Sticking throttle on KDX200

[rocketman1]

On my new KDX 1998 bike, (refer post # 10 as well)
The last ride I noticed the throttle got stuck open a few times, the last time disaster, got thrown off when going over a steep bank, couldn't have happened at worse place, gave me a hell of a fright, I didnt have time to pull in the clutch or hit the kill switch,all happened unexpectantly fast.
Before the ride I had cleaned the sand out of the foam air filter, by washing in kerosene, and washing in soapy water.
and oiling up as usual. Thought I had done the right thing.
BUT on getting home and removing the filter, I found that in recleaning it in a (white bowl) so I could see particles coming out of the filter, a bit more particle of sand appeared. I suspect that one or two of these particles have worked loose and got stuck in the carb throttle slider gate, holding it open.
I have got it clean finally (after about 30 rinses)
I also am a little suspect of the KDX having no return throttle cable unlike my XR250,which has a return cable that pulls the throttle closed when throttling off, I would consider this is a much safer option to the KDX which seems to rely on a spring to push it the gate closed.
The throttle cable seems to be in good condition and trying to get it to get stuck again, off course it just wont happen, Hope it never happens again. But if i does, I hope I'm quick enough on the clutch & brake.
I also note that the air box and air intake, carb side of air filter was dirty and oily, I have since cleaned all this up real tidy. Anyone know of any other reason the the throttle should stick?

 

[MaicoCPA]

Work your throttle while turning the forks to all positions. Sometimes the cable will bind only when the steering is at a certain position. Also, if you haven't already, open up the throttle and make sure the cable inside is in the right position and there is no sand or grit inside. Also, slide the thropttle off the bar and make sure no sand or grit is inside the throttle tube. Lastly, inspect the place where the cable enters the carb. Sometines the cable end will jump out if something is worn and there is not enough tension to hold it in.

 

[Green Hornet]

Lube The Cable

 

[John Harris]

Sounds like you think the problem was in the air intake system. You have cleaned it all and then the filter again and again. Why not throw the filter away and start over? Oil a new one good and use grease to seal it where it meets the air box. I suspect your cable may have sand in it also. Might take a new one. Cheers John

 

[Chili]

Start at the throttle assembly and work from there.

 

[rocketman1]

Sticking throttle

Yeah I have cleaned it up now 100% and have no more problems, boy! I don't want that problem again!
I would say keep your air cleaner CLEAN and oiled and make sure you dont dislodge any sand into the carby side of the filter, when removing the filter. I did also reposition the throttle location a bit and it seems to snap shut a bit quicker , but I dont think this was the problem.

 

[blackduc98]

Lube The Cable

Yes. At the risk of stating the obvious, disconnect the cable from the slide first, or else the pressurized lube spray will just push whatever dirt is hiding in the cable right into the carb slide.

If my throttle got stuck, I would have disassembled the throttle assembly and the carb immediately upon getting home. I would be looking for sandy grit and for wear marks on the slide and carb body. And of course clean the heck out of everything.

 

[ridejunky]

Yeah I have cleaned it up now 100% and have no more problems, boy! I don't want that problem again!
I would say keep your air cleaner CLEAN and oiled and make sure you dont dislodge any sand into the carby side of the filter, when removing the filter. I did also reposition the throttle location a bit and it seems to snap shut a bit quicker , but I dont think this was the problem.

I have to ask, when you noticed this sticking was it usually on a steep hill? If so, are you sure it was really stuck? Improper float level adjustment may cause a lean condition on longer steep hills which may cause a temporary "lean runaway" scenario.... Just a thought.

 

[Rotorranch]

The throttle tube was pushed in on the bars.

That's my guess.

Rotor

 

[rocketman1]

Well just when I thought it was all OK, it happened again, yeah going up a hill again this time my son was riding the bike, and he got on the problem by hitting the kill switch quickly, like this only happened once in 3hours riding. I have today pulled the carburettor completely to bits, cleaned and reassembled it. there was some dirt in the area on top of the slide, I can only guess this has worked loose from inside the throttle cable and dropped down onto the slide.
I cant think why this should happen only when going up a steep hill other than the throttle is wide open for quite a while??
I have cleaned the cable out with CRC and it appears all clean.
The throttle snaps shut quickly, it is free and not being pinched anywhere, the bike doesn't have any thing wrong with except for this dangerous intermittent fault.
The air cleaner is spottless clean no sand getting in and the petrol tank has been cleaned out as well.
I opened & shut the throttle about 100 times to see if I could get it to stick, my wrist got sore, It didn't stick once. beats me?? any more good suggestions

 

[AwfulSmokey]

I have to ask, when you noticed this sticking was it usually on a steep hill? If so, are you sure it was really stuck? Improper float level adjustment may cause a lean condition on longer steep hills which may cause a temporary "lean runaway" scenario.... Just a thought.

It seems you didn't read this. Sounds logical to the problem you explained.

