Hi there, I think I might be able to help here. I rebuilt a set of Mikuni VM32s carbs on my GS(X)1100S Katana, and have spent sometime sorting out the slow running/idle circuits. I posted the following on the Katana Australia Forum, but it's not visible to non-members of that forum, so I thought I'd paste it here. I couldn't find anything that gave straight forward instructions on how to tune VMs that have both an air screw (side screw) and fuel screw (bottom screw) beyond trail and error. So after about a year of tinkering I've arrived at the following system that has given me good low end carburation.
Incidentally, one thing I also discovered was that off idle richness that can't be dialed out using the screws is a sign that the needle jets are likely to be worn, or corroded. These flow fuel just off idle and if are bad will create blubbering. These are the corroded culprits (the carbs sat for 30 years...).
So the post on idle/slow/pilot circuit tuning is as follows:
"I think I have finally got the the low end carburation sorted. I couldn't find anything definitive on setting up the slow circuit on carbs that have both air screw and fuel screw adjustment. Interestingly, it seems that this two screw system was used on the VMs fitted to Suzukis, but not so much on Kawasaki fours of the same era. I had been using the general instructions for VMs from Basscliff, and other relevant posts on GS Resources, but this all pretty much boiled down to initial screw positions and then juggling between the screws until it runs right and the plugs look good. I did this and got a better result by leaning off the fuel screws, and leaning off the air screws too. But it was all a bit hit and miss and I got the feeling that although the idle and off idle was now better the idle was uneven, indicating that the settings weren't uniform across carbs. There was also a little roughness at 90-100 kph at cruise, but above that it was crisp.
Looking at the carbs and diagrams of the VM32 slow running circuit, showed that the fuel mixed by the air screw and idle jet feed two outlets, the pilot and the by-pass. The fuel screw only meters the pilot outlet -the furthest downstream orifice which is on the engine side of the slide. The bypass sits under the slide forward of the needle/needle jet. The layout is the same as the diagrams below (although the air for the slow running circuit in the diagram is air jet rather than an air screw).
A bit of further research revealed that the bypass is a multi-function orifice (Yes Glen, its true). Admitting extra air into idle mixture that is exiting the the pilot outlet when the slide is shut, and as the slide is opened it stops admitting in air and begins flowing fuel mix, effectively becoming a secondary pilot outlet. Apparently the bypass also flows more fuel as the throttle is opened, effectively enriching the mixture, which is why these things get away without accelerator pumps.
The accepted knowledge on tuning the slow running circuit on two screw VMs seemed to be that air screw adjusted mixture, and the fuel screw (preset at the factory) was to adjust for minor differences between the cylinders. Quite frankly I think that's bollocks. The diagrams suggest is that the VM fuel screw has most effect setting the mixture at idle only when the bypass is adding air. After all, at idle it's metering the only outlet flowing fuel. At all stages when the slow circuit is boss the mixture is set by the air screw. At idle this mixture is modified by the bypass and adjusted by the fuel screw.
So armed with this knowledge, out with the vintage Colortune plug for about the sixth or seventh time on the Kat. I'd ridden it about 15-20 Km and it was fully warmed. Set up the cooling fans, and set to it. For each cylinder I first set the idle knob to have the engine running at about 3,000 rpm and adjusted the air screw rich to an orange flame and leaned it out until it turned blue. Then I dropped the idle to 1,100 rpm and adjusted the fuel screw the same way. Checked the flame colour while slowly dialing it up with the throttle and it was uniform. Tried blipping and the flame colour went orange then back to blue, which is what its supposed to do. The initial reading on the colortune confirmed my suspicion that the idle mix was all over the place. Two cylinders were richer that the other and one was quite a bit richer (this also matched the plugs as I took them out). The engine was now way smoother at idle and responds to blipping from idle much more cleanly.
I took it out for a road test and my ass dyno tells me that its pulling harder from 2,000 - 4,000 rpm. It feels crisper. Cruising at 100 kph, it felt may be a tad lean (slightest hint of surging, although I might have been imagining it). Any how I dialed in an extra 1/8th of a turn rich on the air screws and she seemed to be all good. I'm closer than I've ever been and hopefully my theories are close to the truth."
