Chasing a sticking idle........Need carb help!

[1978GS750E]

Just finished ultrasonic cleaning and replacing all the O-rings on the carbs from my 1978 GS750. I set the fuel and air screws back to where they were before the cleaning. (3/4 turn and 1-1/2 turns). Now the bike is very slow to start when cold, and when warmed up it idles around 2,000 rpm unless I "blip" the throttle, then it goes back down to 1200 rpm. Once warmed up, the bike starts fine and the choke will raise the idle to around 3,000 rpm when applied. I have ridden the bike a few miles and it pulls and accelerates well throughout the entire rpm range. Adjusting the pilot air screws do not seem to have any effect on the idle at all??? I have not synced the carbs with a set of gauges yet. My question is, will a carb sync cure this, or do you think that I have other issues? Any idea of where to start???

[Nessism]

Sync may cure the idle issue but not the starting issue. Did you clean the float bowl choke passages and make sure they are open? Check the choke pick up tubes to assure they are open? And did you replace the intake boot O-rings or better yet replace the boots entirely?

[1978GS750E]

Replaced the intake boots, all O-rings, even the boots going to the airbox. Checked and cannot find any air leaks. Choke passages are clear. Choke increases the rpms when warmed up so I think the choke is working as it should. Guess that a carb sync is in my future???

[Nessism]

Guess that a carb sync is in my future???

What's with the ??? Of course you should sync, regardless of how it affects this particular issue. Maintenance is always good.

[azr]

Anytime you rebuild the carbs or do a valve service you HAVE to do a vacuum sync.

[Steve]

A carb sync certainly won't hurt anything, but probably won't cure everything.
If nothing else, it ensures that each carb is pulling the same amount of mixture, so that adjusting the screws will actually make a difference.

How did you decide on your 3/4 turn and 1 1/2 turn settings on the screws?
If your intake and exhaust are stock, the 3/4 turn on the fuel screws is probably close, but the 1 1/2 might be a bit too much, making it run lean. Running lean makes it act a bit like you are describing. 1 1/4 turn should help to get it running better, but that is only to get it to a point where you can do the fine-tuning. It runs better with a too-rich mixture than it does with one that is too lean. Of course, it runs even better with the proper mixture, but take it one step at a time.

.

[roeme]

Hmmm, does he have VM or BS?

I had a sticking idle on my VM's once that was caused mechanically, not by an air-leak.

[1978GS750E]

I set the fuel and air screws back to where they were before the cleaning. (3/4 turn and 1-1/2 turns).

They are VM carbs.

Roeme, what was the mechanical issue that you experienced causing the idle issue? Maybe that is something to look into.

Going to get gauges and do a sync in the next couple of weeks after a valve adjustment.


Thanks for the help!

[standswithabeer]

all four evenly hot on start-up? i'd do a plug chop, to confirm a lean setting. if adjusting the pilot air screws does nothing (as stated in first post), idle circuit(s) maybe still be plugged. there's info on the web somewhere with a nice pic of where the carb spray should exit, when sprayed into each circuit.

[Gorminrider]

Adjusting the pilot air screws do not seem to have any effect on the idle at all???

Yes -see above (per standswithbeer) because no effect? that's just wrong if the bike is idling 1200 or below-they should have effect.

Plugging very common I found...then broken tips and improper assembly of the tiny spring, oring and washer...

[roeme]

(...)
They are VM carbs.

Roeme, what was the mechanical issue that you experienced causing the idle issue? Maybe that is something to look into.

(...)

The bit that connects the throttle shaft to the throttle cables got loose, which allowed for some of the plungers not to return to their idle position when closing the throttle slowly - blipping helped a bit. Sounds familiar, eh? Here's the old thread where I chased that one: Clicky. Scroll down to get a video illustrating the issue.

[LAB3]

Back to the original question: Check the routing of your throttle cable for any tight bends, sounds like it's hanging up and not returning all the way to the off position.