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Idle too high, Suzuki GS405

  • Thread starter Thread starter dbursell
  • Start date Start date
Looks can be deceiving.

Some people spray stuff on the outside of the carbs looking for a change in idle to confirm vacuum leaks. This method works on gross leakage, but not smaller leaks.

I suggest removing the carb boots from the engine again and flexing them to see if there are any cracks that open up. Also make sure the airbox is sealed on the back side of the carbs and that no false air can enter the system other than through the normal airbox openings.
 
Nessism said:
Looks can be deceiving.

Some people spray stuff on the outside of the carbs looking for a change in idle to confirm vacuum leaks. This method works on gross leakage, but not smaller leaks.

I suggest removing the carb boots from the engine again and flexing them to see if there are any cracks that open up. Also make sure the airbox is sealed on the back side of the carbs and that no false air can enter the system other than through the normal airbox openings.
Click to expand...

Alright, back at it again. Any advice on dealing with leakage, should I find it?
 
However is there another part inside that can be stuck? Do i need to disassemble it again to inspect?
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here is a diagram from a suzuki shop manual. If you have dis-assembled it once, you'll know it's pretty simple. Maybe it's possible it's not doing it's job, but in my experience, if you are getting "effect" by operating it, it's ok. IE cold bike stumbles to very low idle when you take "choke " off....and can otherwise adjust rpm with it.

and yes, you should get a good result from a bench synch. They are "open" the merest sliver at idle so a large portion of the bikes fuel comes through idle circuit when idling.

still,( and I'm speculating because I haven't tweaked my carbs in an effort to make them NOT work properly )..Perhaps your main jet needles (controlled by "CV" function) are set too "high" I seem to recall these Mikunis have three positions on the needle ....

Really, if you are not sure you have everything at a close-to-factory baseline, people (like me!) are going to suggest a million and one things off the wall. Baseline is the surest way to deduce the changes that cause this. ..IE: Adding a piece of pipe to your short mufflers is a fast way to cross them off the list as cause.

You are already on the air-leaks trail with Ness.
Alright, back at it again. Any advice on dealing with leakage, should I find it?
Click to expand...
Apart from buying parts not really ...cracks on boots can be gooped as temporary but only applied outside...no-where it can be sucked into engine.
 

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Could a "wrong" ignition coil make it idle too high. I believe one of my coils might be from an old gsx400. Could this make any difference?
 
Ok, so I disassembled the carb again. The choke plunger seems to work fine. However I spotted something I didnt see before. On the left hand side carb the butterfly valve seems to brush against the housing which causes it to not close as tight as on the right hand carb. As you can see in the picture, this is the bottom position, in other words the most closed I can get it. This also causes the other carb to open up just as much when they are linked together. Could this be the cause of the high idle? Or should it be open that much? I'm not sure how closed it should be.

IMG_6770.jpg
 
Is there a throttle stop screw keeping the throttle plate from closing all the way or is the butterfly binding in the carb bore?
 
Nessism said:
Is there a throttle stop screw keeping the throttle plate from closing all the way or is the butterfly binding in the carb bore?
Click to expand...

The idle adjustment screw is completely disengaged and is not touching the throttle plate. The butterfly seems to bind in the carb housing. I think I could almost see some small metal shavings.
 
You may have to crack the butterfly screws loose a little, then exercise the throttle shaft until you get the butterfly shaft centered in the carb bore, then snug the screws back down.
 
Nessism said:
You may have to crack the butterfly screws loose a little, then exercise the throttle shaft until you get the butterfly shaft centered in the carb bore, then snug the screws back down.
Click to expand...

Thanks, i've done that now and reassembled. Gonna try and start it up tomorrow again and see if that works some magic. Let you know how it goes.
 
Set the sync screw so both butterflys are open the same amount when the throttle stop is set. That way the sync is close the first time you start up the bike.
 
dbursell said:
Ok, so I disassembled the carb again. The choke plunger seems to work fine. However I spotted something I didnt see before. On the left hand side carb the butterfly valve seems to brush against the housing which causes it to not close as tight as on the right hand carb. As you can see in the picture, this is the bottom position, in other words the most closed I can get it. This also causes the other carb to open up just as much when they are linked together. Could this be the cause of the high idle? Or should it be open that much? I'm not sure how closed it should be.

View attachment 52323
Click to expand...

Sounds like you found the problem. Maybe someone actually unscrewed the butterflys from the shaft which is a bad idea....

But to synch the plates, use a gauge wire.

IMO..the reason you use anything at all as a "gauge", to synchronise the butterflies opening is because there's no easy way to tell if they are both exactly closing at the same time and deciding on an opening sliver of light is not easy either...but using a "gauge" as a feeler allows you to adjust too large or too small until you get it very close ... ...it doesn't really matter exactly how thin the "gauge" wire is (something that stays stiff like piano wire is good ) because you will simply adjust idle from the main knob to get it right anyways.

On a twin, retesting synch later with vacuum gauges hopefully will not require big changes -or any at all. If it does indicate large differences, it's telling you there's a problem is somewhere else.
 
Okey, a little update. So everything seems to be working properly now and I can get the idle to where I want it to be. The problem was as I mentioned before a misaligned butterfly valve. It would seem as a previous owner has disassembled the valve and when putting it together haven't bothered to align it. The butterfly valve caught in the carb housing, leaving it slightly open, which in turn caused the other carb/side to push open as well. The gap was less than a millimeter but enough to make the idle go to above 3500rpm. You live and you learn. A huge thank you to all the guys here that have taken their time and helped me out with this problem and all the other problems i've had in other threads as well. Very much appreciated. Kudos!
 
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