its a 4stroker! it doesnt foul plugs if it idles for a long time ( as long as they arent chinese plugs!) even if its set so rich it sprays petrol out the pipe... tends not to accelerate too well though.
its an interesting carb in how the fuel is first regulated via the main jet, enters a seperate section machined at the base of the emulsion tube. here, some of it is sucked up to the idle, the rest bubbles up into the emulsion tube and out the jet. as the whole lot fills up with fuel when not running, i guess it would give for a brief super rich condition when first started.
the screw doesnt adjust just air, or fuel alone, but rather, a mixture that is then mixed in with the air bypassing the butterfly. it blocks the port off into the carb throat behind the butterfly. you can poke the tip of it when screwed all the way in.
when the jets the right diameter, idle can be so low that the butterfly is completely closed, and all the fuel/air is delivered via the low speed system. the screw only adjusting the volume delivered, not the mix.
otherwise its a matter of adjusting idle screw and mixture screw for a nice idle as they interact. in this case, air bypasses the butterfly, but doesnt create enough suction to draw fuel through the main spraybar. so this air is straight air. the idle system is delivering an overly rich mix as its jets oversize. you have to adjust the idle flow with the clean bypass flow, to get the correct mix.
the biggest issue is when the orings around the idle jet get damaged, then the low speed circuit is too lean, and nothing will fix it but new orings. my theory is dont remove the thing!
the struggle to fit a vm20 isnt worth it.
a really odd ball bolt pattern that needs a pretty fancy adaptor to work.