VM26SS carb confusion

[Guest]

Dear all,
I am tuning the carbs on my GS850 from 1979, and have read the very fine guide on VM Carb Rebuilds written by Mr. Paul Musser.
In this guide Mr. Musser explains that the two adjustment screws found on the carb are a pilot air screw (easily accessible on the side of the carburettor) and a pilot fuel screw (totally unaccessible underneath the float bowl)
What confuses me is this:
1. Why would there be two possible adjustments to the Pilot circuit, but no adjustment possibilities to the Main circuit
2. If my understanding of Mr. Mussers guide is correct, and both the adjustment screws control the idle circuit, is there then a preferred sequence of adjusting these two screws? As I understand Mr. Mussers guide, you adjust the Pilot Air screw for optimal idle speed. If you then adjust the pilot fuel screw, you have to adjust the air screw again.

All the generic carb tuning information I have found states that you either have a pilot air screw OR a pilot fuel screw, but newer both, so I am in dire need of a step by step guide to dialing in the pilot circuit of my GS850 GN Carbs.

Any advice will be highly appreciated.

[Big T]

1. There are endless adjustments to the main circuit, by changing jets. These are applicable over 1/2 throttle
2. where the pilot circuit is set depends on your exhaust/ air filter combination. Standard is 3/4-7/8 out on the fuel screw and 1 1/2 out on the air screw. You tune with the air screw. You should not change the fuel screw after that

[chuck hahn]

see my answer in your other thread

[erki52]

On my VM's the screw underneath the carbs, only easily accessible when the carbs are off the bike, turned out 1/2 turn from lightly seated. The screws on the sides are set approx 2 x the underneath screws to start and then fettled to get the best revs on each carb.

That made more sense to me before I started typing but no doubt someone better qualified on here will correct me.

[DKGS850]

Dear all,

Thanks for your input. Here is my summary:

1. You set the pilot fuel screw underneath the carb bowl, and leave it
2. You adjust the pilot air screw at the upper side of the carburetor for the optimal idle
3. You adjust the throttle range not covered by the pilot circuit by replacing the main jet and the needle position. There is no external brass adjustment screws to help with that.

So... follow up question:
What do you adjust the pilot fuel screw after. How do you know if that is in the optimal position? Manual says 5/8 th out, but I guess that there is some need for adjusting it since they made a screw for it.

BR Michael

[chuck hahn]

The bottom screws is the PILOT SCREW. The ones on the side at a 45 degree angle are the AIR SCREWS. PILOTS are set lean from the factory and i set them at 7/8 out and my colortune confirms a nice blue flame at idle. Now all you need to do is get a nice blue flame by adjusting the AIR SCREWS...you leave the bottom ones alone...Its that simple. I addressed this in you other thread.

[DKGS850]

Hi Chuck
I understand that the Pilot screw is adjusted once and then left alone, but I am curious why you would adjust it to 7/8 out. I belive that the factory setting is 5/8 out.
Do you have alterations (like changes to air filter or exhaust) that requires this adjustment?

Best regards,

Michael

[chuck hahn]

because japan sent them here extremely lean but still run...this was to pass the EPA bulllllstick back then..7/8 gives them a little more richness and helps the transition from pilot circuit to the main circuit. THATS the explanation as given to me from a 40 year dealership veteran thats done more GS bikes than anyone i ever knew.