Bike runs wide open after closing throttle

[MarkSims]

I had a problem this weekend where my bike would suddenly start running like I was holding the throttle wide open. When it would happen, it was always after I had just got on the gas. When I released the throttle, the bike would continue to run like it was wide open.

The first time it happened, I pulled in the clutch and killed the engine. After that, when I started the bike on the side of the track, it would immediately run like I was holding the throttle open so I had to shut it down. I don't think the throttle was actually still turned - but the bike acted like it was. Is it possible that the throttle returned to closed but the slide stuck open?

After pushing the bike back to the pits, I started it and it acted normal. Took it back out on the track and ½ a lap later it happened again.

I wonder why the bike was stuck in the wide-open state once the problem occurred – but then went away after 10 minutes. Could heat have something to do with it and the problem went away after it cooled?

I took out the slide and it had some milky/greasy stuff on it. I don’t know what it was but it was fairly thick and possibly could have caused the slide to stick. I thought maybe some Belray grease from the lip of the air filter may have been sucked in. I cleaned the slide, drained the gas and mixed up some fresh.

Then I went to a grassy field and got on the throttle several times and had no problem. Took it out on the track and ½ lap later same problem. The only difference between the field and the track is that in the field I would open the throttle and hold it open as I crossed the field, then I would shut it down, turn around and do it again. On the track, I was on and off the throttle much more sporadically.

Here’s a history of what’s happened recently with the bike.

4 weeks ago I put in a new top-end and repacked the silencer. I had one easy ride and one hard ride since then with no problems. Between that last ride and this one, I removed the forks and greased the steering stem bearings. I also retorqued the cylinder and head nuts and cleaned the air filter. I found one cylinder nut loose and tighten it.

After riding I always drain the gas and store it in a metal can. There’s always a bit left in the tank that I can’t get without removing the tank. I’m running VP Racing Ultimate 2 (oxygenated).

The throttle cable may have been under some stress while I was installing the forks because the bar fell from where I had it perched and it was hanging by the cables for a few minutes. I removed the throttle cable from it’s sheath and inspected it. I didn’t see any kinks.

I’m going to clean the carb and throttle and replace the cable. I’m not optimistic that this will fix the problem. It’s very scary to test ride this beast now.

Any suggestions as to what could be causing this? I want to attack the problem from all possible fronts.

My present plan is to address the throttle, cable, and carb. I was wondering if an air leak could exhibit this behavior - only having an affect once the bike gets hot?

Thanks for all your suggestions!

 

[magneto]

Is your slide worn? A new slide for my CR5 was close to $100!

 

[MarkSims]

I did notice some scratches on my slide. I was able to get rid of most of it by polishing it with a cloth. Can the slide stick but the throttle still return to the closed position?

I think it is most likely that it has to do with the maintenance I performed - since it was fine during the last ride but I only made it 1 lap before I had problems this time.

 

[dirt bike dave]

I suspect the throttle cable is damaged or not routed correctly.

If the carb slide or the carb body is worn, it is possible for the slide to stick open - basically it can get wedged sideways in the carb bore. Rolling off the throttle after a wide open run would be the most likely situation (high vacuum) However, this is much more common on a 4 stroke, especially if the slide return spring is too weak, or if the owner has disabled the push/pull cable to give a lighter throttle effort.

If the jetting is too lean or there is an air leak, it could cause the motor to rev with little or no throttle when very hot, but your situation sounds pretty severe. Hopefully, a new, properly routed cable will solve the problem.

 

[MarkSims]

Thanks for the replies. I have my fingers crossed that a new cable will fix things up.

 

[Layton]

Check the throttle spring. They will sack out over time and are cheap to replace.
Also, remove the float bowl and see if there is any water in there. Water will sometimes bounce up into the main jet, which leans out the bike, and make it rev for a second.

Good luck!

 

[rockers109]

Check the oil seal ( fly wheel side) you can check it by spraying carb cleaner on it while it's running and see if it revs up or down. Just a little will do.

