KTM idle is sky-high

[number9]

The KTM has been off the road for a little over a year. It has a Rekluse clutch which failed at Southwick last summer; the basket was torn to shreds and some clutch steels were warped to hell. So I parked it, bought a CRF250R and the KTM experienced some benign neglect.

I got the clutch done late last fall but the bodywork was a hot mess and the electrical system was worse. After doing the clutch, the idle was fine as I had to test the clutch engagement.

I didn't ride it last fall and left it in the garage this summer. I finally gave it some TLC by replacing the wiring harness and replacing the kill switch. I also cleaned the air filter and replaced the spark plug. I tried starting the bike today (first time since the fall) and when I start it the idle is sky-high. It doesn't have a tach but it sounds like it's absolutely screaming.

In all the time I've owned the bike, I've never touched the carb. It starts first or 2nd kick, so I've never needed to touch it, meaning I have no idea what the jetting is. I didn't touch the throttle assembly either.

Could this be as simple as a (big) air leak? I tried pulling the air adjustment screw out a couple of turns, that made no difference at all. The throttle opens and closes cleanly, so I don't think it's binding on anything. Is it worth disconnecting the throttle from the carb to make sure it's not being held open by something?

I know I need to add some more transmission oil but I can't imagine that's related...

[Kurlon]

Can you feel the slide thwack down onto the idle speed set screw on the carb when you let go of the throttle? If not, start chasing the cable routing to see if something is pathed awkwardly.

Also, take a can of brake and parts cleaner and spray around the carb intake boot gasket, boot to carb joint, etc. If you've got a massive air leak the bike's RPM will instantly change when that cleaner hits it. That said, if the RPMs are up as high as you're saying I can't imagine that being the result of an airleak, I'd expect hard starting at this point if leaking that bad?

[number9]

Can you feel the slide thwack down onto the idle speed set screw on the carb when you let go of the throttle? If not, start chasing the cable routing to see if something is pathed awkwardly.

Apologies for my ignorance, but you mean literally put a finger on the idle speed screw and see if I "feel" the throttle opening and closing?

Also, take a can of brake and parts cleaner and spray around the carb intake boot gasket, boot to carb joint, etc. If you've got a massive air leak the bike's RPM will instantly change when that cleaner hits it. That said, if the RPMs are up as high as you're saying I can't imagine that being the result of an airleak, I'd expect hard starting at this point if leaking that bad?

Yeah, I'll give that a try. I agree that such a big air leak should make it hard to start, but I traditionally suck at kick-starters and this thing gets going pretty easily.

[Chippertheripper]

Before you do that, make sure the end of the throttle cable is seated properly in the top of the carb.

[Kurlon]

Yeah, you should feel the slide hit the screw as a vibration through the screw. You should also be able to hear the slide hit the stop through the airbox if you pull the air filter. No snap, start digging. No free play in the throttle, start digging...

[number9]

Before you do that, make sure the end of the throttle cable is seated properly in the top of the carb.

I'll have a look, thanks.

Yeah, you should feel the slide hit the screw as a vibration through the screw. You should also be able to hear the slide hit the stop through the airbox if you pull the air filter. No snap, start digging. No free play in the throttle, start digging...

Yep, I can definitely feel it when I open & close the throttle. It snaps nicely. I took a peek at things, and the only thing I can think of that looks maybe suspicious is the carb intake boot. The bike has a Moto Tassinari reed valve system, probably this one. The OEM intake boot looks a little dry rotted, but I think it's always been like that:



Could that be my problem? Interestingly, the Moto Tassinari kit requires you use the intake boot from an SX bike, even if you've got an EXC. A new boot is only $23, but disassembling everything to get to it would be a pain.

[Chippertheripper]

That boot almost looks like somebody tried to seal one of those dry cracks with a fingerful of silicone, no? Either way, that's only going to cause future woes.

[number9]

That boot almost looks like somebody tried to seal one of those dry cracks with a fingerful of silicone, no? Either way, that's only going to cause future woes.

That's actually a good point. I was poking and prodding in there but I don't know if I made that smudge or if the PO did. He was a hamfisted fuckhead, so the money is probably on him.

I have to order some stuff anyway from RMATV so I guess I'll get another boot. The new (OEM) bodywork doesn't fit properly, but that's another story...

