I have a brass Viton tipped needle on my mk1 concentric. From the below text taken from the Amal web site the idle & pick-up problems I have could be caused by a brass needle ( whereas elsewhere the site suggests a screw in pilot jet ( which I have) can also cause similar symptons !!!
Should we all be dumping them and getting the aluminium needle ?
Needle Valve
If your float chamber is fitted with a brass needle valve, you may find the valve sealing under its own weight, before the float has risen far enough to press it shut. Symptoms of this problem can be that the carburetter takes a long time to tickle, hesitates on pickup and does not idle reliably. A Viton tipped aluminium needle valve is now available that overcomes this problem. It is now fitted as standard equipment to all new Mark 1 Concentric carburetters.
I've just stuck the old plastic needle back in......& the idle is not much better...if better. However the plastic needle is fractionally shorter and therefore the float height higher and above the float bowl so too high ( unless that is you read hitchcocks guide to concentric carb tuning !)
Sounds like the primary galleries need cleaning out going by your idle issues. Lots of threads on this and videos on Youtube.
Yes that was my prime suspect for a while, but having checked/probed/cleaned the idle air circuit there was more or less nothing to clean out. As I have an adapted 2-stroke carb it doesn't have the press in pilot bush that is a prime cause of clogging in 4-stroke carbs.........but does have a screw in pilot jet that is apparently too distant and causes idle problems in some low manifold vacuum bikes.
I've changed just about everything in trying to set up the carb & it's set up to a spec that Dave Smith previously recommended as the 'starting point' for a 627 carb.
An Aluminium float needle will hopefully arrive for the weekend & stay up float.... however float level wasn't that far out, as far as I can work out, so not expecting it to make much difference.
I've ruled out air leaks now. So if the float needle makes no real difference it may be the screw in pilot jet, which seems the only option left......which would require a new carb.
( I seem to have a good spark and I changed the condenser )
A bit frustrating.........but I guess I'll just be more chuffed if I managed to sort it !!.... and might even ride the bike !!
any chance that someone else local would have the correct carb you could try?
to give you an idea Ive got a couple of monoblocks and 600 series carbs in the unit, a classic workshop near you might have similar. may not be an actual match for yours but with 15 mins could be made close.