79 GS850 still idling rough

[mpidsadny]

As an introduction, I’ve been working at this bike for a while. I’ve replaced intake and carb boots, set ignition timing, replaced condensers, set valves, replaced the air filter, sealed the airbox, hot dipped the carbs and replaced O-rings, rebuilt choke plungers, replaced coils, plugs and caps, and replaced the float/fuel height.

I’m right where I was 1 year ago with how the bike is running. The idle is rough, and the adjustments I make to the pilot circuit screws seem to have little effect. I’ve found conflicting information whether to sync before or after you’ve adjust the pilot screws.

I don’t know how to fix this idle and rough running. It’s getting frustrating to not be able to ride!

 

[unfocused]

Did you check compression? Simple test. Take a vacuum gauge and hook it up to any carb - at the sync ports - if the gauge bounces - valve job is needed.

If it doesn't - check compression.

Still running rough - we're missing something

 

[1978GS750E]

Did you check compression? Simple test. Take a vacuum gauge and hook it up to any carb - at the sync ports - if the gauge bounces - valve job is needed.

If it doesn't - check compression.

Still running rough - we're missing something

That's a new one on me! Of course, I haven't seen everything ..........yet!

 

[mpidsadny]

Did you check compression? Simple test. Take a vacuum gauge and hook it up to any carb - at the sync ports - if the gauge bounces - valve job is needed.

If it doesn't - check compression.

Still running rough - we're missing something

I have checked compression and it’s even across the board. Checked leak down as well and was in spec.

Spoke to someone a bit more knowledgeable than me about my issue and we went through the pilot circuit diagram. Turns out I’ve been adjusting the wrong screw for the past year…
The 79 gs850 carbs appear to be slightly different than other VMs of the era. Corrective action is in progress! Will report back here.

 

[unfocused]

If you have 2 screws - one on the rear and one front lower

Rear - air bleed screw - start at 1.5 turns - use this for off throttle hesitation - lean/rich condition
Front, underneath - air/fuel mixture screw - start at 1/2 turn then you might end up at 3/4- set it and forget it.
Slight adjustments needed after
These are currently installed on my bored out 750 and this is where they're set at the moment.

If you have only x1 screw in the rear - early VM26's (Kawasaki) were set up like this - I had mine about 2 turns out - engine running riding but had other issues not associated with mixture screws.

I have both styles of VM26's and have ran them on the same engine. I also had the Kawasaki VM26's on my GS1000 engine.
I prefer the single screw pilot port however theses carbs are harder to deal with the slides/needle height.

 

[Nessism]

The 79 gs850 carbs appear to be slightly different than other VMs of the era. Corrective action is in progress! Will report back here.

All the Suzuki flavor Mikuni VM's are the same, in that they have both a pilot fuel screw (by the float bowl) and a pilot air screw on the side of the carb. A good starting point for adjustment is 3/4 turns out on the pilot fuel and 1.5 turns out on the pilot air. You should vacuum sync of course.

One problem that sometimes befalls VM's with fuel screws, is the pointy tip of the screw breaking off, blocking the carb passage. You might want to check for that...

 

[mpidsadny]

All the Suzuki flavor Mikuni VM's are the same, in that they have both a pilot fuel screw (by the float bowl) and a pilot air screw on the side of the carb. A good starting point for adjustment is 3/4 turns out on the pilot fuel and 1.5 turns out on the pilot air. You should vacuum sync of course.

One problem that sometimes befalls VM's with fuel screws, is the pointy tip of the screw breaking off, blocking the carb passage. You might want to check for that...

I've already removed a broken tip which resulted in a slightly enlarged fuel screw, and I know that this will cause some different settings on that particular carb.

Also just to clarify, I am adjusting the pilot air screw to achieve highest idle? And I'm turning in to make the mix richer? As mentioned, I haven't had the best luck tuning it this way, but I'll keep at it if this is the tried and true method.

 

[Nessism]

Where are the pilot fuel screws set? How far open?

 

[mpidsadny]

Fuel screws were set at 1 when I started. I moved them in to 3/4 yesterday and noted an improvement in throttle response.