@chris846 wrote:Apologies if the following is a bit laboured, but you keep mentioning that you don't fully understand the functions of the various adjustments and stuff. Hope this helps.
The fast idle is just a simple mechanism to hold the throttle butterflies slightly more open than normal tickover. It does exactly the same as holding the throttle twistgrip slightly open. Best thing to do when you are about to work on the throttles is back it off so's the cable is as slack as, and there's no chance of it affecting anything. I don't even have one on the K75.
The Throttle Stop is what you are referring to as the 'Main Throttle Adjustment Screw'. You're quite right to think that it should come to rest on the circular metal anvil. If you back it off so that it doesn't (i.e. you can see a slight gap even when the throttle twistgrip is fully released) then that means that one or more of the butterflies has closed so much that it is jamming in the throttle bore. You should avoid doing this as it can cause wear. If the throttle stop screw has already been twiddled with, then a best guess base setting can be got by backing it out to give a gap, then screwing it down so's it just takes the 'weight' of the throttle spindle springs. Open the throttle a small distance and release it - you should be able to hear a definite click as the throttle stop screw strikes the metal anvil.
Tickover.
When the bike is ticking over, it is getting just a tiny amount of air from 2 sources: 1 a small leak past the butterflies themselves (because the throttle stop screw has been set so's they don't fully jam in the throttle bores) and 2: a small amount from the brass bypass screws.
The function of the brass bypass screws is mainly to compensate for the different 'leaks' past the individual throttle butterflies - due to manufacturing tolerances and wear. This balances the air flow into each cylinder and gives a smooth tickover. More importantly, it ensures that the bike will pick up revs cleanly for a smooth set off.
The linkage screws (the ones you are best not touching) between the throttles are used also to balance the air flows, but more in the region of small throttle openings. Again, this is for smooth running and clean pick up. If these are twiddled with, it will also screw up the ability of the butterflies to close in a synchronised manner - one or more will stay open whilst the others shut. If this happens, then it can be difficult to get the tickover slow enough - the 'open' butterfly will allow air past, even if the Throttle Stop Screw is backed right off and the other butterflies are jamming in the bores as a consequence.
As you have identified, the first thing to eliminate is any air leak. If a leak occurs after the AFM but before the butterflies, then the mixture could be too lean, owing to unmetered air entering the system. The tickover should still be even though, if everything else is correctly adjusted. If the leak occurs after the butterflies (more likely owing to the higher vacuum) then the tickover will be properly screwed because neither the butterfly nor the brass bypass screw will have any chance of controlling the amount of air entering that cylinder. As has already been said, leaks can occur between the mating faces of the rubber inlet manifolds and the cylinder head. I always use RTV on these joints (ever since I spent ages trying to cure a rough & unresponsive tickover funnily enough)
If you have cured any air leaks and your tickover is still poor, then you could try cleaning the brass bypass screws. Before you unscrew them, screw each one in carefully as far as it will go, but don't force it. Count the turns and, if everything's okay, the screws should each have been somewhere in the region of 3/4 to 1 1/2 turns out from fully shut. If not, then this might indicate that somebody has already tried to deal with a butterfly/air leak problem by going at the brass screws. That never works btw.
Once you've cleaned the screws (they can get sooty on the tapered business end) replace them, either with the same number of turns as you found them to have or, if they were haywire, 3/4 of a turn out from fully closed. If you still can't get an even tickover, then you really need a carbtune or similar to see what's going off.
Another thing you can look at is whether the tickover responds to each brass bypass screw. If things are working properly, then screwing the brass bypass screws in and out by a quarter/half a turn should noticeably affect the tickover. If this doesn't happen, then it can indicate either an air leak after the butterfly, or a problem with the throttle linkage screws - i.e. a butterfly being more 'open' than the others.
If everything's looking okay, but the brass bypass screws all need to be screwed out beyond 1 1/2 turns to get the tickover speed high enough, then you should screw the Throttle Stop Screw down a little. This will speed the tickover up, and you can then screw all 4 brass bypass screws back in to get the tickever back down to the right speed.
As already advised though, a Carbtune or equivalent is necessary to do these settings to the degree required for a decent tickover and slow speed pickup.