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79 GS 1000E poor idle

  • Thread starter Thread starter cliffnote
  • Start date Start date
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cliffnote

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My bike runs great on the open road and responds well to the throttle. However, the bike idles poorly and will vary between 900 and 1500 RPMs when it is warm. I have rebuilt the petcock and installed transparent tubes for both the vacuum and fuel lines as well as installed an in-line filter. Since I can see fuel enter the carbs, it appears the petcock is working correctly. I have had the carbs off twice; once as a cleaning and a second time to replace the gaskets and float needles (only parts of the K&L kit that seemed to fit for my bike). There is a strong gas odor when I park the bike in the garage and when I completely fill the gas tank, I will get a leak out of the #2 carb float bowl hose until the bowl empties. Someone suggested that my #2 float may need adjusting but can this be done without removing the carbs yet again?
 
Have you adjusted the valves ? Replaced the intake boot O rings ?. Sealed the air box ? Done a carb sync ?
 
Hi,

The carbs have to come off the bike in order to adjust the float height.

To get your bike running correctly, READ THIS and take no shortcuts. Let us know what maintenance steps you have taken, i.e. Have you properly cleaned the carbs? Have you replaced the float needles and seats? Sealed the airbox? Replaced intake boot O-rings? And so on.

(CLICK HERE for the VM carb rebuild guide.)

Unless you're using a gravity feed part (like for a lawn mower), inline fuel filters can cause fuel starvation. Keep us informed.

Thank you for your indulgence,

BassCliff
 
cliffnote said:
My bike runs great on the open road and responds well to the throttle. However, the bike idles poorly and will vary between 900 and 1500 RPMs when it is warm. I have rebuilt the petcock and installed transparent tubes for both the vacuum and fuel lines as well as installed an in-line filter. Since I can see fuel enter the carbs, it appears the petcock is working correctly. I have had the carbs off twice; once as a cleaning and a second time to replace the gaskets and float needles (only parts of the K&L kit that seemed to fit for my bike). There is a strong gas odor when I park the bike in the garage and when I completely fill the gas tank, I will get a leak out of the #2 carb float bowl hose until the bowl empties. Someone suggested that my #2 float may need adjusting but can this be done without removing the carbs yet again?
Click to expand...
To check float height you must remove the carbs. The factory setting range is .90 to .98" for your model, I set them to .94 and that gives you that margin for error if you were a tad off. Check both sides of the float as they are sometimes tweaked (one higher than the other) before adjusting. If tweaked, gently straighten. You need verniers to accurately check. Remove the bowl gasket when measuring.
I can't vouch for K&L float needle valve quality. I recommend genuine Mikuni parts. I assume the K&L valves came with a new brass seat and a new seat gasket? All valves and seats wear as a unit and shouldn't be mixed. Valves can sometimes have a small burr on them. Dirt or varnishing (fuel build up) can interfere with the tip sealing. A valves spring can be weak.
After you know the fuel spilling problem is fixed and the floats are set correctly, re-check the idle.
If still running high, be sure the throttle cables are adjusted and lubed and routed correctly. Side to side movement of the bars shouldn't effect rpm. The throttle should snap back quickly. Your PULL cable should have about 1/8" of slack, max' and the RETURN cable zeroed out lightly. Lube the throttle pulley at the carbs too. If all is OK and you still have a high idle then it could be a vacuum leak, most common at the manifold o-rings and sometimes the manifolds themselves. If you're saying the idle is approx' 900 when cold/warming up but increases to 1,500 as it reaches full operating temp then it sounds like a vacuum leak.
Let us know if it appears you have a vacuum leak.
 
79 GS 1000E poor idle

Keith and all,
I want to thank you for all of your help. I removed the carbs and made adjustments to the floats so they all sit where specified. In addition, I dug up some old strings of Keith's on adjusting the pilot and air screws (you know, the ones we weren't supposed to touch) back to 3/4 out on the pilots and about twice that on the air screws. What a world of difference! Idles like a champ now. One remaining question: Other than going by the sound and feel, is there an objective measurement one takes when setting these two adjusting screws?
I plan on buying a carb sync device in the very near future but understand that this meter is not for adjusting the air and pilot screws?
 
