Revs from hell -- stuck throttle or bad mojo?

A

AZRickD

New member
I was having a ball on my 2005 EC250 at this weekend's race. A 52 mile GP north of Tucson. Rocks and sand. Nothing particularly technical except for lots of ingress and egress from silty sand washes.

I was on my second of two laps with five mile to go when I exited some single track and came to a kinda fun, long, rocky jeep road with a modest incline. I was motoring up nicely when I noticed (the whole thing transpired over a period of about 4 seconds -- or less) my revs begin to build, slowly at first, then faster. Like a Cox model airplane that had just run out of fuel. Lean condition.

I was well aware that I hadn't changed throttle position and was wondering about a stuck throttle. But this "stuck throttle" resulted in more and more revs until it was screaming beyond the point that my front tire started to loft (pre-loop).

I _attempted_ to grab a fist full of clutch and brake but I kept moving as I tried to find the kill switch and lost some leverage on the clutch lever. Moments from disaster (heading to a rock pile), I decided to do a strategic dismount and fix things later. As I dropped the bike I slammed my pinky into a rock (broken finger, rock is doing well). The sweep and course worker heard my still screaming motor and rushed to my aid as I finally found the kill switch on my now looped and pointing-downward bike (musta looped it when I let go of the clutch).

I re-started it and it was still acting funny. I couldn't tell if my throttle was sticky or if my hand was numb. I rode it up to the top of the hill with it still acting strange. It was revving mid-high, but not like a banshee as before. The sweep told me to shut it down. He said he was going to find a short cut for me but I told him I'd finish. With no real solution in sight, I backed off the idle screw and re-started. It behaved normally but I doubt the idle screw had anything to do with it.

I rode at sub RickSpeed with no real problem accept for the double whoop section and some particularly rocky sections flexing my stiff-as-a-board pinky. I was spooked that I might tip in a wash and prang my finger but that didn't happen. I did get my pinky thwacked by three tree branches though. :eek: Glad I had big bark busters on.

Not sure what happened to my revs. Stuck throttle or stuck slide doesn't quite answer it since the revs slowly increased to the moon without any grip input from me. *New thought* I wonder -- maybe the float got stuck partially closed and caused the carb to go lean.

I'll be at the doc's tomorrow getting x-rayed and set.

Typing with one hand sucks.

Thoughts?

Rick
 
You know where the throttle cable enters the the throttle body on the handle bars? Does yours have a little cap-like thing that you can slide on and off? Ill get a picture of it if you dont know what Im talking about. Anyway, I find that mine occasionally slides off and catches, causing the throttle to twist and the bike to rev harder.

I'd check this out, seems really simple, but it catches me out often
 
Safety Wire:eek: The cable to the throttle and the same on the carb...cheap insurance.....if the cable came out of the housing...
 
This was already safety wired. I'm almost certain this area of the system had nothing to do with the problem I experienced.
 
Sorry to hear about the pinky finger, broke my right one also on a bad get off a few years back..Was out at Moonrocks, NV, anyway, I think I'd pull the filter and see if there is any dust or very small grains of sand they may have somehow got by the filter and into the air boot to the carb.

Check your airbox to carb boot to make sure it did not "come loose" or "slip off", which have caused a grain of sand to get in a jam the slide.

With airbox off and the carb bell expossed, if there was any dust that may have by passed the filter you should be able to tell due to a little premix being on the carb bell and a tell tell sign of dust, ect.

Check your throttle cable for SMOOTH opertion.

Did you fall prior to the this happening? If so, then maybe some dirt worked it's way into the throttle tube between the bar.

Pull you spark plug...have a look there and see how it is burning.

I know I'd go over the entire fuel system from craddle to grave, throttle cable/ throttle tube / cable routing / carb mounting / carb vent hoses / airbox boot clamps and filter. I also would pull the carb apart and check the slide and that plastic part that hold the needle in place and spring.

Good luck hope you figure it out. Please keep us updated on your findings.
 
While I will do all that (when my pinky allows), those item don't explain the behavior. Without any input from me, the bike began to build revs. Then, a few minutes later, all was well.

Sand in the boot wouldn't match up with that.
 
sand in the fuel could cause it to rev up from running lean, then it could have worked itself out.

might have been an air leak, but I've never heard of a self-curing air leak.
 
had a similar incident,racing in NY dusty conditions,cruising in third gear having a great ride,throttle just started revving up on its own,killed it and loosened the clamps and spun the carb and pulled out the slide,as my row went by:mad: looked ok in there,didnt see anything too bad,but put it back together and it never happened again,Im thinking a little dirt got in mine and jammed the slide,I also had the throttle stick once and changed out the cable to a longer one,one full throttle experiance was enough!
 
Fury,

Dusty? Very !!

