Nearly at my wits end with this bike. Idle takes off like a rocket with the tank on.

[cp___32]

Hey All,

I was really hoping I'd be putting the seat and side covers on this thing and taking it for a safety soon but unfortunately another gremlin as popped up that is beyond my limited knowledge and I haven't found a solution to point me in the right direction using the search.

Cleaned the carbs (Ultrasonic, carb cleaner, set floats), new o-rings, new carb boots, airbox boots, filter (an OEM style foam one), petcock rebuild, all new electrics including a SSPB. I got the carbs synced up and the bike seemed to be running pretty well. Throttle response was decent and it held idle where I set it. I put the tank on, connected the vacuum tube, made sure I was flowing fuel by installing a temporary run of clear tubing. All seemed well, but with the tank on, it would struggle to start, wouldn't hold idle and the RPMs would take off suddenly.

So I took the tank off thinking I must have disturbed something while installing it. Resynced the carbs, double checked for air leaks and that was all good, put the tank on again and had the same issue. If I get the bike running without choke and set the idle to 1000 or so RPM, if I blip the throttle one of two things happen. Either the bike bogs and nearly dies, or the revs take off upwards of 4000. I dial back the throttle adjuster to get the RPM to fall back down and then the idle goes through the floor and it either lumps around at 500-600 and dies, or just dies right away. The adjuster is coming to rest on the lower stop when fully turned out, the throttle linkage is free and has a slight bit of slack before it picks up. I'm really at a loss for what to do next. I had set the timing for the Dyna-S way back so I hooked a timing light up in hopes I could dynamically tune it, but since it won't hold a consistent idle with the tank on it's a lost cause.

Where do I go next?

 

[Guest]

Are your throttle cables run correctly? Do you have one or two? Could you post some photos?

 

[cp___32]

As you can see, the cable has a bit of slack, when I twist the handle it quickly goes away. Throttle response was smooth and behaved as expected with the tank off. When it's released it snaps back right to the original position every time. There's nothing that seems to be hanging it up. When I turn the knob to adjust the resting position the RPMs increase or decrease as expected. With the tank on the bike can idle really low and then suddenly climb to redline without any throttle input, and when the throttle is turned the bike either bogs and stalls or the revs climb abnormally high and then hang long after the throttle has returned to the resting position. The only way to get them to come back down is by turning the idle screw till the butterflies close entirely.

 

[cp___32]

I also pulled all 4 plugs and they all look like this. They aren't very old (just had the sync and tinkering time on them. Maybe an hour tops). I could put brand new ones in I suppose but that doesn't explain why things seemed to work much better when the tank was off. Good news is it doesn't seem to be running lean.

 

[loud et]

Intermittently functioning petcock. Hmmm... That's a possibility.
Try running it with the gas caps off first. See if that helps. It could be struggling to draw fuel from a vent clog. (Not likely but possible)
If it's not that, the petcock is wonky and operating sporadically or you have a partially collapsed fuel line.
Just a few weirdo possibilities to help troubleshoot.

 

[Guest]

What I'm wondering is where the cables are routed. Is the tank pushing one of them down? Did you run new cables or are they the same ones as before? If new, did you route them exactly the same way? If you don't have a manual, here is the one from Bikecliffs webpage. On page 684 shows the proper routing of you throttle cables.
http://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS550_77-82_all.pdf

 

[Runeight]

Intermittently functioning petcock. Hmmm... That's a possibility.
Try running it with the gas caps off first. See if that helps. It could be struggling to draw fuel from a vent clog. (Not likely but possible)
.

Thats where I would go first.

 

[BigD_83]

How much fuel is in the tank, and when the tank was off, were you running the bike off an auxiliary fuel source, or the tank in a higher position than where it would sit on the frame?

 

[Runeight]

Thats where I would go first.

Although a vent issue would either work or not work I would think. Been thinking on this myself.

 

[Guest]

Intermittently functioning petcock. Hmmm... That's a possibility.
Try running it with the gas caps off first. See if that helps. It could be struggling to draw fuel from a vent clog. (Not likely but possible)
If it's not that, the petcock is wonky and operating sporadically or you have a partially collapsed fuel line.
Just a few weirdo possibilities to help troubleshoot.

I could see the bog coming from a bent or collapsing line or a weak feed from the petcock, but how would that make the rpms sky rocket? I've seen this with a cable that was routed in the fork wrong, where it would get pulled in a turn and make the rpm shoot up. The cable routing in the manual shows the cable should go over the right tank perch, but without seeing how it is routed in reality, it's still a guess. Also did you change the handlebars?