 

[rocketman1]

Awful smokey

What you say, I cannot quite understand, yes it only seems to stick badly going up a steep bumpy hill, fast.
Shutting the throttle to off position on the throttle grip makes no difference, it just screams away like a crazy thing, by flicking the throttle doesnt fix it. You have to hit the kill switch, start it up and she's sweet for the next few hours steep fast hills included, then, out of the blue it will scare the sh-t out of you again.
Can you please explain this lean runaway scenario as I dont quite understand what is meant by that & how do I adjust / fix it.
I know the previous owner adjusted the carb to make it run leaner, and make it a very snappy/ sharp throttle response, which it is, prior to that it bogged down a bit, he said it was running to rich, would this have anything to do with it?

 

[ridejunky]

What you say, I cannot quite understand, yes it only seems to stick badly going up a steep bumpy hill, fast.
Shutting the throttle to off position on the throttle grip makes no difference, it just screams away like a crazy thing, by flicking the throttle doesnt fix it. You have to hit the kill switch, start it up and she's sweet for the next few hours steep fast hills included, then, out of the blue it will scare the sh-t out of you again.
Can you please explain this lean runaway scenario as I dont quite understand what is meant by that & how do I adjust / fix it.
I know the previous owner adjusted the carb to make it run leaner, and make it a very snappy/ sharp throttle response, which it is, prior to that it bogged down a bit, he said it was running to rich, would this have anything to do with it?

What you are describing has all the symptoms of lean run away, adjust your float higher so your bowl has the correct amount of fuel when ascending steep hills and your problems will disappear. You also may want to check to make sure your petcock is clean, this alone probably won't cause the problem but it may be contributing. Lean jetting is not the culprit.

 

[rocketman1]

lean runaway scenario, motor has a mind of its own?

I trust what you say, but I have never heard of a lean runaway situation, but checking out the internet, it appears it is quite common, on aircraft engines, I was wondering does anyone know why a motor should want to self destruct itself in this fashion thank god for the kill switch.
What are the mechanics/ physics behind it all?

 

[ridejunky]

I trust what you say, but I have never heard of a lean runaway situation, but checking out the internet, it appears it is quite common, on aircraft engines, I was wondering does anyone know why a motor should want to self destruct itself in this fashion thank god for the kill switch.
What are the mechanics/ physics behind it all?

The kill switch wont work!! BTW air leaks in carb boot can cause this situation too, most often it happens more consistently though. Think diesel andyou'll understand why the kill switch doesn't work

 

[blackduc98]

rocketman, can you answer the following question for us: when you say "throttle stuck open" are you really sure that the throttle slide itself was stuck in the open position? In other words, when you rolled off the throttle, did it feel like the silde was closing? Or did it feel like the spring which is supposed to push the slide down has somehow disappeared? We know your motor was revving high, but what did your right wrist feel? I'm probably asking a lot of you to analyze exactly what was happening in that terrifying moment, but the answer is really important.

 

[rocketman1]

When I rolled of the throttle it was completely as normal, no pressure at all like it did not want to close, it felt completely normal, no sticking or jamming, it was snapping shut, except the bike just took off like a crazy thing. Almost like its going faster than if I opened the throttle up, normally.
The kill switch stopped it in both cases
It felt like the spring was not closing the slide.
There is a reasonable amount of spring force closing the throttle, so it seems hard to believe it wasnt shutting.

 

[blackduc98]

When I rolled of the throttle it was completely as normal...

It felt like the spring was not closing the slide.

These 2 statements are contradictory. If it felt "normal" to your right wrist, then the spring did close the slide. So is the 2-nd statement describing what your wrist was feeling, or is it describing your guess of what caused the engine to rev? Assuming the spring really closed the slide, this leads us to the "lean runaway" described by ridejunky.

What are the mechanics/ physics behind it all?

I have my carb apart at the moment, so I tilted it as if on a steep uphill, and here are my observations. First of all, the jets end up sitting in a shallower pool of fuel because the float bowl is severely tilted. This is the beginning of a lean condition. However, leverage on the float needle is decreased, so this may compensate a little by allowing more fuel flow into the bowl. When you're running at WFO for a while, the fuel flow rate may become an issue depending on potential restrictions in petcock, fuel filter (if you have one), and float needle. Oh BTW, how much fuel did you have left in your fuel tank? After 3 hours of riding I bet you were running a bit low. If the float valve and/or needle side surfaces are rough then the needle may drag and not open all the way on a steep bumpy hill. Simply put, if the engine is able to consume fuel faster at WFO than the bowl gets refilled, then eventually it will get really lean. So it's very important to start by making sure your float bowl fuel level is correct.

Pilot jet is towards the front of the bike, so it will experience even more lean-out due to tilt. Don't forget that the pilot jet is always delivering fuel regardless of throttle opening. If incline is really steep, the pilot jet can be completely in the air, so now the overall mixture gets even leaner. If your bike was jetted a little on the rich side, this tilt-induced leanout will result in more power.

