KTM 125 SX issue

[Corban_White]

I got my son a 2006 KTM 125 SX a bit ago and he has been in heaven. I have also been enjoying the bike and having a riding buddy. So much fun! Anyway, on Saturday he was out on a ride with his friend and it suddenly fell flat on it's face. Won't go, won't rev without dying. It starts real easy just like always, but as soon as you rev it (even in neutral) it coughs and sputters and dies. Starts right back up. I have changed the plug, inspected the reeds and am cleaning the carb now, but nothing looked amiss when I opened it up. At this point I'm thinking CDI. What say the RME crowd? I hate CDIs because the only way to test them is "replace with known good unit." And this particular CDI is discontinued and hard to come by. But from my research I think I can use one off of a 2007-2015 bike. I just hate to spend $250 on a hunch. Anyone care to confirm or refute my gut feeling?


As a side note, it's not fun to tow a dirt bike with another dirt bike in technical terrain...

 

[Tonkaman]

It kinda sounds like it’s gone into a lean condition. It could be cracks in the intake book or a bad seal/gasket. I would do a leak down test to be sure

 

[Rot Box]

^ Yes base gasket comes to mind as well.

From what I’ve seen CDI’s either work or they don’t. If you’ve got spark you should be good there.

 

[anderson750]

Check grounds also. I highly doubt it is a CDI. As @Tonkaman said, they are usually dead in the water when they fail.


How many hours are on the top end? I have seen bikes with high hours run decent until the compression reaches a point where it won’t and it happens quickly.

 

[Corban_White]

I did physically check the grounds, I will try a leak down test. No idea how many hours are on the top end, or the bike for that matter. As far as the rate of the failure, it went from running great to not running at all (but starts and idles good) in ~1 second.

 

[anderson750]

I did physically check the grounds, I will try a leak down test. No idea how many hours are on the top end, or the bike for that matter. As far as the rate of the failure, it went from running great to not running at all (but starts and idles good) in ~1 second.

Usually with low compression they will be hard to start, but I have seen bikes that will start just fine. One other thing to look at is the power valve. if it is all gummed up, it will not be moving correctly and choking the motor out.

 

[jeeper]

KTM.


I sometimes have this same problem. For me it's a built-in timer for when to bang the dirt out of my air filter.

 

[Gravy]

My guess is an air/fuel mixture problem.
If it dies when you rev, that means adding air makes it too lean to run.
So likely culprits are:
clogged jet.
Clogged fuel filter/strainer
Low fuel
Airboot leak
Base or head gasket leak
Not enough vacuum signal to pull fuel (rings/ compression)

If I had to put money on it, dirt in fuel clogged main jet. Since it starts and idles.

 

[rholbrook]

I’ve seen a piston with a hole in it run like that too

 

[85CUCVKRAWLER]

Always, always, always....................... check compression first. It takes 5 minutes and it will tell you where to go.

Bad Compression? Take it apart
Good Compression? Check intake boot, spark, timing, etc

 

[Corban_White]

Update, I did a leak down test, but it was inconclusive. The PV vent is also the transmission vent. So with the vent line pinched off, it passed (that tells me that the magneto side crank seal is good at least) but with the PV vent open the air leaked out as fast as I put it in. So I put the vent in a cup of water while the bike was running and it looks like more bubbles than it should have. So probably drive side crank seal. This was done after a thorough carb clean (which was clean when I took it apart, but I went ahead and cleaned it anyway) and it behaved the same. One thing I did notice, when I started it cold it would rev in neutral for the first minute or so until it started to get warm. This behavior seems to confirm crank seal? I did not do the cup of water test when it was cold, by the time I thought to try it again when cold I had already torn it apart too much. Either way, I'm committed to changing both crank seals.

Moving on, I tested the compression and it was 40-45 psi. I didn't believe it so I tried a couple times with the same result. I squirted some 2 stroke oil down the spark plug hole and it came up to 50. Still can't believe it would even run with that much (little) compression, but it was. Ok, so crank seals and a top end. No problem. Got the top end torn apart tonight and there are some slight scratches in the cylinder. 😢 I can't feel them with my finger, but I can with my fingernail. What do the experts think? Good to run or do I need to send it off to be plated?

