When you've got that far and it's still playing up it's back to square one. What about a heavier return spring on the throttle cable?
Can you run it without the air filters and with a mirror, remote camera, etc, look in the inlets to see if any of the slides are still raised after the throttle is closed. As Hugo says, could be one or more sticking slides?
If you don't mind me saying WCP, that "petrol" looks more like poitîn to me !!
Regarding the carb problem you now have on that Honda, have you checked the four diaphrams very carefully for pin holes and/or cracks ? You could try putting them over a bright light source & giving them a gentle pull/stretch to see if any light appears through the rubber anywhere.
If the diaphrams leak that can cause your rev issues I seem to remember from my past with similar type CV ish carbs. Failing that you might need to split the carb bodies & drop them in your ultrasonic cleaner.
I don’t know why I didn’t get these notifications
I must have been asleep.
To respond, throttle snaps shut, bang !, a heavier spring couldn’t close it any quicker.
Yes, I ran it without the airbox, no slide stays up but this is an issue with the slow circuit anyway so the slides aren’t even opening. Which leads me on to …
The slides open once the throttle is turned beyond 1/4 opening, as they should, and they open fully at full throttle and close as they should. All diaphragms are fully intact, visually checked and also blown upwards in order to remove them which wouldn’t be possible if there was a pinhole air leak,
Here’s the latest on the Hornet …
O-rings arrived, turns out two of the original ones did have tiny breaks in them, most likely due to age as they're hardly moving parts,
and the new ones were inserted. Carbs back on, connected, started, slightly better but not perfect. Carbs off again, bowls off, 45 jets taken out and 40s put back in, reassembled, reinstalled, reconnected (Lordy Lord, this is tedious but at least I've got it down to a fine art now
), bike restarted. It now ticks over much better
, it clearly prefers the 40s as the slow jets. Great, I think it's done, hoo-fecking-ray.
Dream on kiddo.
I leave it to tick over and warm up, can knock the SE valve off, all good, ticking over at c1300 revs, nice. Blip the throttle, revs rise to 5-6K, release the throttle, it snaps shut, revs drop instantly but then hang around 2.3to3K. Adjust the throttle idol screw - decking fiddly to get to with the airbox on, revs drop, but too much, and bike stalls. Now I know readers might think I adjusted the idle knob too far but I didn't we're talking about a 30 degree rotation that's all, which has to be reversed in order to get the bike to start and idle again.
Seriously, I give up. It's not even my bike, just trying to help out a friend. It's rideable, I know as I took it out for a test ride, as long as you can live with the fact the revs don't fall below 3K until they feel like it.
It really is perplexing. It's symptomatic of an air leak but where?
I read on AN Other forum of someone who had a similar problem which his mechanic eventually traced (allegedly) to "a leaking oil seal in the engine" but I'm at a loss to think what oil seal or gasket could be leaking that would permit air to enter the intake manifold and lean out the mixture.
Anyway, it’s now been given back to its owner, slightly better but nowhere near perfect
.
I get a weeks reprieve now after this weekend to get out more then the super hot Mrs Viada arrives for a week. Sometimes life is a bitch but I just roll with it 
