1981 GS850 Carb Tuning Help

[Guest]

Finally got the bike up and running after having it for nearly a year.

Disassembled, cleaned, and rebuilt the carbs only to discover one of the tips of the mixture screws - actually 3 of them, but two I was able to remove - was broken off down inside the carb body and could not be removed; Replaced the carbs; adjusted the valves - 7 of 8 were out of spec; replaced the crappy eBay air filters with some UNIs; rejetted, and voila.

Felt great to finally ride it!

However, now the real 'fun' of tuning the carbs begins.

I purchased a 6 sigma jet kit, and followed their instructions which called for 127.5 main jets for carbs 1 and 4, and 130 jets for carbs 2 and 3. Replaced the pilot jets with the supplied 47.5 jets. Adjusted the needle jet with two shims to richen the mixture (as my needle is not the adjustable kind). Set the mixture screws at 2 turns out. And checked the float height, which were all at 22 mm (give or take .5 mm).

Still have the dreaded flat spot in my throttle response. I also have 125, and 132.5 main jets that I could swap out with what are in the carb now. Any thoughts on a next step?

[Steve]

I purchased a 6 sigma jet kit, and followed their instructions which called for 127.5 main jets for carbs 1 and 4, and 130 jets for carbs 2 and 3.

I have no experience with a 6 Sigma kit, but why do they suggest different jetting for the inner cylinders?
Suzuki did that with the '83-and-up 550 and with the 1150, but I have not heard of any others suggesting it.

Replaced the pilot jets with the supplied 47.5 jets.

Why would you need more fuel at idle? The need for more fuel is when you remove the restriction of the air box, but that basically only affects full-throttle. The pilot jets supply fuel at idle and very LOW throttle settings, when the airbox is not a factor.

Adjusted the needle jet with two shims to richen the mixture (as my needle is not the adjustable kind).

Please describe, IN DETAIL, exactly how you adjusted the needle jet with two shims.
This is often done incorrectly, just want to see if you did it right.

Set the mixture screws at 2 turns out.

Generally, we suggest 3 turns out as a starting point, but with your larger pilot jets, 2 turns might be plenty, but don't take that as an absolute, that is just a starting point.

And checked the float height, which were all at 22 mm (give or take .5 mm).

That is good for a static setting, did you happen to also check the actual fuel height?

Still have the dreaded flat spot in my throttle response. I also have 125, and 132.5 main jets that I could swap out with what are in the carb now.

How many jets of each size did you get in the kit? Just wondering if you HAVE to mix the sizes or if you got four of each.
Also wondering if they are Mikuni jet sizes or their own creation.

Any thoughts on a next step?

Plug chops.

.

[PardonMe4687]

I have no experience with a 6 Sigma kit, but why do they suggest different jetting for the inner cylinders?
Suzuki did that with the '83-and-up 550 and with the 1150, but I have not heard of any others suggesting it.

.

I'm not entirely sure. When ordering, they request that you specify year/make/model, intake and exhaust modifications, and altitude above sea level in order to design the jet kit for the buyer's specific conditions... so they say.

Why would you need more fuel at idle? The need for more fuel is when you remove the restriction of the air box, but that basically only affects full-throttle. The pilot jets supply fuel at idle and very LOW throttle settings, when the airbox is not a factor.

.

Again, I'm not certain. This is all new to me.

Please describe, IN DETAIL, exactly how you adjusted the needle jet with two shims.
This is often done incorrectly, just want to see if you did it right.

.

I removed the circlip that retains the guide holder, removed the needle from the slide, removed the spring, installed the new metal washer shims (which, after going back and looking more closely at the installation instructions, I may have incorrectly placed these below the stock metal washer, as opposed to above), replaced spring, replaced needle assembly into slide, replaced guide holder, replaced circlip.

That is good for a static setting, did you happen to also check the actual fuel height?

.

No, I have not checked the actual fuel height, as I do not have the appropriate drain plug adapter. Sounds like it might be a good addition to the tool box.

How many jets of each size did you get in the kit? Just wondering if you HAVE to mix the sizes or if you got four of each.
Also wondering if they are Mikuni jet sizes or their own creation.

.

The kit came with two each of the 127.5, 130, and 132.5 jets. I purchased the 125 jets to use in case I needed to go down another size. But again, I only have two of each. And I'm not certain if they are Mikuni jets or their own, I'll have to check when I get home.