Idle too high, Suzuki GS405

[dbursell]

Hello

I have read the similar threads but haven't found a solution that works for my issue.

I have a Suzuki GS450EX from 1981 with Mikuni BS34 Carbs. The problem is that the idle is too high.
I have had the Carb dismantled, cleaned and all the o-rings replaced. I have tried adjusting the idling
screw (the big one between the carbs) but it is in the bottom position. I have made sure there is slack
on the throttle cable on the carb side, so there is no hang up. I have sprayed the intake boots with start gas
and I cant hear any difference in RPM, when done so.

Help me out guys. Does the idle mixture adjustment screw on the carb play any part in this? I just put it
back as closely to the same position as I had it before I dismantled the carb. However the bike was not
functioning when I bought it so I don't know if that is correctly adjusted to begin with.

Best Regards

David

 

[Nessism]

Have you removed the intake boots, inspected them carefully for cracks, and replaced the O-rings? Is the bike stock, airbox and exhaust, and if not have the carbs been properly rejetted? If the bike is stock are the airbox boots on the backside of the carbs sealing well and is the airfilter clean and well oiled?

You may want to check the newbie mistake link in my signature to see if anything applies. Better to understand the common problems up front and take care of them before problems occur.

 

[Gorminrider]

depends what you mean by "too high" an rpm ? I am supposing its 1800+?

but No, I don't think you can get the idle mix to make a huge difference in rpm...it has it's own air passage and is limited as to its effect, when the butterfly throttle is so nearly closed.

When you took carbs apart, you of course bench-synched "butterfly"throttle plates to be very near closed? If not, can still adjust these with carbs installed...see carb-synch info...
Given your info above, throttle closing, no air leaks, just an idea would be: to check that the enricher/"choke" is functioning correctly. This can cause a high idle if the little pistons are not dropping to shut the passage off... Perhaps the tangs that are supposed to operate these up and down are not in place?

 

[sam000lee]

Have you checked the choke mechanism/plunger? Could be stuck on somehow.
Is the airbox attached?
I think this was just a typo but `bottom' position in the idle screw to me sounds like it's all the way in opening the plate. All the way out so that it is not making contact with the throttle cable retainer would be the `lowest' setting.

 

[dbursell]

Have you checked the choke mechanism/plunger? Could be stuck on somehow.
Is the airbox attached?
I think this was just a typo but `bottom' position in the idle screw to me sounds like it's all the way in opening the plate. All the way out so that it is not making contact with the throttle cable retainer would be the `lowest' setting.

Yes this was a typo. I'll check the choke again.

 

[dbursell]

depends what you mean by "too high" an rpm ? I am supposing its 1800+?

but No, I don't think you can get the idle mix to make a huge difference in rpm...it has it's own air passage and is limited as to its effect, when the butterfly throttle is so nearly closed.

When you took carbs apart, you of course bench-synched "butterfly"throttle plates to be very near closed? If not, can still adjust these with carbs installed...see carb-synch info...
Given your info above, throttle closing, no air leaks, just an idea would be: to check that the enricher/"choke" is functioning correctly. This can cause a high idle if the little pistons are not dropping to shut the passage off... Perhaps the tangs that are supposed to operate these up and down are not in place?

Rpm is about 3000. I bench synced the carbs after putting them together as well and they were as closed as possible. I'll check the choke again to see if there is anything wrong.

 

[dbursell]

Have you removed the intake boots, inspected them carefully for cracks, and replaced the O-rings? Is the bike stock, airbox and exhaust, and if not have the carbs been properly rejetted? If the bike is stock are the airbox boots on the backside of the carbs sealing well and is the airfilter clean and well oiled?

You may want to check the newbie mistake link in my signature to see if anything applies. Better to understand the common problems up front and take care of them before problems occur.

I have removed them and they are still nice soft in the material. I have replaced the o-rings as well. The airbox is stock and the air filter has been replaced. The exhaust is on the other hand, not stock. Could that make the difference? I thought it only mattered what happened before and in the carb. The carb has not been rejetted after that. Is there a good procedure to follow for that?

 

[Nessism]

How far open are the pilot screws and did you remove them when you dipped the carbs? What kind of exhaust is installed? That normally doesn't cause an idle like you say but it can be a factor. Also, you need to vacuum sync the carbs in order to get a proper idle.

 

[dbursell]

How far open are the pilot screws and did you remove them when you dipped the carbs? What kind of exhaust is installed? That normally doesn't cause an idle like you say but it can be a factor. Also, you need to vacuum sync the carbs in order to get a proper idle.

The pilot screw, if i remember correctly is 1,75 turns out. The exhaust is custom with very short pipes. I know I have only done a bench sync but shouldn't I be able to get a somewhat good result out of just a plain bench sync. At least better than what I am getting.

 

[Nessism]

Set the pilot screws to 3 turns out. Short pipes like you mention are typically very loud and annoying. Please tell us you are not one of those guys that only think about themselves and go around spewing noise everywhere.

 

[dbursell]

Set the pilot screws to 3 turns out. Short pipes like you mention are typically very loud and annoying. Please tell us you are not one of those guys that only think about themselves and go around spewing noise everywhere.

I'll give it a shot. Short pipes, efficient muffler

 

[Gorminrider]

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....

The pilot screw, if i remember correctly is 1,75 turns out. The exhaust is custom with very short pipes. I know I have only done a bench sync but shouldn't I be able to get a somewhat good result out of just a plain bench sync. At least better than what I am getting.

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.

 

[Guest]

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...

 

[Gorminrider]

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/_foru...hat-the-hell-did-I-do!!&p=2410615#post2410615

 

[dbursell]

Set the pilot screws to 3 turns out. Short pipes like you mention are typically very loud and annoying. Please tell us you are not one of those guys that only think about themselves and go around spewing noise everywhere.

I tried setting them to 3 turns out and initially it went fine. Once it started up the idle was lower than 1000rpm and the engine died from time to time. So then I tried to adjust the idle screw to give it a slightly higher rpm. Then i couldnt get the thing started again, my guess is that it went sour. After cleaning the sparkplugs i found on one side it was dry and the other a little wet, normal? When I got i running again it was up to 3700rpm in idle. Advice?

 

[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/_foru...hat-the-hell-did-I-do!!&p=2410615#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.