Disconnect it and see if makes a difference

I have found the booster can leak and braking can get longer but the brakes work without the booster, just not as well. If your legs are strong it wouldn't be as noticeable, but a vacuum leak could really throw off that carb.

 

What Madkaw said ^^^^^ It's an easy experiment

 

Blues idea is good too, just a little more work

I swapped the carbs, synced them and now the situation is this:

 

Driving above 1200-1500 rpms is okay, no extra vibrations, accelerates well, some occasional pops from the carb (maybe 1 or 2 in 5 mile trip)

 

idle below 1200 is UTTER CRAP, i feel like im in the wheel of some super cammed V8 that shakes all over but as soon as i raise rpms, it runs steadier.. Could it be manifold leak or something? I tried with and without the booster, no noticeable difference to me. Except in braking.. heh. Im afraid the engine mounts will suffer if i let it idle, with SU's the idle was smooth as baby butt.

i know absolutely nothing about your carb setup, but in my experience w/bike carbs idle shake was often caused by them being out of sync at the idle circuit. i assume you've synced them with a flow meter of some kind? could also be a clog or imperfection in one of the idle circuits (crap in a jet or burr in an orofice) leaning out one idle circuit's fuel delivery even though the air flow was constant at the bellmouth. again, just some basic multi-carb advice - not specific to your setup.

What are you using for the carb insulators between manifold and carb?

I cannot tell for sure by the photo.

Are you by chance using the o-ring type insulator?

set of 3 40 SK/OER racing carbs, yes i have Gunson Carbalancer wich i used to syncronize. If they were out of sync at idle, should they be above that as well? Because they are not, thats the weird thing. Ive spent crapload of carbcleaner, of course i have not injected that among the fuel.

I've located a small inlet manifold leak from cylinder Nr5. If i spray starter fluid at it, i can hear idle rising for a second. There is no visible hole or crack or anything. This could be the problem after all?

I would think any intake leaks could cause issues.

I bet it does. I switched to bigger idle jets (55F9) but i STILL have to dial mixture open for 1.5 turns, just like with the smaller jets (50F9). But i have noticed some difference, idle is a tad smoother (even with the inlet leak).

 

One strange thing was that i drove on the freeway about 20 miles, after coasting to normal speed, i could hear faint "pop" from the exhaust occasionally, like it was burning excess fuel there. SO basically i have mixture on the verge of being too lean, but it still pops in the exhaust`??

If your rpms are high and you take foot off gas to coast, the pistons are sucking hardest but the throttle plate is closed. This causes a huge vacuum in the manifold that sucks fuel from the idle enrichment circuit. If your idle is set to rich then too much fuel will be drawn through and it will build up in the exhaust and ignite making the popping sound.

Sounds like lean-popping to me, due to some/all cylinders partially misfiring during coast-down. Proper idle jets will get rid of that.

Yeah, i guess i'll have to try even bigger jets. Maybe 60F9?