KTM 520 SX -00, sometimes doesn't return to idle.

Hi there.

Just recently bought myself a 520 and have had some problems with it from the beginning.

Problem no. 1: While driving, or just revving it in neutral, the bike sometimes doesn't go back to idle as it should. I'd say it gets "stuck" about 3 times out of 10.

I've had the carb apart and cleaned it. I've checked the throttle cables and adjusted thm. I've checked the valves and they are OK. Tried spraying starter gas around the carb and intake to see if there are any leaks, and nothing happened.

So, I'm guessing the jetting needs to be looked and. And no matter how hard I've stared, nothing has gotten better! :moon:

My current setup:

Original exhaust and air filter.

OBDTM, clip @ 2nd from top.

175 main jet

52 idle jet

85 starter jet

Living and riding at sea level, close to the sea. Temperatures vary between, say, -5°C and 25°C.

I've tried adjusting it today, but it didn't really make any sense. Turning it in or out from the standard 2 turns out didn't make any huge differences. Guess I'm just too big of a newb. :thumbsup:

So, what can I do? I've thought about getting myself a OBDVR needle, since it should improve everything, right? :moon:

Problem no. 2: While driving along, giving a sudden burst of throttle makes the bike bog down. Now I just read about the O-ring mod a couple of minutes ago, so I'm going to try that. But could it be related to the jetting? I'll add it here in case it helps.

Any help would be appreciated!

I had that problem and i see many bikes wit it.

Tell me, when it does not go to iddle if you touch the throtlle it goes immediatly to normal idlle speed?

Normally a small MJ will do, try a 172 or even a 170.

The second problem could be related, all KTM´s is see riding over here run a little lean and in my area i have almost the same conditions and temperature than yours.

Keep us posted :thumbsup:

Ah, the dreaded 'hanging idle'. This is due to a lean pilot circuit. I'd start off with opening the fuel screw a little at a time till it goes away. The needle you want to try is not a good idea as it will further lean the carb between 0 and 1/4 throttle. That needle will richen between 1/4 and 7/8's throttle though. But moving the clip does that too.

Ah, the dreaded 'hanging idle'. This is due to a lean pilot circuit. I'd start off with opening the fuel screw a little at a time till it goes away. The needle you want to try is not a good idea as it will further lean the carb between 0 and 1/4 throttle. That needle will richen between 1/4 and 7/8's throttle though. But moving the clip does that too.

Opening the mixture screw made a little impact if I recall correctly (tried it yesterday, before I posted here), but it also made the bike backfire when blipping the throttle, which seems awkward to me...

The reason I'm thinking about the OBDVR needle is because people recommend it and call the OBDTM needle a piece of *%¤#.

IF I were to go with the OBDVR needle, would I be needing a bigger idle jet then? And what would be appropriate? 54? 56?

Would I also be prompted to go for a leaner main jet then? Like a 168 or even smaller?

If the revs hang, and I blip the throttle, it most often returns to "normal" idle.

The needle will have nearly no effect on the idle. Pilot jet, slow air jet and fuel screw do. If the correct pilot jet is installed and the fuel screw is set right, the idle will not hang unless... it is a mechanical issue that is causing it, but you stated you are sure this is not the case.

Back firing and popping is lean related. The straight diameter of the needle has a great effect on this. The final letter of the needle designation identifies this. The higher in the alphabet, the leaner it is. Therefore, a needle ending in 'R' is leaner here that one ending in 'M'.

Focus on one are of the carb at a time. Get the fuiel screw set right.

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

Keep in mind, just because '...people recommend it and call the OBDTM needle a piece of *%¤#...." does not make it so (though it may not be a good choice for you). You'll notice I am not reccomending any specifc choice, just how to properly determine what YOU need.

Set the screw according to this method.

Thank you for a great post.

Now, before I posted here on this forum, I tried setting the mixture via a method closely resembling the one you suggested.

Setting the screw at 2 turns out (recommended in the manual), letting the bike idle at very low idle, and then turning the screw in, hardly anything happened! Blipping the throttle a few times and then letting the revs fall it would sometimes stall. But trying to slowly screw it in and see if I noticed any changes, that didn't seem to happen...

Probably just me, like I stated before. I'll try again once I get back from work (next monday...) and see if I can get it better this time.

One of two things wrong! The throttle slide is a two piece affair, main body is a big square Titanium block, its troble free, front piece is also TI (be nice in there those pieces effect the forien exchange rate) is called the "vacuum Release Plate" (VRP)

1: The VRP is upside down. It must be installed square end down!!!

2: The VRP is cracked.

When the VRP is fixed you will need a #48 pilot jet and make certain the closing or bottom cable on the carb has more slack than you think it needs.

If you need a slide or give up and want to have the carb serviced I do many and have some used parts

Hi guys. Just got home from a week at work today, and I immediately went out to the garage.

First off, test start of my 660 SMC. Changed to the Rallye camshaft the evening before I went to work and didn't have the time to fire it up.

Even went out for a small test ride, was a bit slippery in 20cm of snow with my Pilot Power 2s. :bonk:

Then, I checked to see if the fork legs I had gotten for my CR500 fit. Which they did. Happy, happy, joy, joy!

After this, I went on to remove the carb from my 520 and completely take it apart. Cleaned it out with alcohol and compressed air. Made sure everything was clean and nothing broken.

Then, when putting it back together, I saw a potential problem...

The two "arms" that raise and lower the slide?

Well, they were touching the "ridge" that they are (probably) supposed to just pass, which led to the throttle getting stuck sometimes (not all the time). Looked at it closely and it seemed as if one of them was slightly bent. So, I bent it back to straight, tested it, and everything seemed to be running smoothly.

Put the carb back together, bolted it to the bike, and fired it up. Voila! Now more hanging idle for me! :bonk: More happiness!

This does strike me as an odd problem though. Can't see how it would just bend like that all of a sudden, and it's not like it's exposed to any external forces... Anyone has an idea as to why it got like this? Just out of curiosity... :bonk:

Now the only problem remaining is the off-idle bog that everyone seems to be having. Will try setting the pump to just open about 0.6mm tomorrow using a collar or something, and see if it gets better. Will see if I can do that on my 660 SMC as well since that bogs off-idle too.


Been in the garage about 8h per day for the last 5 days now. Working on my car, my 3 bikes and my friends Husaberg FS650.

Took the time to try my 520 a little more yesterday after putting a collar on the acc pump and setting it to a 0.6mm gap. Now, it shouldn't be bogging down when giving throttle, right?

Well, it still does.

And after riding around a little, I realise that it isn't just off idle that it bogs. Even if I keep steady medium revs (4k?) and whack the throttle, it bogs.

So, the acc pump is no longer my suspected culprit.

But since it is bogging at higher revs as well, does that eliminate the idle jet?

What could it be? Any bright ideas?

Turning the mixture screw doesn't seem to make a big difference, so will try to go down one size and see what that does.

Fresh spark plug might be a good idea as well, to eliminiate other sources of error...

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