[Kurlon]

Oh hell yeah replace that boot.

[number9]

Oh hell yeah replace that boot.

I'll order one tonight. Do we think it's a potential problem? I guess I could put some electrical tape over it and start the bike, that might seal well enough to let me know if it's the issue or not.

I really don't want to run the thing for long because I'm afraid I'll blow the motor.

[Kurlon]

Start the bike, spray that area with brake and parts cleaner and see if it responds.

[number9]

Start the bike, spray that area with brake and parts cleaner and see if it responds.

I'll give that a shot in a second. Any chance the carb could be gummed up because it's had gas sitting in it for a year? I can't think of how that would make it idle high, though.

[number9]

I tried covering up the crack in the boot with electrical tape. It seemed to make a little bit of a difference in that the idle went down. I tried spraying it with carb cleaner after removing the tape, that made basically no difference.

I'll order a new intake boot tonight.

[typeone]

Any chance the carb could be gummed up because it's had gas sitting in it for a year? I can't think of how that would make it idle high, though.

it may not be related but after that length of time, i would def clean the carb fully AND lube/recheck throttle routing. easy stuff. with a boot that dry, you may want to just get a carb rebuild kit for peace of mind as well. modern pumpgas is evil stuffs.

[number9]

it may not be related but after that length of time, i would def clean the carb fully AND lube/recheck throttle routing. easy stuff. with a boot that dry, you may want to just get a carb rebuild kit for peace of mind as well. modern pumpgas is evil stuffs.

I have an ultrasonic cleaner but I've never taken apart a carb before. I'm not really sure that I want to try, but..

I stored it with a totally empty gas tank but there would have been gas in the bowls. I didn't think far ahead enough to use Sta-Bil.

[typeone]

it's not as daunting as you might think. just get a rebuild kit and have at it. very mechanical stuff ... just tiny in some places so you need to be thorough. very simple overall.

gas left in the bowl can create some BS to deal with but it's always a gamble, never know how the fuel will rest. i've flaked and left full bowls without issue, but i've also had carbs that wouldn't stop pissing come spring time because the float was stuck or the needle seat was hard/varnished/crappy.

simple system but it needs to function properly to work.

[number9]

it's not as daunting as you might think. just get a rebuild kit and have at it. very mechanical stuff ... just tiny in some places so you need to be thorough. very simple overall.

gas left in the bowl can create some BS to deal with but it's always a gamble, never know how the fuel will rest. i've flaked and left full bowls without issue, but i've also had carbs that wouldn't stop pissing come spring time because the float was stuck or the needle seat was hard/varnished/crappy.

simple system but it needs to function properly to work.

Well, it would be a good excuse to put a JD jet kit in there, or whatever the current hotness is. But part of the reason I'm so reticent to tear it apart further is that it just lowers the chance I actually throw a leg over it before winter comes..!

Right now I'm focused on Occam's Razor so I'll try the new intake boot and see how that goes. If it doesn't help, I guess I'll pull the carb off.. maybe Kurlon will want to rebuild it for me

A new OEM carb from RMATV is only $270.. while I'd rather not spend that, that's not much shop labor if I end up having to pay someone to rebuild it!

[Kurlon]

Carb cleaning if you've got an ultrasonic to stuff it in is EASY. You got this, the longest part will be waiting on the ultrasonic to do it's thing.

[number9]

Carb cleaning if you've got an ultrasonic to stuff it in is EASY. You got this, the longest part will be waiting on the ultrasonic to do it's thing.

I do have an ultrasonic cleaner, it's come in pretty handy. Mostly I use it for cleaning fasteners that I don't want to replace...

I'll try swapping the intake boot and see if that helps. The boot should be here by Thursday or Friday. If not, I guess I'll tear down the carb...

(Honestly, I should fix this thing and sell it and replace it with something much newer. Sigh.)