So you roll off the throttle hoping to calm the bike down. But a strange thing happens if tilt-induced leanout was too extreme: partially rolling off the throttle will bring air/fuel mixture back to ideal, thus producing more power. Now you are really scared and peeing in your pants, and you shut the throttle off completely. Does the idle stop screw really allow the slide to close completely? Usually not. If your pilot circuit is not set up quite right, then you probably turned the throttle stop screw clockwise so that the bike will idle OK, but this means that even with closed throttle it will draw some air under the slide and some fuel thru the main circuit. But on the hill the mixture is already very lean, plus the bike may be sucking air thru the pilot circuit instead of fuel, so the bike revs much more than expected for such a small throttle opening.

My bet is that when this phenomenon happens, riders don't really shut the throttle off completely because they are caught off-guard. Especially on a steep uphill the unexpected burst of power throws you back, and naturally causes your arm to pull the throttle open.

BTW, running lean for a while will make carbon deposits in the combustion chamber glow red-hot and ignite incoming mixture, so like ridejunky already pointed out, even killing the ignition may not always help.

I would do the following: (1) check float level. (2) buy a new float needle. (3) check condition of float valve and if it is rough then maybe you can polish the walls, but avoid touching the seat. (4) make sure floats don't stick when tilted to extreme angles. (5) always ride with 1 finger on the clutch lever.

 

[rocketman1]

Thank you for your in depth analysis.
Yes you are probably right, at this time you are not thinking clearly as to what is happening with your throttle hand, you are concentrating solely on survival.
By the way we crashed on both occasions, with the bike lying on the ground screaming away.
My thoughts against a full lean runaway condition is reinforced by that fact the kill switch worked on both occasions, surely this would not happening, if there was this self igniting deisel effect?
The tank was 1/2 full both times,... we had refuelled, midway.
I will carry out your recommendations as what you say seems very logical.
Thanks again

 

[blackduc98]

My thoughts against a full lean runaway condition is reinforced by that fact the kill switch worked on both occasions, surely this would not happening, if there was this self igniting deisel effect?

Absence of self-ignition does not rule out lean runaway. Self-ignition requires carbon deposits or other sharp edges to be heated up by very high combustion temperatures for a long enough period of time to make those deposits glow.

Good luck, and keep us updated.

 

[rocketman1]

Blackduc 98
I have taken the carb out and checked the float level, and sure enough contrary to the 16mm float above carb housing as per the tech tips, it was 22mm (low) so from my assumption the floats were sitting 6mm lower when fuel was cut off, ie there was conversly 6mm (1/4") less fuel in the float bowl than there should have been.
I have corrected it to 16mm +/-1mm as specified.
The float needle was in perfect condition and the valve looked all good , I didn't take the petcock off as, petrol was flowing freely when the tap was on.
It started 2nd kick and runs down the street like rocket like it always has but I have yet to take it off road & up a steep hill. I hope this has cured this problem.
Thanks again for your assistance I will keep you posted.
Rocketman

 

[ridejunky]

From what you were describing, I was almost certain that was your problem, now you can ride with confidence knowing you wont starve your float bowl and you bike wont scream away with a mind of its own.

 

[blackduc98]

Rocketman, 6mm is a huge discrepancy in float level, and certainly is a prime suspect. It might be worth a call to the previous owner to find out why he set it up this way. Maybe the floats are leaky and were not shutting off the fuel flow properly?

The manual describes 2 methods for checking the float level. The easy way with the carb removed is to measure height of float just as the tang touches the float needle. The harder way is to use an adapter which screws into the bowl drain, but it measures the actual fuel level. You might want to use the 2-nd more reliable method.

Also keep in mind that raising fuel level makes all circuits run richer, so you may need to revisit jetting.

 

[rocketman1]

Rocketman, 6mm is a huge discrepancy in float level, and certainly is a prime suspect. It might be worth a call to the previous owner to find out why he set it up this way. Maybe the floats are leaky and were not shutting off the fuel flow properly?

The manual describes 2 methods for checking the float level. The easy way with the carb removed is to measure height of float just as the tang touches the float needle. The harder way is to use an adapter which screws into the bowl drain, but it measures the actual fuel level. You might want to use the 2-nd more reliable method.

Also keep in mind that raising fuel level makes all circuits run richer, so you may need to revisit jetting.

Blackduc98
Mate I finally got around to riding the bike after the resetting of the float levels, no more problems, but i do have a problem like you say, its smoking more, running a bit rough & harder to start, can I fix this without rejetting, will the airscrew adjustment, be an easy fix?" The bike is also not quite as responsive on the throttle.
please tell me there is an easy fix.

 

[ridejunky]

Now that you corrected the float you will need to re jet to get it to run better. What is your clip position? sounds like you may want to drop your needle by raising the clip. I go 2nd from top then jet accordingly. There is alot in info on jetting if you do a search. Jet it to the way it suites your ridding style best.