After have initial carb problems, I got my new-to-me KLX250S running great and have been happily riding it for the last 2 years. It starts right up, can be taken off the choke shortly after and idles and runs fine through the rev range.

BUT... it's always been a bit abrupt from on to off throttle (and to a slightly less noticeable amount, from off throttle to on). When I let off the throttle, it shuts the fuel off NOW and the front end dives and it's not exactly a smooth transition. I try to work around it by gently easing off the throttle and using the clutch a bit sometimes but a smoother on/off would be nice.

Can it be improved by fine tuning the air-fuel screw or is this just the nature of the beast? It's currently approx. 1.75 turns out.

 

I think it's simply the fact that a thumper will decel harder than some multi=cylinder engine. You could adjust the air/fuel screw a bit, but I don't think it will have a great effect. I'm at 1000 ft elevation and mine's at 2.5 turns out.

 

Yeah, you're probably right. I was just hoping I could smooth it out a bit.

Is there a procedure for tuning the air-fuel mixture listed anywhere?

 

It isn't easy to reach the screw, there are some that make it easier, like the Kouba idle mix screw. The only method I know of that is fairly simple is the idle drop method. Finding the middle point for best idle.

You know yours is working at 1.75 turns out, turn it in a quarter turn. If the idle drops the mix is too lean, if it picks up it is too rich. Now, knowing it's at 1.5 turns you turn it out 1/2 turn. If idle picks up it is getting closer to best mix, turn another 1/2 turn and on until the idle drops. That is where it is getting too rich. Now count the turns back in, idle picking up a bit since you are right on the edge of correct, until it starts to drop again. Take the number of turns or fractions of a turn and divide it in half. Turn the screw that far back and you're good to go. There is a small range, maybe a half turn total where the idle will hold fairly constant, unless you have a tachometer that reads in 10 rpm increments you won't do better.

The good part about finding that middle setting is that the weather conditions can vary and the engine can function well either direction when weather or elevation changes. Elevation will have the biggest effect if you are going several thousand feet difference. This middle setting will allow a reasonable variance.

Here's a video on it

click here

 

Great instructions, thanks klx678. I watched the video too, so between you and it, I should be able to get it reasonably close.

 

That's the thing, with fixed jetting there is no one perfect setting. It sets one fuel/air mix, but weather and elevation changes will alter the perfect setting. That's why you set the idle between the two drop zones. I don't know if he actually covered that in the video. The mechanics at the shop where I sold bikes were totally wired when they got a special tachometer that read in 10 rpm increments, making their idle drop mixture setting even more accurate - in general, splitting the difference between the two drops (lean and rich).

 

Like you said in your first reply, it might be the nature of a single cylinder engine that it deccelerates a bit harder than a multi cyl. bike. I'm just going to tweak it a bit and see if I can smooth things out. As I said it's already running well, just want to see if I can refine things a bit.