 

[dantodd]

Starting to sound like compound problems. Try running it off a remote tank again. If it runs well like that then put the tank back on but keep running off the remote tank. That should eliminate any kind of mechanical issue with the tank or cable. I've you have her running nicely on the remote tank with the real tank mounted then you should hook the tank back up and see if it is a fuel problem.

 

[Guest]

Sounds like a faulty petcock to me

 

[loud et]

I could see the bog coming from a bent or collapsing line or a weak feed from the petcock, but how would that make the rpms sky rocket? I've seen this with a cable that was routed in the fork wrong, where it would get pulled in a turn and make the rpm shoot up. The cable routing in the manual shows the cable should go over the right tank perch, but without seeing how it is routed in reality, it's still a guess. Also did you change the handlebars?

I was thinking if he has a low or struggling idle issue due to poor fuel flow from the petcock or a bent fuel line and he adjusts the idle up. .....then the petcock starts flowing better or the fuel line relaxes and either one starts filling the bowls then the increased fuel supply would make it run properly but having adjusted the idle up......it would seem like it's racing.
Basically like he is messing with the master idle and sporadic fuel flow making the situation hard to pin down what's happening.
Something like that. Lol

 

[gustovh]

I looked at your first picture, tighten the bottom nut on the throttle cable adjuster, it's loose. I bet your problem will go away.

V

 

[Steve]

View attachment 45744As you can see, the cable has a bit of slack, ...

What I see in that picture is a cable assembly that is not held tightly to the bracket. Note the angle between the cable and the bracket. Tighten one of the nuts to fix that. Can't say whether to tighten the upper one or the lower one, it depends on which one leaves the proper amount of slack.

I looked at your first picture, tighten the bottom nut on the throttle cable adjuster, it's loose. I bet your problem will go away.

you noticed that, too. :encouragement:

.

 

[uk gs nut]

Sucking air from the vacuum feed to the tank is my thoughts

 

[cp___32]

Thanks everyone!

I'll do a thorough inspection of the throttle cable to make sure it's all good and tighten it down at the adjuster. If that doesn't work I've got some spare fuel tubing around so I'll replace the vacuum line that operates the petcock. And if all else fails, I like the idea of leaving the tank in place and still running the aux fuel supply. If that fixes it then I know it's fuel delivery so I'll troubleshoot from there.

Gotta do the strut mount and front brakes on my car but hopefully I'll get to the GS on the weekend and give those things a try. I'll report back soon.

Thanks

 

[cp___32]

Tightened up the throttle cable, and decided to check the plug order. Turns out that I had 2 and 3 swapped so when I changed them to the correct configuration it ran better.
Just to see if it was a timing issue I decided to hook up my timing light and found something curious.

My bogging and random stalling as well as my idle issue might be spark related. I noticed that at idle the timing light was flashing really inconsistently and that matched exactly with the misfire/bogging I was experiencing. Then it would catch and flash as expected and the revs would take off. Then suddenly the spark would go inconsistent and the revs would drop again and it would idle rough. Sometimes the spark would stop entirely (at high RPM and idle) and the bike would die. The best I can explain it was the spark looked like Morse code.

The coils were new OEM replacements as were the plug wires. I'll look up how to test the coils to make sure they are good. I'm guessing I could have a grounding issue? It might also explain why the bike would crank no problem and then sometimes it would be completely dead as if the key wasn't on. Pressing the starter switch would not do anything but I could jump across the starter terminals and the bike would crank. That seems the most logical explanation, no? If I can get consistent spark I think it'll solve my problem. More late nights searching the forum for answers this weekend

 

[salty_monk]

Are you using the stock 80 Spark box? No mechanical advance? They have been known to be faulty..... The fix is to get a Dyna or similar & a mechanical advance (or run a Dyna 2000).

Also the revs could take off with a faulty petcock... if it runs lean the revs would rise at idle.

 

[cp___32]

Oh, I tried running the bike without the gas cap on. Forgot to mention that. I heard the petcock clacking away and when the bike would misfire I would notice the petcock would also not get a pulse. I'm assuming that's normal?

Yeah pretty much all the electrics except the wiring loom itself are new. SSPB, Dyna-S ignition, I can't recall the brand but the stator is new. Picked up one of the fancy R/Rs used on Polaris quads and R6s and such. OEM Coils like I mentioned, NGK plug caps. New Champion plugs. I probably should have sprung for the Green Dyna coils when I replaced them.