KLX250S Stuttering When Warming Up | GTAMotorcycle.com

KLX250S Stuttering When Warming Up

jonpurdy

jonpurdy

Well-known member
Site Supporter
Background: had carb issues in the spring (poor performance and took a long time to warm up). Got carb cleaned ultrasonically and this seemed to correct the issue. Though it came back a couple of months ago.

Issue: When cold starting the bike, it takes a long time for the bike to idle smoothly and run well. When applying throttle quickly (in neutral, not riding), the revs will go up slightly, die way down for a second or two, then climb up (if the bike hasn't just stalled out) to where they should be. Once up in the revs (past 5000 or so) it doesn't have the issue. I thought it was a clogged jet or bad gas so I've replaced the pilot jet but it still has the same issue. Have also tweaked the mixture screw and idle speed so that the bike runs incredibly well once warmed up (pulls strong and revs stay consistent when applying consistent throttle). I've checked the throttle cables as well and they're fine.

Next steps: I now suspect that it's possibly a cracked hose or a leak somewhere. When the bike is cold, the issue exists but once it warms up it goes away and the bike can be ridden normally. Since it's a function of RPMs I figured it was the idle jet since at high revs it doesn't happen. However, there might be plenty of heat at that RPM for whatever material to expand and fill a potential gap. I've ordered replacement hoses for the carb and gas. (Haven't ordered radiator hose replacements since I don't see how it would cause this issue.)

Any input on what the issue could be? Am I on the right track by replacing hoses or should I be trying something else? Just an amateur who is happy to be learning about the bike in a hands-on fashion, so I may have missed something obvious. Thanks in advance!
 
Assuming that you have good compression, I'd say your mixture is off. Cracked hose or leak would make the bike generally run like sh1t whether cold or warm.

How does the spark plug look? Pull it and post a photo if you can.
 
if you blip the throttle and revs drop below idle then come back up, thats a rich pilot/mixture screw, if they hang a bit then drop to idle thats lean. now remember, the mixture screw lets in more air.
 
Did you replace pilot jet with same or next size up? When the bike can idle without choke but still not fully warmed up you could do the throttle blipping while playing with the choke. The choke is an enrichener so dumping fuel in (if it blips better) would suggest a lean condition. Is the airbox snorkel in place? You could try partially restricting the airbox opening with red green tape to create a classic choking effect. If it blips better that means it's lean. You may have a tiny bit of dirt in jets or passages.
 
Bikes like this are jetted lean (leaner than Kawi would like them to be - leaner than they should be to run well) from the factory, to pass emissions requirements.
A bone-stock bike will run better with the needle raised one notch.
Perfect example is the carbureted Ninja250. A Dyno-Jet jetkit will immediately help a brand-new bone-stock carbureted machine!
That said, any other changes at all - even a K&N replacement air filter - will make the stock bike run even worse.
Removing noise-abatement trumpets on the intake, anything at all to let more air in, and of course any slip-on pipe, will trash the tuning.
Trace your air intake system searching for air leaks.
Follow the airstream beginning where it enters the airbox, then checking the connection between the airbox and carb, then the connection between the carb and the motor. Tighten clamps if you're able and check for cracks etc.
Anyway, many bikes like yours take forever to warm up, but then run okay once they are...
You can't snap the throttle open from idle, or they will die. That's not abnormal!
It's what you have purchased. It's all that many riders have ever known.
Over the winter, see if your needle has different grooves (to adjust it up or down) as needles have had for decades.
OR discover that it's one of the newer EPA needles with only one groove, non-adjustable.
The big benefit from Dyno-Jet needles (in their jetkit) is that they are old-school. Adjustable!
From 10% to 90% throttle, the motor is on the needle... so it's a hugely important factor.
Regards
 
When were the valve clearances checked last?


Sent from my iPhone using Tapatalk
 
Thanks for all the responses!


Matt Rain said:
Assuming that you have good compression, I'd say your mixture is off. Cracked hose or leak would make the bike generally run like sh1t whether cold or warm.


