KPX 250 Stalling occasionally? Feedback from owners appreciated!!

AdamK · 08-20-2023, 05:26 PM

Hey guys my KPX 250 has about 500 miles on it and out of the box has had an issue with stalling once and a while when I come to a stop and pull in the clutch. Also bogs at low RPMS with a dead spot that seems like it may have improved after I adjusted valves but is still present when bike is cold. Have you guys had similar issues or have issues similar?? Love the bike otherwise, just seems like something is slightly off. Changed the plug, oil change and break in are on point(I'm meticulous with that stuff). Once I'm cruising in higher gears there is no bogging sensation, just the random stall. Reached out to Venom and they said the delay with the throttle when I give it quick gas is normal... not sure what the deal is and looking for any input.

Thumper · 08-20-2023, 08:48 PM

These all sound like throttle body response and enrichment curve issues. It's an EFI system. If you've adjusted the valves and everything else is good, it's hard to imagine anything other than fuel/air mixture or EFI/throttle body responsiveness.

The stalling is likely to be not enough fuel at idle. Try to find a "tune" or method to modify the enrichment curve. This won't help with the delay, but should help with the bogging and stalls.

**JerryHawk250** · 08-21-2023, 07:50 AM

I haven't looked into it yet because mine runs fine but i did run across a video the other day with the same issue on a KPX. Supposedly you can make an adjustment on the throttle position sensor by rotating it. It looks like the screw holes are slotted so it can be adjusted.

Thumper · 08-21-2023, 09:06 AM

TPS sensor does seem to have an adjustment slot. But they don't seem to have an adjustment procedure other than try a different position (!) and they don't discuss what the effect of adjustment is, or how to decide where to place it.

My guess is that a similar adjustment slot exists on a Honda, or other motorcycle, and that the adjustment procedure is known for that. It might even be described in a manual.

I doubt it is changing the enrichment curve. The throttle body position feedback must inform the EFI software the exact position. This information, combined with temperature, twist grip throttle position and perhaps rpm result in how much fuel is spraying. These operate on the edge (lean) and so it makes sense that TPS position is sensitive.

Megadan · 08-21-2023, 09:54 AM

A poorly adjusted or out of position TPS will affect throttle roll on enrichment. When cold the minor lean spot it can create would be exaggerated just enough to potentially cause a little hiccup or dead spot. Loosen the screws and turn it slightly in the opposite direction of the throttle shaft.

Ideally you would have a voltage range you would adjust it to, but without that information it would be blind guess work. Unless people are willing to test their normally functioning tps voltages and post them here.

letsride · 08-21-2023, 11:22 AM

My KPX has had that lag in throttle response since the day I first started it. And IMHO Venom is completely FULL OF SHAT with that "its normal" crap. It doesn't seem to be hurting the bike though, its just flat out annoying. But maybe we are on to something here. When I posted this same exact problem a while back, some folks said there is an enrichment screw on the throttle body. But there is no such screw on the KPX throttle body that I've seen.

Texas Pete · 08-21-2023, 01:47 PM

TPS is just a linear potentiometer so all the troubles and troubleshooting techniques for potentiometers applies to them. The input voltage you can measure, it should be 5v. The stop is the physical stop for the bottom range so it never can turn fully into a 0v floating reference scenario. So it should be something higher than 0 and when moved throughout the range it should linearly increase voltage up to the reference input voltage.

Edit: Whatever you do, don't fall for the highway piracy of TPS test gear charging hundreds of dollars just to test a plain old linear pot =)

Megadan · 08-21-2023, 02:49 PM

The vast majority of 5v Throttle position sensors I have tested or calibrated usually have a sweep of .5V to 4.9V, give or take a tenth.

There are a few 12v and 24v units out there, but not very likely on a cheap motorcycle.

AdamK · 08-21-2023, 06:13 PM

Thanks to all of you guys who responded. Great info and that makes total sense. It is simply annoying. I am going to try to adjust the sensor and see if that resolves it.

kyle · 11-01-2023, 05:41 PM

Quote:

Originally Posted by AdamK

Thanks to all of you guys who responded. Great info and that makes total sense. It is simply annoying. I am going to try to adjust the sensor and see if that resolves it.

If you haven't had your ECU remapped, do that first.. otherwise I'd just check the TPS position (as stated above.. no real adjustment.. just try a different position)

Bill Hilly · 11-01-2023, 06:19 PM

I am very pleased with the results after sending mine (ECU) into American Lifan. It was less than a week after mailing it in. Cost me around $8.00 to send it, and they cover return postage.

Ratbagger · 11-02-2023, 04:56 PM

Interesting stuff to hear about the ECM guys, thanks.

I got a KPX 250 in August and from day one it had idle issues. It has improved over time a bit but the problem has been that when left idling or if I rev high while riding and suddenly pull the clutch and hold it, when it drops to idle, it may stall. It's improved enough over time that it rarely happens now at ~700 miles but I'd like to fix it.

