knocking noise and runaway throtle??

[Madmax]

I just bought this GS650GL and went through the carbs etc. When starting for the first time it fired up fine then there was a knocking noise that sounded like it came from the top end and the throttle took off on its own to aprox 5K rpm. I shut it down immediately. My thinking is a stuck or open valve that is creating vacuum to open the CV carbs. Would be interested in thoughts from others to know where to start looking.

[Big T]

Air leaks in the intake system

The knock was probably a lack of oil on start up

Make sure your idle screw is not set high and the throttle cables have a bit of slack


A stuck valve is incredibly rare. You did check and adjust your valve clearances to preclude a valve held open?

[Steve]

My thinking is a stuck or open valve that is creating vacuum to open the CV carbs. Would be interested in thoughts from others to know where to start looking.

Vacuum in the intake system will NOT lift the slides in the carbs.

The slides in the carbs do NOT make the engine run any faster.

The part in the carb that allows the engine to run faster is the butterfly plate at the exit of the carb. As it opens, it allows more air to flow through the carb. As more and more air flows under the slide, the pressure at the bottom of the slide is reduced. That reduction in pressure is transmitted through the hole in the slide to the area above the diaphragm, and THAT is what lifts the slides, not engine vacuum.

Try turning your idle screw out (counter-clockwise) about one turn and try again.

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[Madmax]

I bought this bike and rode it home. On the way the idle increased to high rpms. I have done some changes aesthetically to the bike but not to engine except for new O-rings on intake, rebuild carbs and add a MAC 4 to one. Idle screw just touches then 2-1/2 turns. Engine starts to idle at 900 rpm then climbs to 5k. knocking noise is in top end and never goes away.

[Steve]

I bought this bike and rode it home. On the way the idle increased to high rpms.

An engine that increases idle speed as it warms up is running very lean. Usually due to a leak between the carbs and the engine.
Replacing the intake o-rings should have cured that, but the intake boots themselves can also leak.

I have done some changes aesthetically to the bike but not to engine except for new O-rings on intake, rebuild carbs and add a MAC 4 to one.

How did you "rebuild the carbs"? Did you install aftermarket kits or did you do the full "strip and dip" and replace all the o-rings that is recommended here?

Did you re-jet the carbs for the pipe? Do you have stock airbox or pods, which would require further re-jetting?

Idle screw just touches then 2-1/2 turns.

I have never counted how many turns it takes to open the throttles to a proper idle speed, but 2 1/2 turns might be a bit too much.
And, ... when you say "idle screw", are you talking about the idle speed adjuster between carbs 2&3, or are you talking about the idle mixture adjustmenet screws on the top of each carb?

Engine starts to idle at 900 rpm then climbs to 5k.

Do you use the "choke" to start the engine? Starting at only 900 RPM does not seem right.

knocking noise is in top end and never goes away.

"Knocking" in the top end could be any of several things, depending on the actual sound of the "knock". Any chance you can post a video so we can hear it?
One possibility is that the valves need to be adjusted.
Another possibility is that the cam chain is not properly tensioned. This is actually quite common because too many 'previous owners' simply don't know how to properly install the tensioner.
A less-likely possibility is that there is damage to the top end, but being able to hear the noise will help us diagnose it a lot better.

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[tom203]

Madmax said........."I bought this bike and rode it home."
Did it make this noise on first ride home? As others said, very likely cam chain too loose and maybe slapping against one its guides. Pull cam cover and go exploring- you got to check valve clearances in any event.

What's the mileage?