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

[Guest]

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?

[gsrick]

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

[cp___32]

photo 1.jpgAs 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]

photo 2.jpgI 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.

[gsrick]

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.

[gsrick]

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.

[hjfisk]

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]

[ATTACH=CONFIG]45744[/ATTACH]As 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.

.