[Kurlon]

You'll have the carb off to swap the boot, just clean the carb then. Undo all the hoses, noting which go where. Take the top off, two screws, don't both removing the cable from the slide if that intimidates you. Take the bowl drain off, it just unscrews. Turn the carb upside down, remove the four bowl screws and lift the bowl off. You're now staring at goodies. The float is held in place by a pin, and has a needle under it. Push the pin out one side and lift the float straight up so the needle comes with it. Note how the needle is held onto the float tang by a bent wire. Slide the needle off the tang, making sure not to flex the tang so you don't alter the float height adjustment. Needle, bowl and bowl drain go in the cleaner. Next to remove from the carb are the main and pilot jets, 6mm hex and small flat bladed screw driver, the pilot will be recessed into a passage. These go into the cleaner. Leave the idle speed screw alone. Count how many turns it takes to lightly (and I mean LIGHTLY He-Man, you're not securing a rotor to a wheel with that screw, just seeing where the threads stop) seat the idle mix screw, write it down, then unscrew. There will be a spring, washer and o-ring that should all come out with it. Put the screw into the cleaner. Put the carb body into the cleaner.

Let the cleaner do it's thing.

Once clean, remove the carb body first, dry it off with compressed air, blowing out all the little passages, holes, etc, and then hose it down with WD40, pat dry. Repeat for each part you pull from the cleaner.

To reassemble, it's the reverse of how you took it apart. Put the spring on the idle mix screw, then the washer, and o-ring, thread in until it lightly seats. (Remember what lightly means?) Back it off however many turns you counted before. Put the main and pilot jets back, noting that you just need them tight enough so they won't rattle loose, they don't have to be cranked on there. Make sure the lil pop up piston on the back side of the needle is springy (If it sticks, clean again, still sticks, order a new one) and then carefully put it back on the float as it was before. Float gets set back onto the carb, slide the pin through to hold it in place. Bowl goes back on, screwed down (don't over do it!) and then the bowl drain. Finally put the slide back in and screw the carb cap back down.

Done.

[number9]

You'll have the carb off to swap the boot, just clean the carb then. Undo all the hoses, noting which go where. Take the top off, two screws, don't both removing the cable from the slide if that intimidates you. Take the bowl drain off, it just unscrews. Turn the carb upside down, remove the four bowl screws and lift the bowl off. You're now staring at goodies. The float is held in place by a pin, and has a needle under it. Push the pin out one side and lift the float straight up so the needle comes with it. Note how the needle is held onto the float tang by a bent wire. Slide the needle off the tang, making sure not to flex the tang so you don't alter the float height adjustment. Needle, bowl and bowl drain go in the cleaner. Next to remove from the carb are the main and pilot jets, 6mm hex and small flat bladed screw driver, the pilot will be recessed into a passage. These go into the cleaner. Leave the idle speed screw alone. Count how many turns it takes to lightly (and I mean LIGHTLY He-Man, you're not securing a rotor to a wheel with that screw, just seeing where the threads stop) seat the idle mix screw, write it down, then unscrew. There will be a spring, washer and o-ring that should all come out with it. Put the screw into the cleaner. Put the carb body into the cleaner.

Let the cleaner do it's thing.

Once clean, remove the carb body first, dry it off with compressed air, blowing out all the little passages, holes, etc, and then hose it down with WD40, pat dry. Repeat for each part you pull from the cleaner.

To reassemble, it's the reverse of how you took it apart. Put the spring on the idle mix screw, then the washer, and o-ring, thread in until it lightly seats. (Remember what lightly means?) Back it off however many turns you counted before. Put the main and pilot jets back, noting that you just need them tight enough so they won't rattle loose, they don't have to be cranked on there. Make sure the lil pop up piston on the back side of the needle is springy (If it sticks, clean again, still sticks, order a new one) and then carefully put it back on the float as it was before. Float gets set back onto the carb, slide the pin through to hold it in place. Bowl goes back on, screwed down (don't over do it!) and then the bowl drain. Finally put the slide back in and screw the carb cap back down.

Done.

Josh, that's awesome, thanks! I looked through a few YouTube videos and this one seems to be the closest carb to what I have: KTM 125 SX Top End Overhaul (Part 4) - Cleaning Carburator - YouTube

I also made this and was going to put it on 771's Facebook page but then felt bad. But it's also kinda funny, so...

[Kurlon]

Totally post that!

[number9]

Totally post that!

Done!

[Kurlon]

Hrmmm... can't tell if it's showing up or not. FB tells me I need to approve it, but doesn't give me a knob to do so... bah.

[number9]

Hrmmm... can't tell if it's showing up or not. FB tells me I need to approve it, but doesn't give me a knob to do so... bah.

Yeah.. I don't believe you. At all.

Fool me once, shame on you. Fool me twice...