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GSX400F hanging idle issues, losing the plot

  • Thread starter Thread starter Kayak23
  • Start date Start date
K

Kayak23

Guest
Hello all.

Got myself this here motorcycle...

Untitled by Kayak23, on Flickr
....a 1982 Suzuki Gsx400f (not even sure I'm allowed on this forum with my extra letters after GS).
It's been about fifteen years since I last had a bike and I'm a little rusty, as is the bike
icon_biggrin.gif


Anyway, I'm having some running issues and hoped this place might suggest a few avenues to try.
I'm having charging issues but I reckon I can figure that one out (tested reg/rec, seems fine, not sure about alt)

The main issue I'm having and can't seem to suss is a 'hanging idle', in other words, the revs will often not drop and stay at or climb to about 5-6k and stay there until I nudge the clutch out in gear. It's obviously not ideal when you're trying to slow down and the bike wants to keep going.
It appears to be worse when the engine is hot.

The first thing I checked for was vacuum leaks. With the bike idling, I sprayed carb cleaner around the rubber boots to the engine. No change in revs.
I checked the throttle cable which seems to run very smooth and has a small amount of play at the carb end when under full lock either way so I don't think it's that.
Then two days ago I stripped and cleaned all the carbs, using carb cleaner, blowing out the jets with air and cleaning gently with a toothbrush. There was evidence of yellowing deposits in the float bowl but obviously I cleaned the whole shebang.

I inspected the diaphragms and the sliders. Diaphragms looked fine and although the sliders seemed to move up and down pretty freely, there was some degradation of the black coating on them, some of it coming away very easily under my thumb.
Untitled by Kayak23, on Flickr
When I reassembled the bike and started it, the hanging idle was briefly there, but after it settled it seemed good. I took it for a long run of about fifty miles and it seemed to be running great. I'd blip the throttle and the revs would stop right down again like they should. I even had engine braking!

So that was the day before yesterday. Yesterday I took it for a run again and almost immediately I had the hanging idle again.
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Again this morning I've started it up, and hit each carb boot in turn with carb cleaner and seemingly no change in revs.

Today I plan to take the sliders out and polish the bore to see if I can get them running any better.
I'm hesitant to touch the coating on them as it seems very fragile.

Saying that, they do push up and drop down fine. Putting a finger over the slot thing makes a vacuum and they drop slower too.

After reassembling the bike today following checking float height and polishing up the bores a little and wet and drying the slides very lightly to get rid of any flakiness, the hanging idle is back, and if anything, worse than before.

Checked for vac leaks again but it all seems fine.

Can anyone suggest what else I might try?
Bear in mind I've been away from bikes for a long time and I may not know what you're on about... :D
 
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Have you replaced the intake boots and O-rings yet? If they're old the rubber dries out won't seal well.
 
sam000lee said:
Have you replaced the intake boots and O-rings yet? If they're old the rubber dries out won't seal well.
Click to expand...


The intake boots are pretty new I think. They look it anyway.
Also, I've sprayed carb cleaner all around them one by one and not had any change in revs. Would this not suggest that an air leak isn't the problem?

I've not noticed any O-rings in there by the way. Just the intake boots which are a tight fit.
 
Here's a vid of the bike going mental.
You can see that I'm not touching the throttle at all. The choke makes the revs drop but taking it off makes it rocket up again..

 
I've never been real happy spraying stuff to check for leaks...
I'm looking at a parts fiche that suggests that you have carb boots bolted to cylinder head... but you tell me.


image.jpg
 
sam000lee said:
Are you saying that it is missing the O-rings?

I couldn't find the fiche for a 1982 GSX but for the '79 one, it definitely has boots AND O-rings: https://www.cmsnl.com/suzuki-gs400-...-e39_model13423/partslist/FIG-13.html#results
Click to expand...

Ah right. It may well have O-rings then, but they look to be part of the boot 'sandwich' that's bolted to the cylinder, in other words, not just sitting there when you take the carbs off.

I've not actually unbolted the boots and taken them off because they don't appear to be leaking.
Surely if the O-rings were leaking I'd get that change in revs happening when spraying the carb cleaner round there?
 
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I dont think that the 8 valve twins have orings between the boot and the cylinder head, it uses rubber coated boots like the other GSX's.
It is not the head pictured above, it has double the valves and rocker arms.


when the bike is idling high, toggle the kill switch off and on, if the idle returns to normal then hangs again when you blip the throttle then the idle circuits are plugged.
 
It's a 4 cylinder and I hope it has O-rings as I've just ordered some :D

I put it back together again today, checked and double checked all the rubber bits, disconnected the throttle and started it up.

I got it to idle and then went nuts with the carb spray. I found that one of the boots I was definitely getting a slowing of revs when I directed the spray behind the boot where the O-ring would be.

So, I'm hoping that changing the O-rings will cure it. Fingers crossed. I would have ordered rubbers too but they were ?30 each and to be honest I reckon the ones on there are spot on. It's more the O-ring perhaps leaking than the rubber.

Strangely though I've just taken it for about forty miles and it totally behaved with virtually no hanging idle.

Weird ......but anyway, I'm really hoping the O-rings are the cause.
 
You might want to go through the Newbie Mistakes thread linked in my signature. Maintenance is good.

Oh, and plugged idle circuits won't cause a hanging idle.
 
Nessism said:
You might want to go through the Newbie Mistakes thread linked in my signature. Maintenance is good.

Oh, and plugged idle circuits won't cause a hanging idle.
Click to expand...

What newbie mistake did I make? I've only had the bike a week so can hardly be called out for lack of maintenance yet! :D
 
Kayak23 said:
What newbie mistake did I make? I've only had the bike a week so can hardly be called out for lack of maintenance yet! :D
Click to expand...

Did you read the thread? For one it calls out for replacing the intake boot O-rings.
 
Nessism said:
Did you read the thread? For one it calls out for replacing the intake boot O-rings.
Click to expand...

Uuhh yeah, but you know, until I actually found evidence that something might suggest they needed replacing I thought it better to keep an open mind..I've been trying the carb spray for a few days without any apparent change in revs.

I wasn't finding anything that would suggest they were leaking until today, and new O-rings were only suggested yesterday. Did you read the thread? :)

Hopefully it is an O-ring, but it seems strange that it's so intermittent. Fingers crossed they arrive soon and I can rule it out.
 
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Leaking intake boot O-rings are one of the most common of all GS issues. I change them first thing every time I get a new GS as preventative maintenance. Same thing for the carb O-rings. They rot and deteriorate, particularly the O-rings on the fuel transfer tubes. Did you take the carbs fully apart, ungang them? You can't replace the transfer tube O-rings otherwise.
 
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