motor goes wild by starting

[Ronny]

Hi, i have a GS 550 from 1981,
when i start the bike he goes immediately to full rpm (throttle is closed)
to prevent damage (cold motor ) i switch of with the ignition key, so i have no idea how long this phenomenon will last...
has any one of you guys an idea or experience what could be wrong? false air?

thanks in advance for your input and wisdom

 

[tom203]

Your enricher- “choke” - system is probably stuck “on”. Maybe the idle stop screw is set wrong too. The enricher circuit has its own air supply to bypass throttle position

 

[Rob S.]

What you've described is perfectly normal if you're starting using the choke. As soon as it starts, back the choke off before it gets much above 4k rpms. You can let it warm up a bit at around 2k, then slowly ease off the choke.

 

[Ronny]

OK thanks, i will check it out

 

[KiwiAlfa156]

By full rpm, do you mean off the scale? Like 12,000 rpm?

 

[Ronny]

yes, he goes to max rpm

 

[chuck hahn]

Thats not normal RPMs from the choke. Choke may be around 4000ish. Are the cables run correctly? Have verified the throttle butterflies are freely moving? Have you turned the idle knob until it IS NOT toughing the linkage and again verifying the butterflies fully close? Have you lubed the linkage and you can pop the rubber dust caps up off the chokes and put a drop of oil on them then work them to get them lubed good. Does the throttle freely snap back when you twist and let go? Ends of the cables frayed causing them to hang up..especially inside the switch housing? Is the handlebar jamming into the rubber grip grom the inside..may need to move it out a 1/8 inch to be sure its free.
In any case i suspect smething is holding the vutterflies from closing.

 

[chuck hahn]

Tom.. the enrichers bypass throttle and sip gas up the choke tubes into the carbs. Air comes in normally from the front of the carbs.

 

[KiwiAlfa156]

yes, he goes to max rpm

OK.

As others have already posted, its likely that the carburetor butterfly plates are jammed either partially or completely open.

The throttle handle and cable pull on a quadrant that rotates a shaft that passes through the front of all four carbs, close to where the go into the manifold boots attached to the engine. Inside the bore of carb is plate (the butterfly valve). The position of this plate from closed to open, controls the amount of air the engine can take in. On a CV (Constant Velocity) carb such as the ones fitted to your 550, the throttle (and but extension - the rider) only controls the amount of air the engine can induct. The rest of the carb is designed to add the right amount of fuel for that air, whether your opening, closing, or steady, given the rpm and load on the engine at that moment.

At idle you want just enough air to keep engine ticking over. So essentially the butterflies are barely cracked open, literally fractions of a millimeter. The following diagram from a Suzuki manual, shows the fuel and air flow at idle. Note the position of the plate.

So something is most likely stopping the butterflies resting in this position with the throttle closed. Start at the throttle tube and work your way down the carbs. There are adjusters at both ends of the throttle cable. These should be adjusted to give about 1 mm of slack (side deflection on the exposed cable where it is mounted to the carb quadrant.

If you can't see anything obvious, you'll probably have to remove the carbs to inspect and clean. With the stock airbox and old hoses, it's a mission to get the carbs out and in, but it can be done, and is regularly done.
​​