I previously attempted adjusting the TPS as shown in some videos you can find online but that didn't help at all. Actually it made it worse, so I put it back where it was. I've been thinking that I need to adjust the idle set screw and bump the idle RPMs up by a couple hundred, and I've already attempted to do that, but ran into a problem. After taking off the plastics and getting access to the idle screw I found I couldn't separate the grub screw from the nut. The grub screw calls for something like a 2 or 4mm hex wrench, very small, and it stripped/rounded off when I tried to twist it out of the nut. Since then I've been planning on getting a new idler screw and nut. If anyone has any info on a nice replacement, something like a knurled thumb screw would be really nice.

After seeing this thread though I have to wonder if maybe I can send in the ECU and get it fixed that way. I also notice some popping sounds occasionally that seems to happen most often during engine braking. I've heard this is typically overly lean fuel mixture. Maybe connected to the idle issue and the ECU reprogramming would fix it.

**JerryHawk250** · 11-02-2023, 05:05 PM

Quote:

Originally Posted by Ratbagger

Interesting stuff to hear about the ECM guys, thanks.

I got a KPX 250 in August and from day one it had idle issues. It has improved over time a bit but the problem has been that when left idling or if I rev high while riding and suddenly pull the clutch and hold it, when it drops to idle, it may stall. It's improved enough over time that it rarely happens now at ~700 miles but I'd like to fix it.

I previously attempted adjusting the TPS as shown in some videos you can find online but that didn't help at all. Actually it made it worse, so I put it back where it was. I've been thinking that I need to adjust the idle set screw and bump the idle RPMs up by a couple hundred, and I've already attempted to do that, but ran into a problem. After taking off the plastics and getting access to the idle screw I found I couldn't separate the grub screw from the nut. The grub screw calls for something like a 2 or 4mm hex wrench, very small, and it stripped/rounded off when I tried to twist it out of the nut. Since then I've been planning on getting a new idler screw and nut. If anyone has any info on a nice replacement, something like a knurled thumb screw would be really nice.

After seeing this thread though I have to wonder if maybe I can send in the ECU and get it fixed that way. I also notice some popping sounds occasionally that seems to happen most often during engine braking. I've heard this is typically overly lean fuel mixture. Maybe connected to the idle issue and the ECU reprogramming would fix it.

You can send the ECU back to American Lifan and they will reflash it.

Bill Hilly · 11-02-2023, 05:22 PM

Quote:

Originally Posted by Ratbagger

Interesting stuff to hear about the ECM guys, thanks.

I got a KPX 250 in August and from day one it had idle issues. It has improved over time a bit but the problem has been that when left idling or if I rev high while riding and suddenly pull the clutch and hold it, when it drops to idle, it may stall. It's improved enough over time that it rarely happens now at ~700 miles but I'd like to fix it.

I previously attempted adjusting the TPS as shown in some videos you can find online but that didn't help at all. Actually it made it worse, so I put it back where it was. I've been thinking that I need to adjust the idle set screw and bump the idle RPMs up by a couple hundred, and I've already attempted to do that, but ran into a problem. After taking off the plastics and getting access to the idle screw I found I couldn't separate the grub screw from the nut. The grub screw calls for something like a 2 or 4mm hex wrench, very small, and it stripped/rounded off when I tried to twist it out of the nut. Since then I've been planning on getting a new idler screw and nut. If anyone has any info on a nice replacement, something like a knurled thumb screw would be really nice.

After seeing this thread though I have to wonder if maybe I can send in the ECU and get it fixed that way. I also notice some popping sounds occasionally that seems to happen most often during engine braking. I've heard this is typically overly lean fuel mixture. Maybe connected to the idle issue and the ECU reprogramming would fix it.

. It's completely free to send the ECU to America Lifan, except for $7.00-$8.00 postage to send it. I live in West Virginia, and they are in California, and about 7:00PM Eastern Time ,two days after I mailed it they emailed me, and said it was done, and going to be in the next days mail. My bike acted way too lean on idle, and would often die when the throttle was touched , especially when cold. I had some luck with adjusting the TPI, but never got it perfect. I found that turning it just a hair clockwise just about took care of the issue if cold stalling when a bit of throttle was quickly applied, and the bike would idle alright initially after the throttle was released, but then the idle would go down hill. It was ridable, just buy giving it a rev, or two while at a light, but once I saw that it was free, quick, and easy to send the ECU to America Lifan, I decided on doing it. .

Ratbagger · 11-03-2023, 10:18 PM

Cool, so you just sent it in to the address listed on the American Lifan site and they sent it back? I should probably ask them before doing that, haha.

