KTM hard to start, won't idle

[tntmo 911]

Between working on the bikeslut WR450, I'm also trying to help a friend of mine sort out her KTM. It's a 2003 KTM 250SX, two stroke. She started a top end rebuild on it, had the piston and cylinder installed and then decided the rest of it was too much for her to handle at the time. I got the rest of it together and we tried to get it started. After several million kicks it wasn't looking good. We push started it, and it fired right off.

It runs really good, very strong motor but I can't get it to idle at all. I pulled the carb and cleaned it thoroughly, adjusted the float level (not positive it's perfect) and now it starts with 4-6 kicks as long as you twist the throttle as you kick. Still won't idle even with the idle screw turned all the way in. If I adjust the cable length and it will idle then, but it won't start unless you back it off. I've adjusted the air screw from almost closed to 5 turns out with little effect.

What else should I be checking? Keep tinkering with the float level? Push it off a cliff? Ideas???

[Spaugh 1]

check the reeds to make sure they aren't chipped and they are closing all the way.

then try putting a new 45 pilot in it. and unscrew the idle adjuster screw until it falls out. inspect the tip to make sure it is still round. Put the idle screw back in without the spring so you can adjust a little further in. If that works, cut a few winds off the spring and reinstall.

if none of that works, you are welcome to bring it over and swap in a known good carb at my place to verify it is a carb problem. but based on your description, it sounds like the pilot circuit is clogged or your idle adjuster isn't pushing the slide up far enough.

[tntmo 911]

I forgot to mention that the reeds had a bit of a gap on them so I took them off and flipped them over. The jets are perfectly clean, although I have seen a new jet make a difference. The idle adjust screw (black plastic one) is definitely making contact with the slide and raising it, I will try it without the spring to see if that helps it to idle.

I think I also forgot to mention that the only way I can get it to start with the kick starter is to kick while twisting the throttle. Not too much or not too little, otherwise you'll just be kicking forever.

[dirt dame 561]

Who put the piston in? Just asking. A piston that accidentally gets installed backwards in a two stroke motor will exhibit the same symptoms. Is all the powervalve stuff timed correctly? Compression okay? Rings installed with the radiused sides up? Check everything all the way back through the rebuild so that you can be sure that every detail is correct and all doubt can be eliminated.

[Zenosan 2]

Who put the piston in? Just asking. A piston that accidentally gets installed backwards in a two stroke motor will exhibit the same symptoms. Is all the powervalve stuff timed correctly? Compression okay? Rings installed with the radiused sides up? Check everything all the way back through the rebuild so that you can be sure that every detail is correct and all doubt can be eliminated.

+1

I have seen inexperienced two stoke mechanics install the piston backwards. But before you tear it down check to see if it has a flat dome or contoured piston. Piston skirt shape can make a difference also. Many two stroke pistons come with an arrow stamped on the top indicating the direction it should be installed.

[tntmo 911]

I really should have checked the piston to ensure it was installed correctly, I'm ashamed to say I didn't. I set the timing on the power valve, installed and torqued the head. I guess I'll pull it apart and double check it.

[Spaugh 1]

the pistons on my 03 300 have an arrow on top. all you need to do is take off the head, not take off the entire cylinder.

you can also remove the exhaust and look in the exhuast port. You should see the round end of the piston hanging down. If the piston is in correctly, you will see just a round section hanging down. If it is in backwards, you will see a round section hanging down that has another round section cutout of it. You can also probably see the arrow that should be pointed at you with the piston at bottom dead center from the exhaust port.

here is a photo off the web. The non cutout side should be at the exhaust port closest to you when you remove the pipe.

also, i had similar issues with the 380 i bought. the idle adjuster just is a bit shy. had to take off a few winds on the spring. Then worked out the jetting and it runs fine now.

also, make sure your spark timing marks are properly aligned. the ignito has a line on it that needs to line up with the lines on the cases. if the timing is too far advanced, you will experience the same problems you are having. hard to start and idle but runs like a demon on top. For the SX bikes, if you have two timing marks on the cases, align it to the right hand mark. if there is only one mark, make sure it is ligned up with the single mark on the ignito. to check, remove the ignition cover and look for these timing marks.

spark comes sooner, the more you twist the stator clockwise.

[Zenosan 2]

Good info Spaugh. The photos of the piston skirts might save him a teardown to check it. I was going to suggest checking the skirt from the intake side. When you remove the reed block on my 79 Husky 390 the whole intake side of the piston is visible. But I am not familiar with the KTM engine.

[Covered in Dust 163]

You guys are to much. If the darn thing won't start just sell it to me for $100.00 and buy a new one ;-). Good post and replies, this is what makes SDAR a great site.

Covered in Dust

[Spaugh 1]

I think I also forgot to mention that the only way I can get it to start with the kick starter is to kick while twisting the throttle. Not too much or not too little, otherwise you'll just be kicking forever.

this is normal. except it should not be necessary with the choke on.

[dirt dame 561]

On most of the two stroke pistons on my bikes, the intake side has a little window cut in it too.

[Zenosan 2]

On most of the two stroke pistons on my bikes, the intake side has a little window cut in it too.

Yup. On my Husky pistons there are two vertical slot openings on the intake side.

[tntmo 911]

Ok, I pulled the head and verified the piston was installed correctly. I adjusted the power valve, it was about 3mm off. I checked the spark timing, it was spot on. Put everything back together and pulled the spring off the idle adjust screw. It started about the same, but I did get the screw to hold an idle after screwing it in quite a ways. After letting it warm up a bit, it starts in 1 or 2 kicks. I think that's not too bad. Maybe with a bit more fiddling around I'll have it completely dialed in. Thanks for all the advice.

[Covered in Dust 163]

How old is the premix? I bought a KTM 200 from a guy awhile back that wouldn't start. I got him down about $400.00 from his asking price. When I got it home I drained the tank and put in fresh premix and it started right up and ran great.

Covered in Premix

[Zenosan 2]

Do you have a good carb cleaner Tom? Sounds like it needs about a 24 hour soaking in a quality carb cleaner. I use the NAPA brand. The passage ways in the idle circuit clog up when sitting fuel crystalizes. Not just the primary jet. I've noticed that modern fuel doesn't have as much of a shelf life as the older fuels. All the addatives no doubt. I would pull the carb and soak all the components at least overnight. Sorry to direct in the direction of the piston installation.

[SCHWINN 0]

Quote: I'm also trying to help a friend of mine sort out her KTM.

You said " HER " All this work she had better be a good friend or really HOT

I think we need pictures of owner and bike Tom...

[tntmo 911]

She is definitely a good looking gal. I told her to start an account on KTM talk, put up a picture and then she will have more help then she will ever need.

I'm also helping out bikeslut, so I'm not swayed just by a hot body.