340GL carb problem

I have an '89 340GL that has a problem I am hoping someone can recognise and point me in the right direction with. The car starts fine from cold. But it idles erratically - sometimes hunting around 9,000, other times racing up to 50,000.

If I can get the car warmed up, then generally it is manageable, with the revs ranging at idle from 9,000 to 15,000, with just a little hunting and hesitation. The car hasn't lost any power or anything, it can cope with the fast lane of the motorway as well as it ever could.

On braking, however, or changing down (necessitating taking my foot off the throttle), then the revs can drop dramatically to 2,000 or so - with the outcome that they either return to around the 9 -15,000 mark again, or the engine stalls.

If the engine stalls midway between cold and warmed up, I am in trouble. Most times it won't start. It has a go sure enough, but it backfires... and I have to wait 20-30 minutes or so to start it fresh from cold, as it were, when it will start first or second or time, but with symptoms as outlined from the top of the page.

If anyone can help or point me in the direction of a reconditioned or secondhand carb (I live in Manchester), I would be be delighted. The carb itself is a Solex 28-34 CISAC. Cheers.