cliffnote said:
Keith and all,
I want to thank you for all of your help. I removed the carbs and made adjustments to the floats so they all sit where specified. In addition, I dug up some old strings of Keith's on adjusting the pilot and air screws (you know, the ones we weren't supposed to touch) back to 3/4 out on the pilots and about twice that on the air screws. What a world of difference! Idles like a champ now. One remaining question: Other than going by the sound and feel, is there an objective measurement one takes when setting these two adjusting screws?
I plan on buying a carb sync device in the very near future but understand that this meter is not for adjusting the air and pilot screws?
Click to expand...
You are correct in your understanding. The carbsyncing is to adjust the vacuum for the carbs alone, a must do for any tune up on your carbs, makes the world of difference. Your pilot screws are adjusted by doing "plug chops" and your air screws can be adjusted by the fastest idle method or a colortune. Your method of adjusting your air mixture screw by sound and feel is probably just fine. I have a colortune but I find I can do it just the same by listening and feeling the change in vibration. It's a bit more work but adjust the floats with a clear line from the drain plug made the world of difference on my bike, set them up on a level bench in the shed to do it. Glad to hear she's running better, nothing like working on the bike and it actually works!
 
Plug chops and colortune?

Plug chops and colortune?

I'm afraid you lost me on the "plug chops" and "colortune" methods of adjusting. Could you go into a little more detail on that?
 
cliffnote said:
Keith and all,
I want to thank you for all of your help. I removed the carbs and made adjustments to the floats so they all sit where specified. In addition, I dug up some old strings of Keith's on adjusting the pilot and air screws (you know, the ones we weren't supposed to touch) back to 3/4 out on the pilots and about twice that on the air screws. What a world of difference! Idles like a champ now. One remaining question: Other than going by the sound and feel, is there an objective measurement one takes when setting these two adjusting screws?
I plan on buying a carb sync device in the very near future but understand that this meter is not for adjusting the air and pilot screws?
Click to expand...
VM carbs: You set the side air screws for highest idle. If the carbs/motor are in otherwise good tune then there WILL be a sweet spot as you set each carb for highest idle. Use 1,000 rpm's as the base idle. Set one carb and then lower the rpm's to 1,000 by using the idle adjuster knob. Repeat to each carb.
The pilot fuel screws underneath may take some tinkering to get right. 3/4 initially is a good starting point. After testing the pilot circuit correctly just do what the plugs/performance say. If plug colors are a bit dark then turn the screws in just a little. If a bit too white/lack of gray or tan color, then turn them out some. How much depends on the color of each plug. Minor adjustment is all that should be needed. Because they're "upside down", use a mirror and be sure you turn them the intended direction.
A plug chop is just the act of pulling in the clutch lever as you shut off the throttle completely and then turn off the key and coast to a stop.
Shutting down this way gives you a good view of how the mixture is at that moment/throttle position. Since each carb circuit regulates mixture at a certain throttle range you must test at specific throttle openings. Minimal throttle up to about 1/5 is good for testing the pilot circuit. 1/2 throttle is good for testing the jet needle. Full throttle is for the main jet. The previous positions are my suggestion as they have the least chance of having an "overlap" effect from the other jetting circuits. "1/4" throttle for example can have the pilot circuit, cut-away, and jet needle all contributing some and it can make trouble-shooting more difficult. And it does vary a little depending on what model, year, condition, climate, elevation, etc.
For the mains, I get the bike in top gear and go about 1/2 to 1 mile at full throttle or very close, turn around, and do it one more time and chop off. Be sure you're on level or uphill grade, no downhills.
For the jet needles I do the same thing except at 1/2 throttle. If you want, place some tape on your grip and housing and use marks to be sure you're at 1/2 throttle. Be sure to make your marks allowing for throttle slack.
I take a rag and some hose along to help with hot plugs. The hose end fits snug over the plug end to help you re-install. But be careful if you've never done it as you don't want to cross-thread the plug.
 
Last edited:
One other thing regarding chopping off at full throttle...
I won't say it's common but certainly possible that the cam chain tensioner can back off/malfunction momentarily under full throttle and then quickly shutting the throttle. These are older bikes and some in questionable condition so don't tempt fate.
Just don't back off so abruptly/quickly as many may think is the way. Give it 1/2 a second if you know what I mean.
 
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