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Hunting Idle

  • Thread starter Thread starter Tom MLC
  • Start date Start date
T

Tom MLC

Guest
The idle on my '80 GS1100 hunts between 1000 - 1500 rpm.

What I did: I pulled my carbs because I was having a fuel supply challenge. A PO had spliced the fuel line, and he used a soft tube which now easily kinks at the splice. So naturally I figure the fuel supply is getting choked off. I replaced the fuel and vacuum lines with a spiffy looking TYGON tubing rated for gasoline & temps up to 140 degrees. The lines are yellow & transparent. More on this later.

While the carbs were off I replaced all the phillips head screws on the diaphram covers and float bowls with SS allen head bolts. I replaced them one at a time, and did not replace any gaskets. Risky, I know.

After re-installing the carbs and doing a sync, I have this challenge at idle. The rpm goes up...the rpm goes down... I know you will say this is a classic sign of an air leak. I find none using carb cleaner to spray all over the boots. There seems to be no reaction to the spray. The idle still hunts the same way.

I now notice there is fuel in the vacuum line from the petcock (not a lot). Remember it is transparent. I'm thinking there should be no fuel in this line. But because the line is traditionally black, maybe I just never would have noticed it. There is also a large air bubble in the fuel supply line. I can see both these lines react to the increase in RPM when I pull the throttle.

Air in the fuel line would be a problem IMO. Fuel in the vacuum line would be a problem IMO. Sounds to me like I have a problem in my petcock. Maybe that was my problem originally. My petcock has no valve on it. I cannot select prime (that would help get rid of the air bubble) or reserve. I've never seen a petcock without a manual valve before.

Any advice would be appreciated. I'm planning to take this bike on a long road trip. The last run before winter. I hope I can nail this down quickly.
 
I'm pretty sure you should not have any fuel in the vacuum line. Sounds like an internal leak. Obvious answer is change the petcock to a proper one with a lever. Not sure what to recommend regarding patching things together for your trip.
 
Me Too!

Me Too!

Hey Tom,

I have the littler version of your bike - same colour scheme, but a touch older, the 400 engine and a round eyeball in the front.

I got back from a road trip a few weeks ago. The carbs started acting up in the last 50 km, just as you describe. Since I got home, I took them off and gave them a pretty good scrub. They were actually pretty nice looking so I went in as deep as the needles and not much further. I blew out all the carb cleaner with air and am pretty satisfied that the carbs were not the problem (but now I know that my carbs are spotless).

They have now been re-installed and they are acting exactly like yours.The only thing to note here is that I filled the carbs with gas before installing the tank. When it started up, a bunch of gas leaked out the carbs. I assumed that this was normal (vacuum cycle getting started up, too much gas in the carbs...) Once warm (at idle), the bike revs up to about 3000 rpm, then back down to 1200. It takes about 10 seconds for this to happen and then it repeats.

I am also twisting the idle adjustment often to keep it from dropping to low at stoplights (thank goodness for the electric start).

Like you, I am wondering about the petcock and/or air in the vacuum somewhere. Alas, it's dark now and I wanna go riding tomorrow, so I am hoping to borrow my brother's 91 gs650.

If anyone can diagnose this problem, I'd be tempted to post a smiley face.
 
Hey Tommy, Rob here.:) I was thinking if you pinch off the vacuum line and see if there is a difference you may be able to see if it's the vacuum line or not. I'm thinking there has to be a minute or two of idleing time worth of gas in the float bowls. Just a thought. :roll:
 
Nessism said:
I'm pretty sure you should not have any fuel in the vacuum line. Sounds like an internal leak. Obvious answer is change the petcock to a proper one with a lever. Not sure what to recommend regarding patching things together for your trip.
Click to expand...
It does seem like the petcock is an obvious cause of the original symptom, and this new one. I think I'll disect it and see what I find.

By the way, this is the proper petcock. Some models did not have a manual valve handle. The place where there would be a valve handle is covered over, or filled in. I think if I dig this filler out I will find a small screw head. I have heard of this, I just don't recall ever owning a bike with this setup before.

Thanks for the tip.
 
RobGS850L said:
Hey Tommy, Rob here.:) I was thinking if you pinch off the vacuum line and see if there is a difference you may be able to see if it's the vacuum line or not. I'm thinking there has to be a minute or two of idleing time worth of gas in the float bowls. Just a thought. :roll:
Click to expand...
Rob, that sounds like good troubleshooting. I gotta think a bit of liquid in the vacuum line is going to influence the amount of vacuum against the petcock diaphram. I'll try your trick tomorrow. I'm also gonna go for a short trip to see if my original symptom is still there. Thanks pal!
 
Well I figured the easiest way to troubleshoot this is with swapology. I put the tank from my 1000 on the 1100. Doesn't fit exactly, of course. But the purpose was to use a different petcock. Sure enough, there is no fuel in the vacuum line and the idle is steady.

So I'm looking to rebuild or replace the petcock on my big bike. I already ordered a new one, and am curious to see if It comes with or without a manual valve. The parts breakdown at bikebandit.com shows both. I also disected the old petcock. The diaphram and o-ring looks okay to me. No tears that I can see.

I'm confident this will do the trick. Thanks for the tips guys!
 
Sounds like you may have found the problem. Hope so.
If I had reached you in time I would've tried to talk you into a Pingel valve. I think they're a good investment if you plan to keep the bike.
Any fuel in the vacuum line means the diaphragm is failing. A large bubble in the fuel line is common, sometimes even if running. Depends if the bike is on the sidestand, the angle/bend of the fuel line, if it's being revved, etc.
 
KEITH KRAUSE said:
Sounds like you may have found the problem. Hope so.
If I had reached you in time I would've tried to talk you into a Pingel valve. I think they're a good investment if you plan to keep the bike.
Any fuel in the vacuum line means the diaphragm is failing. A large bubble in the fuel line is common, sometimes even if running. Depends if the bike is on the sidestand, the angle/bend of the fuel line, if it's being revved, etc.
Click to expand...
Keith, thanks for the tip. I checked out the Pingle website. Prices are not unreasonable for a quality looking product. I'll keep it in mind for the next time.

If I read it correctly, for our bikes there is only one hose fitting. So they're not vacuum actuated valves? I guess the vacuum line gets plugged off. Do I got that right?
 
Tom MLC said:
Keith, thanks for the tip. I checked out the Pingle website. Prices are not unreasonable for a quality looking product. I'll keep it in mind for the next time.

If I read it correctly, for our bikes there is only one hose fitting. So they're not vacuum actuated valves? I guess the vacuum line gets plugged off. Do I got that right?
Click to expand...
Right. They're gravity feed. Just cap your vacuum intake at the carb.
 
And don't forget to turn the valve off when you kill it! Now you'll know how all those old harley riders feel.....\\:D/
 
petcock

petcock

that was the trouble on mine too.:-|

Up and running again. Vroooooooom!
 
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