Carburation problems

John, 
On a Navigator many years ago a section of the rh gasket. Head to inlet manifold. Broke and got sucked in. 
Fast tickover etc. 
The flat face of the inlet manifold has to mate with two flat faces on two separate cylinder heads which may, or may not, be co-planar. You will understand. Tolerances, hole clearances, etc. 
Worth checking and consider a bit of Hylomar? 

Peter

Test for manifold air leaks by spraying the joints with WD40 while the engine is idling.

If there is an air leak into the manifold, the engine sound will change when any liquid is sprayed on it.  WD40 is usually readily to hand.

How worn are the slides in the carbs you've tried? Surrey cycles sell oversize slides if you fancy giving that a try, id also buy a new needle and needle jet to rule that out. 
Dan 

Worn carb bodies, battery, charging system, all worth checking. Though erratic and ever changing settings to keep the carbs near working suggests they are worn out. 

Advance and retard unit is prone to wear .

float level to high. get it warm .let it tick over . Then turn fuel tap off , and note if it idles good for a short time . It worth trying

We don't know what ignition system, certainly a worn adv/retd unit will do this, ie high tick over, EI cannot do this. The other point here is the AIR SCREW is not to adjust tick over. It is to get the mixture right for tickover and then leave it alone. 
As it happens my Electra, runs on the monoblock it was bought with in 1984 (1st Norton Day) it has not done a high mileage but the monoblock behaves impeccably. Set the TO mixture and haven't touched it for years. The Boyer ignition sorted out a few rubbish moments with the mechanical adv/retd ignition system.

As Al O says above. The tickover speed is governed by the slide stop screw. Undo for less speed. Never failed for me. Aged grey matter tells me that when inlet manifold gasket leaked on my Navi and tickover speed went up, normal service with rather weak running was temporarily fixed with throttle stop screw. (It was 1971)

Is there a problem here? This seems so obvious but.... Cable tight? Obstruction below slide? 

Slide must be hanging on the cable.  If slide is fully down, the engine must stop.

Clutching at straws...might inlet valve guides be very worn?  And I don't know if they should have seals

Hi John,

If the slide can close completely,  ie. the engine will stop/won't idle, then you must have the slide open too much with the throttle stop screw and probably don't have an air leak.
You say that pulling away is 'erratic', which suggests that the mixture is wrong. This could be due to any one of,or a mixture of, partially blocked idle circuit, too small or too large pilot jet, needle jet or needle worn, needle too high/ too low or wrong slide cutaway.
You have to remember that the airspeed through the carb drops substantially as you open the throttle until the engine speed catches up, causing mixture problems.

Have you got a 'Colourtune' plug? If so, using it might give you a pointer as to where your mixture is at when idling.

Another thing you can try is to retard the ignition a little and see if the idle response is better, but you would need to be careful how far you go as WOT/high speed running would be compromised if you left it like it.

Regards, George 
 

I ha this exact same problem and it was ultimately found to be due to the incorrectly set throttle slides. The throttle adjusters on the top of the carbs were not allowing the slides to completely close. This meant they were always running on the main jet with little influence from the idle and air jets. 

Have you tried blocking the filter side of the carb while it is ticking over at 1800 rpm? If the revs don't drop there is an air leak somewhere. If they do drop the carb must be letting air through.
 Have the ports been modified at all? if so it's possible that the tool used has broken through to the rocker box or outside.

One more point, surely tick over speed is set by a combination of throttle stop and air screw adjustment?
Yes/No. The AIR Screw sets the mixture for tick over, but the TS sets the speed. If you try yo use the AS to change the speed from best you will get a bad pick up when you 'drive away' As you have found. Set Air Screw (for best) and try to leave it alone!
What I have seen in such an odd situation was that the cover for the pilot jet and the pilot jet are NOT seating. But you have tried several carbs.
The favourite 'trick' that causes this problem is the slide is 'hanging' on the cable and NOT dropping onto the Throttle stop screw.
Another NO point is the charging system, this cannot effect Tick over like this. 
TRY to remember- the bike runs on a battery quite well until the battery is flat, then the bike stops. Leave the bike for a while the battery recovers and you can run for another few miles (if lucky).

Not giving up on the diagnostics and figuring out the issue. Another very informative thread here.

-Pat

Not sure I understand the 'fix'. Your initial post says:-
            I've tried 3 different carburettors, all behave the same.