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Hanging idle going to stall, NOT carbs. Need ideas.

B

bobgroger

Forum Sage
Past Site Supporter
I will try to make this long story as short as possible without leaving out important info. '81 GS1100e. I have put about 3000 miles on it, has always had minor fueling issues but it would idle and run OK, a bit rich at full throttle and crappy (30-35) mileage. Problem started when I violated the "Change one thing at a time" rule. List of changes:
Removed 4-1 and put stock exhaust back on with new gaskets.
Removed K&N filter and drilled airbox cover, replaced with stock.
Adjusted valves.
Changed plugs, replaced caps with correct NGK.
Set static and dynamic timing (Dyna S with 3 ohm dyna coils).
Cleaned carbs properly, dip & ultrasonic, o-rings, replaced 115 jets with correct for '81 107.5.
Verified proper and clean pilot and air corrector jets, float levels, bench and carbtune sync perfect at 2500.
Problem:
Starts OK, come off choke and idles OK until it warms up. The hotter it gets the more hangy the idle gets. It will idle at 2200 all day and run fine from 2000-9500RPM. The idle will eventually settle down and keep going down until it stalls. When it stalls it starts right back up, but stutters and misfires until 2000 again. The hotter it gets (96 today) the more it stalls, maybe I have just not noticed before as it hasn't been this hot. I have had the carbs off 3 times. Finally put a set of carbs off a perfectly running '83. Started and idled OK, by the end of a 20 mile ride exactly the same symptoms. Have also tried running on prime. I am running in circles here, need some fresh ideas. The more I work on it, the worse it gets.
Already checked: Everything. Well, obviously not and maybe I am overthinking this. but it is not a common problem or maybe I am screwing up the same thing over and over. Things I am going to try next: Put K&N back in, check around coils after dark for arcing. Any other ideas? I was thinking Dyna module, but timing mark is steady and fires all plugs with timing light attached. When it stalls it sounds like the motor is trying to turn backwards as it dies and it blows the #4 hose off the carbtune as the vacuum goes to 0. Runs WAY too good to be cam timing. Carb boots look OK but I am going to swap them, airbox boots OK as well although one is missing the metal ring that secures it. Too hot to work on it again until morning, hopefully I will have some fresh ideas....
 
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Have you changed your Boot O-rings or boots? It's a common place for vacuum leaks.
 
OldVet66 said:
Have you changed your Boot O-rings or boots? It's a common place for vacuum leaks.
Click to expand...
No o-rings on 16-valve motors. I have not changed them but swapping with the bike I borrowed the carbs from is on the list. They are very pliable.
 
How many turns open are the pilot screws?
 
One thing to do is lube your mechanical spark advance unit. If that doesn't do it I'd start looking for vacuum leaks. Maybe replace the rubber carb runners even though the old ones seem okay. There could be a crack somewhere you don't recognize. Also check the petcock vacuum line and even the petcock diaphragm. A hanging idle is more often than not the result of a vacuum leak.
 
Maybe you over-adjusted a valve rocker, so the valve don't quite shut?
Or a poorly seated valve?
Check the compression after the engine is hot..
 
Sorry I can not offer any advice on this subject. I am following along to find the cause of your hanging idle as I am fighting the same thing. Have just gone through my carbs again looking for the cause but as yet I have not found any trouble spots.
Do keep us posted when you learn what it will take to fix yours.
Thanks
 
I would again double check for leaks.

but also I just had a similar issue and it ended up being a bad spark sender in the dyna s. So it it would start up somewhat crappy. But it would end up dying because one of the lobes ended up being faulty. I would give the dyna s a test
 
Almost every time I have issues with a bike that has a dyna s on it, it's the dyna s that is failing. I've bought more than one that was faulty out of the box. I no longer use them, to me they aren't worth the headache.
 
The OP sounds like he knows what he is doing but there is another thing that isn't mentioned. Most hanging idle issues are dirty pilot circuits or pilot jets. I always change the pilot jets as I don't care to attempt cleaning with a little wire or something of the like. Also, if the throttle cable is adjusted too tight, the idle can hang. You should be able to take your finger and deflect the cable where it is between the cable holding bracket and the place where the barrel inserts into the bellcrank and deflect the cable a bit... in other words it should have a bit of slack. Easy to check.
 
All great suggestions, I am going to eliminate as many as I can before it gets too hot today. Also learned the pros use propane to hunt down vacuum leaks so I am going to wave my torch around the carbs and see if the idle improves. Also going to hook the carbtune back up and see if one cylinder is markedly different with the known good carbs, that may help isolate the bad cylinder. Might also run to Radio Shack and get a can of freeze spray so I can cool down the Dyna modules when it acts up. I have to remember owning old bikes is an adventure....
 
DohcBikes said:
Almost every time I have issues with a bike that has a dyna s on it, it's the dyna s that is failing. I've bought more than one that was faulty out of the box. I no longer use them, to me they aren't worth the headache.
Click to expand...

What are you buying as a replacement to Dyna?
 
What about that missing carb boot clamp?
Seems like air could get around that?
Maybe try the stock ignitor/mechanical advance on the 83 bike and see what happens.
Hopefully the Dyna rotor magnet doesn't look like this
 
I now know a lot more about what it is NOT. Still looking for the cause but I need to take a couple days off to clear my mind. I am pretty much down to Dyna or jumped cam timing, although it runs so well above 2k. I have a Dyna on another bike, so that may be next.
 
I wanted to update this in case someone stumbles on it in the future. It WAS the carbs, and I feel like an idiot. However, it was NOT too lean, it was too rich. I had that fleeting thought early on, but dismissed it. It ran fine with the 4-1, the stock exhaust has more backpressure = richer. If the idle circuit is too rich, you have to increase the idle speed to keep it running, which uncovers more of the 3 ports under the butterfly. Once you do that the mixture screw really doesn't have much effect. I pulled the plugs after idling for a while, #1 was black and 2-4 were darker than normal. Looked at #1 and found the mixture screw when turned in was stopping on boogered threads instead of bottoming out. I carefully coaxed it in another 3 turns or so. I also changed the pilot air jets from 170 to the correct 160. Turned the mixture screws out 4 turns and slightly better, but not right. Turned the screws in gradually to about 2 turns which sped the idle up enough to turn down the main idle screw to ~1200 and it is steady as a rock. This may or may not be factually correct, but it explains it pretty well: http://www.mychinamoto.com/forums/showthread.php?344-CV-carburettors-They-re-different . Moral of the story, hanging idle with CV carbs can be too rich as well as too lean. A WB AFR meter is on my list.

Glad to have this finally sorted, a great learning experience and the new owner picks it up Sunday. But he is bringing another GS trade.....
 
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