 

[MXTex]

Mark,
Whatever you do, be very careful on that thing until you have confirmed resolution. I've crashed due to a stuck throttle and miraculously walked away virtually uninjured, but it could have been bad; real bad. It's the worst feeling I've ever had on a bike.
With all this said, an engine needs air to rev as you have described. Yes, air leaks via gaskets or blown seals can make a motor surge and run lean; but not rev like wide open throttle. Simply not enough air. To this end, I would focus on what could inhibit that throttle slide from closing. Sounds like you're on the right track. Replace the throttle cable and ensure proper routing. Clean the thottle tube assembly and ensure free/smooth operation. Make sure the carburetor is surgically clean and that the slide and slide bore are in good condition. And for added safe measure, I now install vent tube canisters on all my bikes in order to eliminate the possibility of sucking dirt into the carb and possibly creating a jam in the needle main jet bore interface. With all this done, ride very carefully with your finger on the clutch at all times until you're positive of resolution. Then ride with confidence but keep your finger on the clutch anyway. It's safer and better riding technique anyway.

Good luck and be very safe. Stuck throttle condition must be respected.

 

[MarkSims]

I went ahead and ordered a new cable, throttle return spring, throttle rotor, and throttle pulley. The throttle tube is an oberg sports twister and has a bearing inside. It turns smoothly and looks clean inside. I cleaned the carb well and I'm going to clean the tank, too. I'm going to pick-up a fresh can of U2 before the ride.

I had a problem like this once before where the bike was acting the same way. The gas I was using was mixed a couple of weeks earlier. When I mixed up some fresh, the problem disappeared. I always thought that was due to water that got in the gas and was causing a lean condition by sucking up water when I would open the throttle - as Layton suggests. A new mix of gas didn't help this time - but it may still be due to a lean condition. When the bike surges, the front wheel comes way up and I think it is more powerful than it is normally - which also makes me think it is a lean condition.

The pilot may have had some water or dirt in it. I couldn't tell what it was but I couldn't see light clearly through it until I blew it out. The carb bowl had some dirt in it also.

It'll be the 17th before I can test ride it. I'll let you all know how it goes. If that doesn't fix it I'll have to start thinking about an air leak.

Thanks for the help!

 

[23jayhawk]

Only other thing I can think of that is not mentioned above is make sure the tang on the nylon collar at the end of the throttle spring drops into the slot on the fastener above the needle.

 

[DEANSFASTWAY]

Maybe that milky greasy stuff froze on the slide when you had it running.?

 

[Detonator]

I'm reviving this post because I'm experiencing the same throttle problem Mark describes. Mark, were you able to resolve the WFO problem by replacing parts, or did you discover water/air leak issues?

I had two close calls this weekend, both in strangely similar conditions; after powering up a hill in 3rd/4th gear, I would close the throttle as I crested the top. In both instances, the engine kept revving, and when the rear tire contacted mother earth again, the bike pummeled me to the ground and took off. It seems the impact of looping out would cause whatever was sticking/happening to stop, because the motor would stop revving and quit on its own. I'm now debating whether I was experiencing a momentary surge (air leak?) or a jammed/sticky throttle cable or slide. I do know that I looped out hard during a woods race two weekends ago, and the throttle housing on the bars received the brunt of the force. I've had two buddies take a look, and they both agree that the throttle action is smooth, there's lots of return force on the spring, and there is no hint of binding while playing with the throttle. Visual inspection of the throttle housing revealed nothing unusual, and everything is nice and clean. I hate to be a wuss, but I won't ride again until I solve this problem. If you've never experienced a motor surging unexpectedly, I can promise you it's one of the most humbling 2 seconds in your life. I consider myself blessed to be able to type this message!

Please help!

 

[va_yzrider]

I I hate to be a wuss, but I won't ride again until I solve this problem. If you've never experienced a motor surging unexpectedly, I can promise you it's one of the most humbling 2 seconds in your life. I consider myself blessed to be able to type this message!

Please help!