 

[rholbrook]

Do it right while you have it apart and then you won't worry about it again. Send the cylinder out since it already has a B piston in it. Get you a new A piston for it and have the rod bearing replaced as well. Just do it right while you are at it. I believe I may have an electronic manual for that bike. Message me your email and I will find it and forward it to you.

 

[anderson750]

On a 2 stroke a bad crank seal will usually result in the transmission oil getting into the crank valley and you get a lot of white smoke. Unless you are seeing that I don’t think you have bad crank seals.

 

[Gravy]

Those Pittsburgh compression testers are notoriously useless I don't think that a 125 will run less than about 80 PSI.

But if you can feel it with your fingernail it's too deep

 

[85CUCVKRAWLER]

Might as well resleeve it. It looks like it needs it and it cheap.

 

[Gravy]

Might as well resleeve it. It looks like it needs it and it cheap.

Don't install a sleeve on a 2 stroke. Especially a 125. It won't transfer heat like the nikasil plating, it will have more friction and it won't expand and contract at the same rate as the AL piston and they're more prone to heat and cold seizing.

I send all my cylinders to Millennium to get them replated.

 

[Corban_White]

No sleeve. Nikasil plating. $400 plus shipping to get it redone from Millennium. This is a kids play bike, not a competition race bike. At the end of the day, I want it to be reliable and work, but I'm not after peak performance and "might as well" or "while I'm in there" need not apply. I want good enough. If I have to do it to make it work, I will, but I'm not interested in spending $1000+ on an engine rebuild just to "freshen it up". I'm more of a run to failure kind of guy. Unless the failure is going to ruin a bunch of other stuff. So I'm not going to split the case and do any bottom end work and I'm leaning toward making sure the marks are smooth with a Scotch Brite pad and putting a piston kit in it. But I will have to inspect the cylinder better tonight, I was in a bit of a hurry last night.

@Gravy do you have a compression tester you recommend? IMO mechanical pressure gauges are relatively simple devices, I'd be surprised to see one that hasn't been damaged to be off by almost 100%. +/- 10% sure, but I don't need a compression check to be that accurate anyway. I will compare it to a bunch of other gauges I have tonight.

@anderson750 so if you don't think it is the crank seal, are you thinking the suddenly not revving is just due to the low compression? My son was riding along, everything running fine and all of a sudden it wouldn't run (other than at idle and just above). Leakdown test (with the PV/transmission vent pinched) holds pressure. So that tells me the gaskets and intake boot are good. Maybe I will see if I can adapt the leakdown tester to the bottom end now that the cylinder is off.

 

[Gravy]

I have had two harbor freight compression testers that read so far off that they were basically useless. They weren't even within 30% of each other. They would read large percentages off in simultaneously readings.

Motion pro makes an excellent well priced unit.

If you can change the crank seal without splitting the case- go for it- it's a cheap seal.

I've had 100+ of 2 stroke top ends apart. If you can feel the scoring with a fingernail, it won't run right without getting it fixed. A new ring won't seal that deep of a nick.

That piston that you pulled out of it does not look like it has run very long. I have a few educated guesses about the situation but I'll keep them so myself unless you ask.

 

[Corban_White]

I just bought the bike a month ago so your guesses are at least as good as mine! I really just want to get it going without spending a mint. But, if it needs to be done, I'm willing to do it. I'm just closer to the farmer fix than pro racer kind of guy. Generally the only parts I replace before they cause a problem is oil. I have friends that replace their tires when the knobs get rounded instead of square. I run my tires until they are worn in the spaces between the knobs. And they usually aren't new when I install them. Good enough is all I'm trying to do, and usually more than I have the budget for!

 

[Gravy]

It sucks to have stuff break.

Look at it as an investment, when you go to sell it you can rest assured it will be worth $400 more with a good cylinder. Motorcycles at that age kinda bottom stop on value and if they're in good shape- they are never worth less since someone will always want a good value quality running starter bike.

I've got an account with millennium, if you'd like we can use it. Another option is finding a good used 150 or 200 cylinder for an upgrade. Unfortunately neither cylinder works nor Athena make an aftermarket cylinder for KTM small bore (125, 150, 200) pre 09.

I built an 02 (similar era motor) before and interesting fact they sometimes ran a 2 ring piston (odd for a 125).