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Hail Mary pass before putting on C.L. : 1981 GS650G racing idle issue

  • Thread starter Thread starter mhobryan
  • Start date Start date
M

mhobryan

Guest
Since rebuilding the carbs, I have a racing idle issue: will race up to 4k, I blip it once or twice and it will come back down to 1500...rinse repeat.

This is everything I've done:

- slightly less than 16k miles
- Completely stock; has air box, stock exhaust
- added foam around the airbox to ensure a good seal
- Rebuilt carbs...completely took them down and soaked each carb body in Beryman's
- confirmed all passages clear
- replaced the float needles
- Completely re O ringed (twice now)
- replaced the rubber intake boots (and yes the O rings behind) as well as the clamps
- Adjusted valves
- Rebuilt petcock
- needles are all adjusted about 2.5 turns out
- Carb's synched with a Morgan Tune
- Fresh oil and filter
- fresh gas
- none of the carb adjustment needles are broken

She starts right up, and has plenty of power - was surprised how well it pulled getting onto the on ramp. Sometimes it will sit and idle perfectly if I don't mess with the throttle, but as soon as I start to use the throttle (load or no load) it will just start racing.

I've had the carbs off and on at least 3-4 times. I even pulled the carbs back apart and cleaned them again (didn't do Berman soak, as I thought it would be overkill the second time) and re O-ringed.

I'm really at a loss .... is it possible I'm looking under the wrong rock, and it is potentially related to the ignition / timing advance /retard? I bet I've got a good 40 hours invested in trouble shooting the problem.

I'm located in Saint Clair Shores, MI if someone is up to assisting me with tackling this challenge.

Any thoughts /opinions welcome.
 
Choke works fine ... even replaced the choke cable. Fires right up. In fact, clogged choke passages was the reason I rebuilt the carbs in the first place.
 
I think Rob was alluding to the possibility that the choke was not shutting off fully for you. Or, you may need to adjust the idle mixture screws
 
I am pretty sure have BS32 carb and there is no choke on a Mikuni BS32 assembly. The things are enricheners and when opened, allow extra gas to combine with the gas from the pilot circuit to enrichen the starting mixture. When not needed they shut against a raised seat in a well. They seal because there are rubber pads inserted into the bottoms. If the seal is bad, idle speed will rev to an uncomfortable level till they seat again; you can sometimes jiggle the choke cable and they will reseat; a little harder to do with a cable rather than an arm with a lever on the end as some earlier VM carbs had... just jiggle that lever and the idle will settle. If this is the case, the rubber needs to be replaced OR you can pick up new plungers...

You may also have a vacuum leak. Vacuum line to petcock is most likely candidate. Last, the throttle cables may be a tad tight; make sure they have slack... you should be able to pull sideways on a cable down at the carb side and it should deflect perhaps a 1/4" indicating it isn't taut.
 
Just for kicks how about opening up the pilot screws to 3.5 turns open? My old 550T had a hanging idle that went away at 3.5 turns.
 
Throttle tube sticking? (Slow to return)
Bent throttle cable (slow to return)
Sticking butterflies (
 
Plug the vacuum connection on #2 carb, run petcock on "PR" spot and see if this issue goes bye bye on next ride. Don't waste time rebuilding petcocks, new ones can be had for $50
 
Thanks for all of the suggestions. After reading them all, my hunch was the petcock. I did the test suggested; turned onto prime, and plugged the vacuum line.... alas, no difference.

Next I will try turning the pilot screws open a bit....although I will say, when I attempted to adjust these using the 'high idle' method for adjusting them i could not discern any change in idle speed regardless how in/out the screws were (hence the 2.5 turn default). And yes, I double checked that none of the pilot screws were broke, and also replaced the tiny little o rings.

After I turn the pilot screws out a half turn, then I will throw a timing light on it. If neither works, will spring for a new petcock. Although the test did not prove it was at fault, I do suspect it, even though I rebuilt it.

Forgot to add...it's not the throttle cable; works smoothly, and know that it fully closes, as I adjusted it so that there is a slight bit of slack when it's shut. When I had it off, I could tell the all the butterflies were closed....so not the throttle cable.
 
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Try running an auxiliary fuel source.
(Small bottle with a nipple to feed the fuel line to the carbs.)
10 mins to whip up a little gas feed system.
That will help rule out/ and or point to certain potential issues.
 
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