You're not being a wuss! I have been there and it is not fun at all. I seperated a tendon after quadding two rythm doubles when my throttle stuck wide open. Needless to say, I came up a little short and nose dived into the last landing.

There is a recall on your throttle pulley. If you haven't had it replaced yet, do so at once!!! This is exactly what caused my wreck and now I'm stuck with a bum wrist (2001 YZ 125).

Nine times out of ten the problem lies in the cable or pulley wheel. A new cable every now and then is a small price to pay for a little extra insurance. Also, be extra cautious when re-assembling your return spring. If you hear it clicking, try again.

 

[Detonator]

I appreciate the information about the recall...I was totally unaware. In fact, if you know of a recall site or model specific list, I'd be much obliged to receive it.

Okay, throttle pulley is one possibility. Any other ideas out there?

 

[dune-addiction]

Do you have a stock needle? What clip position? Is it worn out? I bet the needle is not returning through the needle jet. It doesn't hurt to take a look.

 

[MarkSims]

My problem seems to have gone away. I wasn't going to mess around with it - so I did everything I could think of (replaced throttle parts and cable, cleaned the carb and throttle assembly, etc). The needle had a couple of nicks in it so I replaced it too.

I'm not sure what fixed the problem, but it seems to be gone now. I've only ridden a couple times since then and I'm just getting my confidence back. It is indeed very scary to have your bike surge like that!

Good luck and let us know what you find!

Mark

 

[SMAKme2X]

try just lubing the cable at the point where the cable meets the carb and the point where the cable meets the throttle, a simple chain lube will work for the cables

 

[Detonator]

The needle jet condition and position was my next investigation. I haven't had to fiddle with any carb internals since I bought the bike (how rare is that!), and one of my buddies suggested the same idea that the needle isn't returning to position. It would be nice to have the luxury to go through each item one by one and then do track tests to see which of the areas is the culprit, but wisdom dictates to do it all at once to be certain to flush out the problem. I'm with Mark...I'm going to be nervous for the next few rides. I hope I find a mangled needle/ kinked cable or some really obvious problem so I'm not just hoping I've accidentally fixed the issue. I can't seem to reproduce the effect by playing with the throttle in neutral.
One question I have to ask is the effect water in the float bowl might have. There is a strong possibility with the type of weather we've been having that I have water in the float bowl. Given that my "surges" both occurred after a hillclimb, could there be any relation?

 

[freshroostcr]

check the carb vent tubes, as silly as it seems dirt can enter float bowl . look at needle for tiny scratches . if a grain of dirt is in the bottom of the float bowl on flat ground it will stay put , get in the bumps fuel starts sloshing it can go up the needle jet and wedge needle to stick!!!!! chased that problem too!!!!

 

[Detonator]

I'm doing a follow-up on this post, just for people who may encounter this problem and are doing a search.

1. There is, indeed a recall on throttle pulleys for 1996-2001 YZ125's and 250's, and, if I'm not mistaken, the 4-strokes (any clarification?). A lot of folks are buying these bikes used, and won't get any notification about the recall. That, and used bikes are more likely to have developed grit or kinks in the throttle cable, which can create enough slack to cause wedging between the pulley and the throttle housing. There's a ton of these bikes still out there, and I encourage you to ask owners if they are aware of the recall. Link: http://www.recall-warnings.com/cpsc-content-02-02505.html

2. I discovered quite a bit of dirt in my float bowl and some sludgy buildup on the walls of the slide. In addition, the little rubber cable cover on top of the carb was rotted and useless.I'll have to invent a replacement, since my new cable doesn't come with one. A buddy informed me that a lot of guys cause carb trouble by sticking a WD-40 nozzle in the cable at the handlebar end and hosing a bunch of oil down the cable in an attempt to remove kinks. This is fine for a clutch cable, but disaster for a throttle cable. If you think about it, all that oil and any accompanying grit will dump directly in the carb. If you're guilty, clean that carb! If you have to lube the cable, remove it from the carb, do your thing, and then hang it vertically for a couple of hours to let excess drip out.

