Hi guys. I just bought myself a second hand 2019 150SX as a project build and it’s had a bit of a tough life. I’ve only ridden it a few times just to feel it out before throwing a bunch of money at it and I’ve discovered that something is very wrong with the motor and I can’t figure out what so I’m searching for some answers. Here is where I am at.
Bought with 89 hours on it, I just replaced the top end, barell is fine, no play in the bottom end, reeds are sweet, originally thought it was the power valve but it has been pulled out and totally cleaned and replaced all seals/orings, mikuni carb has been switched out for a 38mm PWK Keihen carb, stock exhaust and muffler.
So the issue is that, the bike start first kick every time, idles perfect and revs up clean and decent enough on the stand. However, once out on the track and under load, the bike runs like absolute shit as is so rich that it barely runs. I got blown passed today on a straight in drag race by an 03 KX125! I then rode said KX and it felt like a 450 compared to my 150.
I’ve played around with jetting and changing carb back and forth, swapping the needle position makes no difference at all. The bike simply doesn’t rev out and the exhaust note is totally wrong.
Anyone have any thoughts or ideas? cheers.
Your powervalve is not opening. They tend to get a little sticky. I was never a fan of the design.
Maybe it pulls some oil from the transmission into the engine?
My 150 powervalve was always a pain in my ass compared to my 125. Take the LH powervalve cover off and rev it on the stand to verify your powervalve is opening all the way.
Any suggestions for making it not so sticky?
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I’ve done that and it seems to be working decent after I cleaned it all. The bike revs slower than a tpi it’s just awful
What jetting are you running?
Keihen Pwk 38. Either N1EF or N0ZF needle tried on both 3rd or 4th clip, tried 45 and 48 pilot and 180 main air screw 1.5-1.75 out. I also had the stock Mikuni set up in there as well and it did the same thing. Like i said, nothing seems to change how rich it runs regardless of the jetting changes. The muffler has oil running out of it.
Did you change the mikuni to a keihin or did you buy it like that?
Bought it with the stock mikuni and played around a little bit with that. Same thing with the power. Then bought a Keihen as I’ve been told they’re more predictable and are just easier to live with than the Mikuni but unfortunately, both carbs regardless of their jetting, the bike just has absolutely no mid to top end power. It feels and sounds like the choke is on the whole time.
Easy fix! The arrow on the piston goes toward the front…..your welcome
Is there any restriction going on? Maybe the silencer is clogged up? Or the pipe?
Gas tank cap is venting properly?
Drain your trans fluid, measure out the correct amount and refill. Run it for a while and then drain and measure, almost sounds like you’re sucking trans oil.
Aj catanzaro had a straight rythm bike that was doing the exact same thing and he brought it to his engine builder to figure out what was going on. It ended up that the powervalve wasnt working correctly and the guy literally just pulled one of the two springs out and it gained 7hp through the mid range.
The bike is a “cr125” but its a custom built bike from italy with a ktm 150sx engine in it, same exact engine you got.
Skip to about 9:20 in the video and see if this matches what youre seeing. I have a strong feeling youre having the exact same problem he had.
https://youtu.be/uR-zupK9Jzg
This would be funny. Doesn't take long to pop a head off a 2 stroke and check. Good thinking.
Ive seen it 2x, my niehbor did it once and so did my bro in his teens. Symptoms were as you described.
https://www.vitalmx.com/forums/Motodrive/Race-Shop,42
There's a tech help section of the forum. Seems like a lot of "help me fix my bike" etc questions in the general moto section.
For the OP, I think some basic diagnostics are in order. Seems like just a guessing game at this point. There are so many possibilities that could cause poor running, it's hard to diagnose over the internet. I'm sure your KTM service manual has some sort of troubleshooting section that will run through the basics to check.
You say the power valve "seems to be working decent"... is it opening properly or isn't it? Might be a good idea to just put the stock carb with stock jetting for one less variable. Verify float level and that the float needle valve is not leaking. Verify proper premix ratio. Leak down test is always a good idea on an unknown engine, or after assembly.
Sorry it's hard to offer any more specific direction without knowing more / checking things one by one.
Had same issue on my 2018 125 and as others stated, it was the powervalve not opening. I dremmeled them to clean them up a bit. There is a thread on vital I think about the issue. Someone makes (or did make) aftermarket arms as well.
I can confirm after questioning myself all night that I did indeed put the piston in the correct way. I’ll check the transmission oil next.
Pit Row
Thanks mate. Appreciate the feedback. I’ve done a fair bit of process of elimination stuff after putting the bike back to complete stock form and it still ran awful. I’ve changed all the basic stuff around but it hasn’t worked so I’m just looking for other issues out there that I may have never heard of myself.
A bad wiring harness will have the symptoms your getting. My sons 125sx's would only get about 40 hrs on a harness. Pull couple bolts and move CDI/ECU around while idling. It will usually bog out when doing this if that's the problem. I've seen this on a lot of 2 stroke KTM's. At least it's only a $45 dollar part
I feel like we just went through this in this thread? Could we have two bad ECU’s in the same week?
https://www.vitalmx.com/forums/tech-helprace-shop/mc125150-wont-get-pow…
Those guys are lucky they didn't kill their engines. I've seen where a backward piston ring snags on the exhaust port and trashes the cylinder.
Im telling you man at least try this out. At the very least its a simple fix that you can diagnose right away and either find out if this is the problem or eliminate it as an option. If youre bike isnt sputtering until you go out and put it under a load thats most likely the power valve. This is a known issue with this exact bike. Id hate for you to replace a million parts on your bike just to eventually find out its the power valve. The bike aj had in this video was a brand new engine and even then the power valve didnt work with that second spring. The second I saw the symptoms I immediately thought back to this video with the exact same probs.
Take it to a professional mechanic.please.
It's a 2 stroke not the space shuttle
Crank oil seal I'd go with
Goldmember with the hot take in every thread!
...yet here he is.
Man, I would just double check the reeds.
Check to make sure the reed black is sealing and check the wiring harness as well. I’ve had the reed block not seal and run erratically as well as a wire on the harness ground out on the frame where the bike would sputter right before it started to get to the Powerband. Plug was always super wet when that was happening.
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