Can't lower my idle - 1980 GSX400

[Guest]

I'm needing some of your wisdom once again. I have a 1980 GS400 / GSX400 that I am trying desperately to get on the road. The problem (one of the problems) is that I can't get my idle to go lower than 3500 - 4000.

Things I have done so far:
- Cleaned and bench synced the carbs
- Adjusted the valves
- replaced the vacuum petcock with a manual because I was getting fuel in the vacuum line
- capped the vacuum line
- replaced the carb manifold o-rings
- tried the idle adjustment screws from 1/2 turn out to 3 turns out
- fresh gas
- new air filter
- yelled every swear word I know

Any ideas?

[ukjules]

Are you sure you have exhausted every swear word ?
I know the feeling getting these bikes back in the road !!!

Possibilities :
Throttle cable jammed (happened to me)
Choke cable issue
Have you cleaned them carbs properly ? All jets ?
Right jets in the carbs ?
Air leaks around carbs , air filter and box standard ?

While you have the spanners out check timing.
Get everything right , plugs , leads ....
and then get back into the garage and do battle.

[Oldspooneye]

Possibilities :
Throttle cable jammed (happened to me)

I'm going to be replacing it this weekend if the dealership has one in stock.

Another thing I forgot to mention, when it's running and I pull the choke, the RPM drops to around 1800 for a few seconds then climbs back up. I'm not sure if that points to anything in particular.

[steve murdoch]

Throttle cable might not need replacing, it might just need rerouting.
An air leak can cause an erratic idle, not sure if it can cause an idle to climb.
Make sure the boots around the carbs are secure and/or not folded over.
Did you replace the manifold o-rings?

http://www.cycleorings.com/

[Oldspooneye]

The manifold o-rings have been changed. The throttle cable has a crack in the casing so I was going to replace it anyway. I can't see there being any air leaks I haven't addressed except for maybe around the airbox. Would that cause it to idle that high though?

[TxGSrider]

I know its not a popular idea around here, but you might try spraying carb cleaner around the carbs and boots with the motor running and see if the rpms change any. I suppose for the safety conscience folks, you could get a spray bottle full of water and use that instead (Ive never tried it, but it should work).

[Gorminrider]

guys above gave the checklist you need....maybe add:
Did you check the choke/enricher for proper operation...Its a pull knob on these (no cable) . Might be not closing or otherwise wrong-oh...your symptom may indicate...

[Sandy]

Are the sync screws too far in not allowing the butterflies to fully close?

[Oldspooneye]

I'm still having the problem. I just replaced the throttle cable because it wasn't snapping back like it should. Now it snaps back perfectly. I have a bit of slack on it so it's not causing the high revving. I started the bike and it ran awesome so I thought the problem was solved but once I took it out on the road I had very slow acceleration and then the revving climbed up to 4500 and I couldn't get it to go down.

Any other suggestions? I'm about at my wits' end.

[tom203]

"I took it out on the road I had very slow acceleration and then the revving climbed up to 4500 and I couldn't get it to go down."

At this point, I'd remove carb diaphragms, look for pinholes and make sure that the slanted hole thru diaphragm piston is on throttle side of main jet area.

"Another thing I forgot to mention, when it's running and I pull the choke, the RPM drops to around 1800 for a few seconds then climbs back up. I'm not sure if that points to anything in particular."

this seems real odd to me.. The choke enricher circuit gets its air supply thru underside of diaphragm

[John Park]

Is the mechanical advance free and working properly? Probably is, but it also probably hasn't been lubed in a decade or three.

These motors are known for a head cracking problem; shouldn't give you this symptom, but maybe check down the plug hole threads.

[1948man]

Did you adjust the idle screw (assuming that model has one?) Is it possible that when you put everything back together after cleaning the carbs you got something wrong so that there is no adjustment that will shut the butterflies enough to drop the idle.

[Oldspooneye]

I seemed to have solved my problem, although I'm not 100% what fixed it. It could have been one thing or it could have been a combination of things. I'll reluctantly list what I did in case it helps someone out even though it will show my foolish newbie mistakes.

1. Pulled out the carbs, checked for holes in the diaphragms, and cleaned and reinstalled the original jets. The new jets came with a carb kit so I installed them when I first cleaned the carb. I have no reason to think the new jets are bad but I figured since I'm in there and there was nothing wrong with the old ones I might as well put them back in.

2. I reset the idle speed screw so the butterflies were just barely closed. This is what I think caused my problem in the first place. My only other bike I've owned was only a couple years old. It rarely had any need for me to use the choke to start it. This bike is 37 years old and it needs the enricher to start. I had the idle screw set so that it was open enough to start without the enricher pulled out. Once the bike warmed up that caused it to rev super high.

3. tightened the hell out of everything when I reinstalled the carbs. This may have also been a contributing factor. I may have had an air leak somewhere.

4. replaced the petcock. The original one had fuel going into the vacuum line so I switched it to a manual.

5. replaced and rerouted the throttle cable. It was old and cracked and didn't snap back as it should.

So there you go. That's 2 problems down. 98 to go.

Thanks everyone for your help.