Idle too high, Suzuki GS405

[dbursell]

I don't know if this would do it, but could the mechanical advance of the ignition be stuck on full advance? Usually they rust at retard, but maybe the springs aren't enough to return it.

Easy to check also. Good time to oil it anyway...

I am not quite sure I understand?

[dbursell]

for all of this you might want to revisit the cable adjustment...slack off every cable adjustment entirely be sure the knurled nut is indeed allowing your throttle plates to close. Take the tank off to really see.
oh, and this from a parallel thread per the synch ports and the petcock's vacuum port....
http://www.thegsresources.com/_forum...15#post2410615

Checked the slack several times and i can see that is not close to being pulled.

[dbursell]

3000 rpm is pretty substantial! I can't see you getting much joy from turning idlemix in and out......1 3/4 turns is maybe a little lean on idle mix screws, but it varies a fair amount bike to bike. idlemix adjustments are small and proportionate to the main throttle. The wider the mains are open, the less the proportionate effect this little passage has.
Does it make a noticeable effect turning them in and out? (why I like twins! much harder to notice on 4 cylinder bikes!) I would think NOT given 3000 rpm.... Just a thought, if you have the needle assembled wrong per tiny o-ring,tiny washer, and tiny spring...OR if one or both idle passages are NOT clean and again, you are compensating by having to open throttle butterflies..

Without mufflers....maybe not so much the lack of mufflers as the length of pipe? Something i can't help with... On the occasions I've run the bike by accident without a muffler, or tried out a set of short Screamin Eagles, the bike backfired on deceleration a fair amount but rpm didn't go up. the opposite, I think I recall...

like air-leaks at carb boots, one thing that can cause a high idle is advanced timing. Someone is always going to remember this, -my turn today! You might want to have a look under the small cover-right side and the signal coil assembly to see if the centrifugal weights are not stuck. usually they are ok and very reliable, but since this bike was not happening when you bought it, you pretty much have to go through all the motions to be sure....





well, sure. being able to close the butterflies together is going to be important. but Synching is just a tweak for small differences and if the bike needs huge differences between butterflies to run smoother, it's a clue as to other issues per that ONE maladjusted side's piston,carb,exhaust etc.

You said you could see slack at the cable, and that you have the BigKnurled screw adjusted to minimum...Closing the butterflies entirely or nearly should still get you into a place where you are tweaking the idle mix jet within a 150 (?) rpm or so together, but if you have to have the butterflies open wider to keep the bike running, well, then it's going to be harder to do and again, is compensating for the Real Problem....


You can see why a couple of us are thinking about the choke/enricher right away, because it can give so easily give a pretty high idle...and it's easy to check.

So I gave it another try and adjusted the idle mix adjustment screw out to 3 turns. Initially when I started it it was hard to start but as it started up the idle was lower than 1000rpm, even too low to keep it running. After dying on me a few times I couldnt get it started again. So I cleaned the spark plugs out and gave it another go. After a while it started up again with the choke but all of a sudden the rpm was back at 3500 rpm. Maybe it could still be the choke plungers?? But visually I can see that the mechanism goes up and down. However is there another part inside that can be stuck? Do i need to disassemble it again to inspect?

[Nessism]

It's normal for the bike to idle at 3500 rpm when the choke is on. Once the bike is warmed up turn off the choke and the bike should settle down to a normal idle speed. Once you get to that point then try tweaking the pilot screws until you achieve the highest idle speed. It should be within 3/4 of a turn from 3 turns open or so. If you are getting 3500 rpm when the engine is warmed up then either the idle speed is set way too high or you have a vacuum leak somewhere causing the mixture to be too lean.

[dbursell]

It's normal for the bike to idle at 3500 rpm when the choke is on. Once the bike is warmed up turn off the choke and the bike should settle down to a normal idle speed. Once you get to that point then try tweaking the pilot screws until you achieve the highest idle speed. It should be within 3/4 of a turn from 3 turns open or so. If you are getting 3500 rpm when the engine is warmed up then either the idle speed is set way too high or you have a vacuum leak somewhere causing the mixture to be too lean.

It is idling at 3500rpm WITHOUT any choke. The big knurled idle adjustment screw isnt even touching. Have checked for vacuum leaks several times but haven't found any, seems to be tight.

[Nessism]

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.

[dbursell]

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.

Alright, back at it again. Any advice on dealing with leakage, should I find it?

[Gorminrider]

However is there another part inside that can be stuck? Do i need to disassemble it again to inspect?

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?

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.

[dbursell]

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?

[dbursell]

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

[Nessism]

Is there a throttle stop screw keeping the throttle plate from closing all the way or is the butterfly binding in the carb bore?

[dbursell]

Is there a throttle stop screw keeping the throttle plate from closing all the way or is the butterfly binding in the carb bore?

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.

[Nessism]

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.

[dbursell]

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.

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.

[Nessism]

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.