Revs from hell -- stuck throttle or bad mojo?

[gaspipe]

Originally Posted by ramcclure200

I don't know what causes it to do that, air leak, stuck slide?

I've had that happen and almost always it's the crank seals going bad.

[cladoo]

I was going to say that you may have a leak at the base gasket or crank seals.

[GMP]

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.

[AZRickD]

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.

[dave948]

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.

[AZRickD]

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.

[iancp5]

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!

[AZRickD]

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.

[GMP]

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.

[AZRickD]

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.