American Lifan Inc
9272 Hyssop Dr, Rancho
Cucamonga, CA 91730

Based on this video it will probably help:

Mine tends to idle around 1200-1400. The manual states 1400. The video shows the remapped bike doing what appears to be more like 1800. Mine does run better than the stock bike in the video though.

I would just try and up the idle RPM since I'm pretty sure that alone would keep it from conking out but there's still the issue of the popping. Doesn't happen that often but every now and then I get a pop in my exhaust note. Sounds like what I've heard from guys putting a slip-on on their bike. I get the sense these bikes are running lean and this is also making the idle a bit weak. This does seem a bit different for my bike though maybe than other people's since I never have issues with it stalling when I give it gas. It's only when idling or when RPMs suddenly drop when riding. Probably it's just a bit different for everyone but basically same thing.

I guess now I have to decide if I can stand to not ride it for a week. Thanks for the info and also the valve adjustment video JerryHawk. I need to do that too!

08-20-2023, 05:26 PM	#1
AdamK Join Date: Jul 2023 Posts: 5	KPX 250 Stalling occasionally? Feedback from owners appreciated!! Hey guys my KPX 250 has about 500 miles on it and out of the box has had an issue with stalling once and a while when I come to a stop and pull in the clutch. Also bogs at low RPMS with a dead spot that seems like it may have improved after I adjusted valves but is still present when bike is cold. Have you guys had similar issues or have issues similar?? Love the bike otherwise, just seems like something is slightly off. Changed the plug, oil change and break in are on point(I'm meticulous with that stuff). Once I'm cruising in higher gears there is no bogging sensation, just the random stall. Reached out to Venom and they said the delay with the throttle when I give it quick gas is normal... not sure what the deal is and looking for any input.
	0

08-20-2023, 08:48 PM	#2
Thumper Join Date: Feb 2021 Location: Kansas City Posts: 2,242	These all sound like throttle body response and enrichment curve issues. It's an EFI system. If you've adjusted the valves and everything else is good, it's hard to imagine anything other than fuel/air mixture or EFI/throttle body responsiveness. The stalling is likely to be not enough fuel at idle. Try to find a "tune" or method to modify the enrichment curve. This won't help with the delay, but should help with the bogging and stalls. __________________ -2022 5 speed Templar X Orange, OEM 51T rear sprocket, 14T front sprocket -NOS 2020 KTM 250SX (2-stroke motocross), less than 10 hours on it
	0

08-21-2023, 07:50 AM	#3
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,553	I haven't looked into it yet because mine runs fine but i did run across a video the other day with the same issue on a KPX. Supposedly you can make an adjustment on the throttle position sensor by rotating it. It looks like the screw holes are slotted so it can be adjusted. __________________ 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S 2004 Honda ST 1300 2016 Black Hawk 250 (sold) Keihin PE30 carb,125 main,38 slow.Pod filter,ported & decked head 10:1 CR,Direct Ignition Coil,15/40Sprockets,NGK DPR8EIX-9,De-Cat,Dual Oil Cooler,Digital Cluster 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
	0

08-21-2023, 09:06 AM	#4
Thumper Join Date: Feb 2021 Location: Kansas City Posts: 2,242	TPS sensor does seem to have an adjustment slot. But they don't seem to have an adjustment procedure other than try a different position (!) and they don't discuss what the effect of adjustment is, or how to decide where to place it. My guess is that a similar adjustment slot exists on a Honda, or other motorcycle, and that the adjustment procedure is known for that. It might even be described in a manual. I doubt it is changing the enrichment curve. The throttle body position feedback must inform the EFI software the exact position. This information, combined with temperature, twist grip throttle position and perhaps rpm result in how much fuel is spraying. These operate on the edge (lean) and so it makes sense that TPS position is sensitive. __________________ -2022 5 speed Templar X Orange, OEM 51T rear sprocket, 14T front sprocket -NOS 2020 KTM 250SX (2-stroke motocross), less than 10 hours on it Last edited by Thumper; 11-03-2023 at 04:25 AM.
	1

08-21-2023, 09:54 AM	#5
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 7,985	A poorly adjusted or out of position TPS will affect throttle roll on enrichment. When cold the minor lean spot it can create would be exaggerated just enough to potentially cause a little hiccup or dead spot. Loosen the screws and turn it slightly in the opposite direction of the throttle shaft. Ideally you would have a voltage range you would adjust it to, but without that information it would be blind guess work. Unless people are willing to test their normally functioning tps voltages and post them here. __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	2

08-21-2023, 11:22 AM	#6
letsride Join Date: Oct 2013 Location: Leesville SC Posts: 832	My KPX has had that lag in throttle response since the day I first started it. And IMHO Venom is completely FULL OF SHAT with that "its normal" crap. It doesn't seem to be hurting the bike though, its just flat out annoying. But maybe we are on to something here. When I posted this same exact problem a while back, some folks said there is an enrichment screw on the throttle body. But there is no such screw on the KPX throttle body that I've seen. __________________ My Stable- 2022 Lifan KPX250 , 2023 Royal Enfield Himalayan, 1992 Honda GL1500 Goldwing, SSR 70 Auto, Kymco Mongoose 90, Honda Four Trax 300
	1

