One thing to check are the bleed pipes, they tend to perish and throw derv out.
They are on top of the engine by the injectors. All link across the engine and back into the derv supply.
Got new pipes for my one (54 reg) just got to get round to fitting them. Just pull on, pull off.
As always, there is one difficult bit that may require removal of one of the metal pipes that goes across the engine, (thats why I've left my one).
Must do it before Zed10, or I know what will happen!!!!