gas. 

Alright thanks a lot guys, and I was also thinking of spark knock but wasn't sure. I mean the bike idles smooth and runs smooth everywhere else except down very low when lugging, and I don't mind the noise as long as I know it's not something bad. And I'm running 91 octane as it's the highest I can get around here.

I run 91, fresh top end, bike has that knock.. My old 200 had the same thing with a new top and bottom end.. Wouldn't worry about it

Alright thanks a lot guys!

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