General jetting question.

(I was hoping it would be a sticky on the top of this page).

In general, how do we know the jetting on an old 1991 KX250 is correct?

I just bought the KX250 for $700 (and a 2004 KX65 for $900).

They were both taken care of and start right up and run far as I know.

There is so much talk of jetting, I'm starting to wonder what to look for if the jetting is off. I have only riden the 250 aroud the 2-acre yard. Only get to touch 3rd gear breifly. It pulls like a raped ape and my arms are tired in just a couple laps.

But I now have at least a tank of gas through it and have the feel for it.

WOT sounds awesome. Anything less sounds kind of I just WOT as much as possible :thumbsup: . When it's gurgly at 1/4 and 1/2 throttle, it still is accelerating and pulling good, just sounds kind of stumbly like....compared to the awesome WOT scream.

Other than five cylinders of snowmobiles, these are our only 2-strokes motors(well, there is the chain saw). But for those of us that are not real experienced, what should we look for in a bike being to fat or lean? Just go by feel and sound? There has to be something more to it than that.

Is there some way to shut it down after a hot lap and check the plug to see if it's to lean or rich? Some way to look for piston wash? Something about the smoke of it?

Or, is it "runs good and starts and doesn't foul a plug it's fine". I can't imagine that's the case.

So. As I'm tearing up my lawn, what should I consider and look for and feel for that might indicate it's not as good as it should be in jetting? How do ya'll know if it's to rich or lean?

I'll strap on the noggin cam and run a lap and post it here so you can hear it.

Please advise what thread I should be reading on basics of jetting.



Edited by CooHead

I'm finding more to read on the subject.

Here is a vid of a couple laps around my house (click on picture).

At the end I go kind of slow.

The wind noise isn't to bad.

Thanks for listening :thumbsup:

Let me know what you think....not about my awesome track and/or riding abilities, but how it sounds :confused:



Gurgly at 1/4 or 1/2 throttle? A single cylinder 2t dirt bike thats made to rip through the rev range quickly will sound different to a 5 cylinder snow mobile and a chain saw that has basically two throttle settings (idle and WOT) Stumbly down low? thats where the engine isn't using its expansion chamber, in fact at that stage the expansion chamber does more harm than good (think of it like turbo lag) once you hit WOT the expansion chamber starts doing its thing, which is scavenging and port blocking. Look up expansion chamber on wikipedia to get a better understanding, you will see the shockwaves that travel back and forth, outside of "powerband" these shock waves are out of time and will inhibit the engine slightly, giving you the ring.......ding, ding.....ring,ding, ding you hear at idle. That would be my guess anyway as to what the problem is, your jetting may be fine, hope this helps!

Just had a listen to the vid, sounds pretty normal to me, maybe some one will tell you its bogging a bit, but I think its pretty good, with a bit more riding you might start ripping it up a bit more and that will fix it!

Edited by kiwi86
Watched the vid

Yea. I'm an engineer and totall understand the pipe attributes. It's made for an RPM range and at it, the wave expands, speeds up, gets squished at the other end and waves back, shoving some exhaust air back into the jug. I get how it all works.

I am trying to keep it up there and breathing right. It's just new to me (two weekends now) and am not super confident with it, nor do I have the room to really rip.

Add to that and I had a basball cap on with the camera mounted and it felt scarry to not have my helmet on.

I know it's not a 4-stroke. I did use to have and ride an Xt600 4-valve and that thing can churn down low RPMs.

Just wondering how we should know if we are off on the jetting? What I should be looking for and listening for and feeling for that would indicate that it's to rich or lean.

(btw: One sled is a three cylinder and the other a twin=five cylinders of two strokes :thumbsup: But yea, those are bigger and more modeslty tuned and not so hi strung...and hold WOT for many seconds at a time.)


There are a few good 2t jetting threads on here somewhere that get real detailed, how to read the plug, what noises it makes etc, try a search for 2 stroke jetting.

Tonights sites of surfing are set on the actual act of changing the jets. least looking at them and seeing what ones are in there.

I'm guessig it should be pretty quick and easy of we are suppossed to often change them to get it to run best.

We do it while it's all mounted on the bike? Like maybe loosen the intake boot strap so can rotate the carb a bit and take the four screws out to take the bowl off and there it all is? Maybe 10-12 minutes? it more involved than that?


On my bike it's pretty much just that, loosen clamps, remove throttle cables, twist it to the side and undo the float bowl. I'd imagine yours wouldn't be anymore dramatic.

If you do that you would do well to pull the jets out and make sure they are all clean, try the bike a gain after and see if it makes a difference. And yeah, check the numbers on the jets and find out if they are the right ones for your altitude et cetera.

Anyone else out there?

Same principles apply as your chainsaw and sled. They really aren't that different at all. You can do plug chops, using a fresh plug, for your main jet (top end). For lower settings it's more trial and error/ observations. A good place to start is find out the factory settings, and then realize these are typically a bit rich especially if you don't ride in 70 degree weather at sea level the whole time. Worn rings, reeds, etc effect jetting so this is assuming yu have a motor in good shape.

Hard to say from a vid but it sounds like you may be a hair rich on the bottom. First try an airscrew adjustment, see were your pilot is, then possibly drop the needle a clip.

... You can do plug chops, using a fresh plug, for your main jet (top end). ......, then possibly drop the needle a clip.

"Plug chop" ??

"drop the needle a clip" which would lean the lower rpms a bit??

Thanks a ton.

Still reading and learning.

When I get back from vacation, I'm going to put a new top end it, check the reeds and intake boot and bearings present jetting and in general, take it all apart and put it back together.

Will advise here what I find.

Thanks for any and all input.


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now