MonzaMan wrote:I gather that rubber seat is not replaceable because I did install a carburettor kit from Dellorto and there was no such part in the kit...
Correct. It's not in the overhaul kit. It's sold as an integral part of the enricher piston, part no. 0615.27.238. Cheap and still widely available - Ducati Paddy, Ducati Vintage Store, Moto Guzzino etc.
MonzaMan wrote:Any suggestions as to how one would check if it is seating correctly?
Attach a small bore tube to the pick-up jet where it protrudes into the float bowl and try to blow up it. If any air passes, the seal of the piston is cooked.
MonzaMan wrote:Does anyone use a VHB carb with these singles?
(Not sure what you mean by "these singles", but ...) Yes, on my 450 SCR.
Idle jet size? 45.
MonzaMan wrote:On some forums (Laverda, Guzzi), I have read about the air passage being fouled and constricting air and causing a rich idle mixture or flooding. I am going to have a look in from the top and try soaking that passage with carb cleaner...
This is a problem most usually associated with the PHF range of carbs, and normally manifests itself by a lack of response when tuning the mixture screw.