Bike over-reving...

[hikermikem]

What would make a bike suddenly rev up to the redline (and beyond)??

My son just called me to tell me he was riding his 650GZ when it out-of-the-blue went crazy; meaning it started reving. He hit the kill switch and pulled over, but every time he re-started the engine immediately took off again.

I walked him through checking the throttle cable & throttle linkage for snags, but all that seemed ok - nothing was stuck.

He let the bike cool down & it started normally & away he went again!!

My question is, why did trhat happen? would a stuck slide make it rev like that?? Perhaps a small piece of smegulia caused the slide to be stuck up??

The carbs have been dip-cleaned & rebuilt last year, & I did raise the needles a few weeks ago - everything worked smooth then.

What do you guys think??

Mik

[Dave8338]

What would make a bike suddenly rev up to the redline (and beyond)??

My question is, why did trhat happen? would a stuck slide make it rev like that?? Perhaps a small piece of smegulia caused the slide to be stuck up??



Mik

That would be my guess. If the cable is free, there is little else it could be. Good heads-up by the son, on the kill switch!

[Fast Doc]

First don't ride until it's figured out. My experience is that when things 'fix' themselves they are fixed temporarily.

It still sounds like a throttle cable issue or perhaps the linkage. Something is hanging up.

[Skyboy8950]

I agree. Even if a slide was stuck open, the butterfly is closed. I would say sticky cable.

[morpheus666]

Was he turning when the crazy revving happened?

[chef1366]

A main jet might of come out into the float bowl.
I would check throttle cable and stuck slide first.
Also intake and airbox boot hose clamps sometimes get caught in the throttle linkage.

[hikermikem]

the throttle cable and/or linkage checked out fine - I'm leaning towards a stuck slide. Kinda hard to trouble shoot unless I look when it's stuck again i suppose.

at the moment, all is fine..

thanks for the suggestions

[focus frenzy]

did you ask who or what he was racing? for a throttle to stick wide open, it has to be twisted wide open first!!!

gunk build up on the shaft is possible, but just as likely the linkage could have hung on a intake side clamp that is positioned wrong.

some GS carbs sync adjustment brackets between the carbs can catch on the clamp screw if they are in just the right spot.

[Pacman]

If you have the trans in neutral and put the throttle and the physical 1/2 mark you will likely be hitting or close to hitting red line. It isn't necessary for the throttle to be at full twist for the motor to hit red line unless in gear and moving.

The carbs on this bike are CV I believe. Therefore, there has a butterfly valve and that butterfly is what the throttle operates. When the butterfly is opened it pulls a vacuum on the little hole at the bottom of the slide which pulls air from the top side of the slide and that pulls the slide up. On this carb the slide going up just provides more gas but not more air without the butterfly being open. That means that even if the slide was stuck up all that would happen is that it would run very rich on that carb but would not rev much higher than normal. In fact it would likely kill the motor if attempting to idle.

The way these carbs are synch'ed is that each butterfly can open sort-of individually and there are screws that adjust the relationship of each butterfly to the others. This is set up so that the throttle can open all but each can be held open without necessarily ALL of them being open. That means if you adjust the screw for #2 to open it more then that also opens #1 more because they are connected.

I think what happened is that the butterfly valve stuck open on one or more carbs and that caused the bike to rev high. (It wouldn't have to be stuck wide open as just 1/2 would be enough). This would also still allow normal operation of the throttle cable on the carbs because the cable is not a direct link to the butterflies.

I suggest you pull the carbs and check for smooth operation of all the butterfly valves individually and I think you will find at least one has a hitch in it's movement. You will probably need to pull the carbs from the backing plate to check this properly because when you open on butterfly the linkage will open some of the others depending on how you do it and it will be difficult to tell if one carb's butterfly is binding up.

Chris

[robertbarr]

I'd guess carb sync. Once the bike warms up, the effect of carbs out of sync can easily cause the engine to rev way up, especially when the 'idle' has been set higher than it would need to be if the carbs were synchronized.

I suspect that raising the needles just compounded the out-of-sync condition.

Once you examine the linkage for obvious causes, I'd sync the carbs. For that matter, after raising the needles, I'd sync the carbs anyway.