I've been going through the bike replacing this and that over the last few weeks. I've always thought my throttle cable was too short. Time to Call GoFasters. :o

Got my pinky set today. The magic of novacaine. :p
 
Sounds sort of like when you kick start a 2stroke and with out throttle input it just starts revvving to the moon and puts you in a frantic effort to hit the kill switch.
I don't know what causes it to do that, air leak, stuck slide?
Ron

04 ECDE200
 
Besides the obvious:

How are your carb vents routed? On a KTM 300 with the same carb I've pulled sand through the vents, into the carb bowl, and had it jam the needle into the nozzle at various throttle positions. Scared the crap out of me. Check the bowl drain for sand and the needle for scratches. Route a pair of the lines to the airbox and use filters if you ride in sand a lot.

Check the rubber carb boot for cracks that would cause an air leak. Good idea to replace this when you do a top end and/or reeds. Mine have shown signs of cracks starting (from UV I suspect) after a couple years.
 
Dale Speaks.

I just got off the phone with Dale at Gas Gas USA.

He said he once had a similar problem on a trials bike. This is his theory.

I was motoring along up the slight incline at my medium throttle setting. By this time, sand has already frozen up my slide, I just don't know it yet. Since I am going over rocks, the traction ebbs and floes as I bounce along. When I have traction, the load on the bike is high enough to keep the revs down. But when I get a bit of a kicker, the rear wheel comes off the ground long enough for the motor to rev bit higher. Then I bounce again -- more revs. Rinse and repeat until the bike motor is zooming.

When this happened to him, he had just cleared an obstacle and his bike was screaming. As he had seen with other people, when he went for the kill switch, it didn't kill the motor. He said that once that bike starts to rev out for a few seconds, the carbon inside th combustion chamber begins to glow. The 2-stroke motor begins to "diesel." The kill switch might not work. So when I thought I didn't have enough force on the kill switch or enough time on the kill switch to stop the motor, I was actually fighting the effects of a little temporary carbonized "glow plug."

He said that the technique he used to stop the motor was to pull in the clutch and reach back and put his gloved hand over the exhaust pipe exit. This slows down the motor enough to cool down the carbon thus allowing the kill switch to do its duty.

He said that since my bike was able to maintain lower revs as I restarted twice means that the sand that locked up the slide was able to work its way loose resulting in a normally acting motor. He also said that a kinked throttle cable could act similarly, although it wouldn't be "working its way loose" the way a grain of sand would.

He instructed me to go through the system from throttle tube to airbox. I'll be very happy to do so.

I hope this has been instructive.

Rick

PS, my pinky finger is back in place and I should be fine for next month's Grand Prix.
 
I had the same thing happen to me back in September, I posted some pics on here showing the damage it caused. (sub frame, rear plastic, bars the list goes on till it gets over a $1000.)

It felt the same way, revs just kept building till it was screaming. I couldn't find anything wrong with the bike. I cleaned the carb, new cable, throttle tube and housing, none of the parts looked bad. I wasn't getting back on till I had confidence in the bike (it hurt bad). To this day I don't know what caused it but it hasn't happened since.

If you find anything else let me know.
 
I'm going to the Maxxis Enduro Cross in Las Vegas this weekend.

I'll tear into the Gasser on Monday or so and see how much grit is in there.
 
I'm surprised there's enough carbon in modern 2 stroke, i.e. modern oils, to act as a glow plug. Whenever I pull heads I am impressed how clean they are compared to the old days of 20:1 mineral oil!
 
The few heads I've seen are always clean. The top surface of the piston, on the other hand...

Anywayz, I checked the throttle for free movement, and even moved the cable around in an attempt to "bind" the cable. No sign of snagging.

I opened up the airbox and checked out the seal on the (Moose) air filter. It looked tight from the outside as I attempted to lift the rim while the filter frame was still screwed down.

However, the black rubber ring backing had separated on the top right corner (opposite the tear-drop corner). It was folded on itself. My guess is that this is where the dust came through.

I am very careful (paranoid) when I install the filter. I take off the seat and both panels and check it all around. I don't know if this happened during installation or during the race. I even use grease on the seal, even though Moose says it's not needed.

I continued my trek along the intake system. I removed the sub frame to get good looks at the airbox and carb. I found no dust/sand inside the boot which surprised me. But I did see a thin streak of brown on the intake side of the venturi. I removed the slide and found slight scuff marks that rubbed away with my finger. No dirt was evident in the float bowl.

Bummer.
 
The sand jamming the needle can be intermittant, and when the throttle is open far enough it can be eaisly drawn through the nozzle/main jet and you never know what happened. It doesn't take much.

Reroute the vents with filters and finnish going through the system. Get the longer throttle cable from GoFasters. Check for air leaks with starting fluid, including the area under the ignition cover.
 
Going through the bike again.

I replaced the throttle cable with a MotionPro unit (same length, it appears), which the former owner tossed in a box for me. New pads front and rear (in the box as well).

Re-jetted to 178/45 for Phoenix "winters."

Going to the track this weekend.
 
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