Thanks for your suggestions, but no to all:

Head has been rebuilt.
Have checked for leaks - none.
Converter cleaned out recently following LPG contamination.

Perhaps someone could tell me how to tune the damn thing.

Mark

maybe needs new diaphragms in the converter or mixer

tuning is done via the 2 srews on the mixer....you have a n idle screw and a full load screw...

mixture is also controlled by the FCV valve...when these die it can lead to very poor mixture control....listen to it closely and you should hear a rapid clicking

i had a pin hole in the idle diaphragm and had similar symptoms, i duno what yours looks like but there should be a hole on the housing next to the idle screw. try covering that up with your finger and see if the idle levels out, if it does your diaphragms shagged. full load screw should have nothing to do with it, have a fiddle with the idle screw and see if it gets any better.