How does the spark plug look? Pull it and post a photo if you can.
Click to expand...


I'll pull it this weekend and post a pic. The last time I checked it looked a bit lean. I put a fresh plug in since then so we shall see the result soon.


Matt Rain said:
Hmmm, I believe that the KLX is like my bike, in that the screw is on the engine side of the carb. If so, it's a fuel screw - opening it up lets in more fuel.
Click to expand...


That's correct; the Keihin CVK34 has a fuel screw. I found that backing it out from the factory recommended 1 5/8 turns to 1 3/4 turns or 2 turns made the bike idle and respond better once it was warmed up. Didn't help with cold starts though.


inreb said:
Did you replace pilot jet with same or next size up?
Click to expand...


Jets are all stock. Was thinking of rejetting based on how lean the bike comes from factory but this issue came up first.


inreb said:
Is the airbox snorkel in place? You could try partially restricting the airbox opening with red green tape to create a classic choking effect. If it blips better that means it's lean. You may have a tiny bit of dirt in jets or passages.
Click to expand...


I have tried that, as well as removing the cover to see if it was being restricted.


Gryphon said:
Trace your air intake system searching for air leaks.
Follow the airstream beginning where it enters the airbox, then checking the connection between the airbox and carb, then the connection between the carb and the motor. Tighten clamps if you're able and check for cracks etc
Click to expand...


I've done that and suspected that a connection might be loose or leaky, which is why I'm replacing those hoses and such. It's a 2009 so the rubber wouldn't be as flexible and seal as well, even with a clamp, as it used to.


Gryphon said:
Anyway, many bikes like yours take forever to warm up, but then run okay once they are...
You can't snap the throttle open from idle, or they will die. That's not abnormal!
It's what you have purchased. It's all that many riders have ever known.
Click to expand...


I've owned the bike since Fall of 2014. It didn't have any of these issues when I first got it. It would take forever to warm up but while it was warming it wouldn't have such an issue with throttle response in neutral. It used to take about a minute or two to warm up before I could accelerate without it dying, but now it takes a lot longer.


Iceman said:
When were the valve clearances checked last?
Click to expand...
Last checked this July. I had everything done that was required for both the 2-year and 5-year service intervals, since it was five years old and I didn't know if the last owner was observing the 2-year intervals.


Matt Rain said:
It's fuel.
Click to expand...
Thought it might be as well. It always started much more difficultly and ran poorer at first if I left gas in the carb float than if I drained it after riding. Fuel filter looked okay but replaced it anyway. I suppose I should give the tank a good cleaning as well.

Should be getting my parts next week and will post back once installed (as well as once I grab a photo of the spark plug to determine lean/rich condition).
 
I've had stuff brought to me with carbs that were ultrasonically cleaned, and they had issues. Took them apart and cleaned it the old fashioned way and reset the float and jets and they ran like champs after.

Carbs are simple, but you have to make sure everything is working properly, no vaccume leaks , boots are tight, air filter is sealing well, floats are correct height, fuel filter is clean etc.


 
Very late update for those still interested.


I've realized that the bike really is just finicky when starting. I was planning on leaving it stock but I came across a new FMF Q4 muffler for $200. Picked up a Dynojet kit as well.


I bumped the main jet to 120 (per the instructions), adding the Dynojet needle, mixture screw out 3 turns, and installed the exhaust. The bike now starts incredibly easily. When it's hot, there's no need for choke. When it's cold, a tiny bit of choke gets it started. No need to run the engine until it's warm anymore.


A side benefit is that the throttle response is much smoother now. No weak spot around 2000 rpm anymore. The girlfriend even mentioned that she noticed a difference in how smooth the acceleration is.


Most (or all) of the improvement is probably due to the rejet, not the exhaust. I should've rejetted a long time ago. Highly recommended on this bike!

YouTube video with the sound of the exhaust, in case anyone is curious: https://youtu.be/QVKVDuFZh4M
 
You must log in or register to reply here.

Back
Top Bottom