08-21-2023, 01:47 PM	#7
Texas Pete Join Date: Nov 2022 Location: Dallas TX Posts: 578	TPS is just a linear potentiometer so all the troubles and troubleshooting techniques for potentiometers applies to them. The input voltage you can measure, it should be 5v. The stop is the physical stop for the bottom range so it never can turn fully into a 0v floating reference scenario. So it should be something higher than 0 and when moved throughout the range it should linearly increase voltage up to the reference input voltage. Edit: Whatever you do, don't fall for the highway piracy of TPS test gear charging hundreds of dollars just to test a plain old linear pot =) __________________ 2022 1/2 Templar X 250 - 6 gear model - 13 Front / 40 Rear Sprockets - #42 / #120 Jets - 1mm thick nitrile O-ring needle shim (removed) - Kenda K761 Dual Sport Tires - Sedona Standard Thickness Inner Tubes - Stock OEM battery, carburetor, spark plug still going strong - https://youtu.be/dhAYEKH-jFQ Texas Pete's Templar X 250 Torque Specifications Sheet Texas Pete's Engine Displacement Calculator Texas Pete's Tire and Rim Compatibility
	1

08-21-2023, 02:49 PM	#8
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 7,985	The vast majority of 5v Throttle position sensors I have tested or calibrated usually have a sweep of .5V to 4.9V, give or take a tenth. There are a few 12v and 24v units out there, but not very likely on a cheap motorcycle. __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	0

08-21-2023, 06:13 PM	#9
AdamK Join Date: Jul 2023 Posts: 5	Thanks to all of you guys who responded. Great info and that makes total sense. It is simply annoying. I am going to try to adjust the sensor and see if that resolves it.
	1

11-01-2023, 06:19 PM	#11
Bill Hilly Join Date: Feb 2021 Posts: 929	I am very pleased with the results after sending mine (ECU) into American Lifan. It was less than a week after mailing it in. Cost me around $8.00 to send it, and they cover return postage.
	1

11-02-2023, 04:56 PM	#12
Ratbagger Join Date: Nov 2023 Posts: 4	Interesting stuff to hear about the ECM guys, thanks. I got a KPX 250 in August and from day one it had idle issues. It has improved over time a bit but the problem has been that when left idling or if I rev high while riding and suddenly pull the clutch and hold it, when it drops to idle, it may stall. It's improved enough over time that it rarely happens now at ~700 miles but I'd like to fix it. I previously attempted adjusting the TPS as shown in some videos you can find online but that didn't help at all. Actually it made it worse, so I put it back where it was. I've been thinking that I need to adjust the idle set screw and bump the idle RPMs up by a couple hundred, and I've already attempted to do that, but ran into a problem. After taking off the plastics and getting access to the idle screw I found I couldn't separate the grub screw from the nut. The grub screw calls for something like a 2 or 4mm hex wrench, very small, and it stripped/rounded off when I tried to twist it out of the nut. Since then I've been planning on getting a new idler screw and nut. If anyone has any info on a nice replacement, something like a knurled thumb screw would be really nice. After seeing this thread though I have to wonder if maybe I can send in the ECU and get it fixed that way. I also notice some popping sounds occasionally that seems to happen most often during engine braking. I've heard this is typically overly lean fuel mixture. Maybe connected to the idle issue and the ECU reprogramming would fix it.
	0

11-03-2023, 10:18 PM	#15
Ratbagger Join Date: Nov 2023 Posts: 4	Cool, so you just sent it in to the address listed on the American Lifan site and they sent it back? I should probably ask them before doing that, haha. American Lifan Inc 9272 Hyssop Dr, Rancho Cucamonga, CA 91730 Based on this video it will probably help: Mine tends to idle around 1200-1400. The manual states 1400. The video shows the remapped bike doing what appears to be more like 1800. Mine does run better than the stock bike in the video though. I would just try and up the idle RPM since I'm pretty sure that alone would keep it from conking out but there's still the issue of the popping. Doesn't happen that often but every now and then I get a pop in my exhaust note. Sounds like what I've heard from guys putting a slip-on on their bike. I get the sense these bikes are running lean and this is also making the idle a bit weak. This does seem a bit different for my bike though maybe than other people's since I never have issues with it stalling when I give it gas. It's only when idling or when RPMs suddenly drop when riding. Probably it's just a bit different for everyone but basically same thing. I guess now I have to decide if I can stand to not ride it for a week. Thanks for the info and also the valve adjustment video JerryHawk. I need to do that too!
	0