3. I do woods racing, so every vent tube except the float bowl drain is tucked up high in a dryer environment and taped (not zip-tied, which can crush the tubes). If you keep your tubes hanging in the traditional way, at least clip the open ends at an angle so they have less chance of plugging up with nastiness. Air vent tubes should last quite a while, but tubes that contact fuel have a shorter life span. If they're crusty or cracked, pitch 'em.

4. After seeing the amount of grit and communism in my throttle assembly and tube, I will now add this to my list of maintenance areas. It'll take less than 5 minutes to clean and remount.

5. If you're going to clean your carb, take an extra minute and pull your reed cage at the same time (4 strokers exempt). Apparently grit in the reed assembly can cause weird engine problems. I had some evidence of dirt in the area, and I'm glad I pulled and cleaned it. Check the rubber boot for cracks/holes. That would be a bad place to have a compromised entry point.

6. Take a degreaser to the airbox and rubber boot while the carb is off. If you notice a lot of tackiness or grease, change your air filter technique. Sometimes people have good filter prep, but they stick the thing in there and go riding right away; often just letting the filter sit overnight lets the oil soak deeper into the foam and is less likely to get sucked into the boot. Ditto for the grease ring around the filter; if you're greasing too close to the inner edge of the filter rim, grease can spooge and get sucked into the boot and/or carb. Sounds anal, but those little jets in the carb make you realize that a spoonful of grease could really spoil a day of riding.

7. If you're going to go to all the trouble of yanking and cleaning a carb, you'd be wise to drain your gas tank, rinse the bottom areas with some fresh fuel, dump that, too, and start with some fresh premix. Ditto for your fuel can. Common sense, but it's exactly the kind of thing we get lazy with.

Anyone want to add items?

 

[muddy226]

This sort of thing is usually a maintenance/worn parts issue, with the big problem being finding out which part is to blame. However, I have heard of it being caused by "icing", where the incoming air freezes the slide open momentarily, and this would happen where the throttle has been wide open and under load, e.g climbing a hill. A friend of mine once experienced throttle sticking on his CR125, and after being advised to use castor based oil the problem went away.

 

[JTurn36178]

Hmmm. Sludgy stuff on the slide walls.

Sounds like the carb is getting water in it. I've had the same expirience a couple of times, It was the boot at the top of the carb. Mine had split and when I got in water or washed it after a ride it was letting soapy dirty water in at the base of the cable. It would work fine then just hang. I lubed the cable thinking it might be dry. That's when I found the prob. By the way, I was going riding and didn't have the opportunity to get a new cable and it didn't hang again.

Good luck. Hope this helps.
Jim

 

[Detonator]

I appreciate all your responses, and I hope we're creating a good resource for anyone who encounters the same. The only other thing I haven't mentioned is that there was a good bit of crud around the spark plug area, indicating that it wasn't sealing properly (air leak?). I cleaned everything up and put a new plug in ( the old one looked great, and my jetting is fine). I also removed the exhaust pipe and silencer and removed as much garbage as I could. I repacked the silencer and siliconed the joints. New exhaust gaskets and springs. Then I took the subframe off and took a degreaser to the airbox and boot. I did discover a pocket of sludge in the boot area, more than likely the waterproof grease from the air filter. I am ashamed!
Anyway, I just went top to bottom cleaning everything and replacing all necessaries. Drained the gas tank and swished some fresh fuel around, drained it, too, but there were no visible deposits in the fuel. My first ride was about 4 hours long, and I experienced nothing unusual (except the bike was a lot quieter after the repacking and fresh plug!) I will keep you posted if the problem reappears.
Q: the carb slide wasn't "pristine," in the sense that there was some light scuffing on the contact points. One side of the slide was like new, the other had the light marks. The carb barrel was perfect, though. I had a local mechanic look at the slide and he felt it for burrs or irregularities, then told me it was just fine. His feeling was that I was getting a surge due to crud blocking one of the jets temporarily. Should I be concerned about the slide condition? It was as smooth as butter when I tested it during reassembly.