hanging idle

[Spoonz]

Have a 78 GS750. cleaned the carbs about 6 times through an ultra sonic tank. New stock jets, running stock air box and filter. Bike will sit and idle all day long. Give any throttle and RPMs jack up to 3000 and hang there for about 30 seconds before returning to idle. Carb boots have all been replaced on both sides of the carbs. sprayed to check for vacuum leaks and non present. Set air screws at 3/4 turn and pilot screws at 1 1/2. Air screws out and it starts popping through the carbs. pilot screw doesn't seem to make a difference. Any advice or suggestions would be greatly appreciated.

 

[Redman]

Have a 78 GS750. cleaned the carbs about 6 times through an ultra sonic tank. New stock jets, running stock air box and filter. Bike will sit and idle all day long. Give any throttle and RPMs jack up to 3000 and hang there for about 30 seconds before returning to idle. Carb boots have all been replaced on both sides of the carbs. sprayed to check for vacuum leaks and non present. Set air screws at 3/4 turn and pilot screws at 1 1/2. Air screws out and it starts popping through the carbs. pilot screw doesn't seem to make a difference. Any advice or suggestions would be greatly appreciated.

Welcome to TheGSR.

Sounds like you are familiar with the basic idea of the hanging idle and its causes, and you have covered the basics.

One of the basic causes can be the oring in the intake boot (up against the head). You didnt specifically mention those, but did say have new boots and have tested for leak there.

I dont know about the VM carbs, but with my CV carbs I had a bit of hanging idle that might or might not come back down, more of a increasing idle... anyway, I covered all the basics like you have and did not completely take care of the problem, until I broke down the carbs enough to also replace the oring on the choke (enricher) mechanism.

And......Any discusion of carb work is not complete without mentioning Cycle O-Ring for carb orings and new bolts for the boots.
http://www.cycleorings.com/
A fine GSR member.

 

[Spoonz]

Orings at the head have all been replaced. I’ve tried running another set of carb, but the results were similar Bumped the jets up one size on both pilot and main with little to no affect. Float levels are all within spec as well.

 

[Buffalo Bill]

An additional issue with Mikuni carbs is wear between the needles and emulsion tubes. Check for needle wear and out of round tubes, will need a magnifier.

 

[Nessism]

Did you fully disassemble the carbs, unganging them, before cleaning them? Did you remove the pilot fuel screws from the bottom and make sure the sharp point is still present and not broken off into the carb body? Did you replace all the various O-rings? Did you sync the carbs, at least visually, to make sure they are close?

BTW, the pilot fuel screw should be set to about 3/4 - 7/8 as a start point, and the pilot air screw about double that. You have that part backwards.

 

[Spoonz]

Carbs were completely disassembled for cleaning. Screw settings were swapped thinking I may have had them reversed and just created popping through the carbs. Pilot screws were removed checked and had to replace one carb body as the PO buried one and broke it. All pilot screws were replaced and new orings installed. Carbs were synced with a syncing tool.

 

[Gorminrider]

How about the throttle cable itelf? at the handlebar or the carbs...sometimes these can be sticky to release that last 1/4 inch... also the choke-enricher might be something to look at

 

[Spoonz]

Choke plungers were all replaced due to the rubber being dried out. The bike will do this even with the throttle cable disconnected and operating the throttle by hand.

 

[Gorminrider]

Ok, thought it was too easy. So that's all I've got except to confirm that

Have a 78 GS750. cleaned the carbs about 6 times through an ultra sonic tank

means you have taken the carbs down to this If not download the VM teardown guide at BikeCliff's Website

 

[Spoonz]

Yes carbs were taken down to this point about 6 different times to clean and reclean.

 

[Guest]

Just throwing this out there. . . how is the timing on this bike? And the dwell angle of the point sets? I recall tweaking this stuff on naturally-aspired cars way back when & discovering how idle decline/duration could be modified with these two adjustments alone.

Food for thought.

 

[Nessism]

Just throwing this out there. . . how is the timing on this bike? And the dwell angle of the point sets? I recall tweaking this stuff on naturally-aspired cars way back when & discovering how idle decline/duration could be modified with these two adjustments alone.

Food for thought.

Good point. If the advance mechanism is binding that can cause an inconsistent idle too. That said, I've never seen a VM equipped bike that required 1.5 turns open on the pilot fuel screw. That's normally results in a pig rich condition.

 

[Spoonz]

Never thought of a timing issue. I’ll check tonight and make sure everything is good and not binding or sticking.

 

[Spoonz]

So I checked the points advance seemed ok, but it got cleaned and lubed just to be on the safe side. Still no changes.

 

[Spoonz]

So for ****s and giggles, while I had the points cover off, I decided to check the points gaping. Spec is 0.018”. Rear point is at 0.010 and front point is closer to 0.005. Reset the points to spec and now the idle is returning in about 5 seconds instead of 30 seconds. Does this make sense?

 

[Nessism]

The gap effects when the coil is firing so yes, it can cause poor running. Do you have a timing light?

 

[Spoonz]

I’ll get a timing light on it tomorrow to make sure it’s in time and the adjust the carbs and see where I’m at. So far, thank you everyone for your input.

 

[Guest]

Glad to hear you're making headway, Spoonz. Have you gotten any timing details? Forgive my ignorance, but timing is pretty much fixed by the cam chain & sprockets, right? If so I'd think that being off by one tooth on either cam would present a bigger issue than a hanging idle. I should study that more for my own benefit...

 

[1978GS750E]

Following this closely since I still have the same issue! I'm at my wits end with this thing! Guess I'll check the timing also.

 

[Spoonz]

So, final verdict is the points were causing the issue. Set them to spec and the hanging throttle almost disappeared. Did a final adjustment on the carbs and the problem is gone. Thank you everyone for your wisdom and advice.