Thanks for your replies. The accelerator linkage seems like a likely candidate. As for plugged-up banjo bolt and line to the ALDA, maybe I don't understand the operation quite right, but in the case I describe, the intake manifold pressure is at atmospheric (or really slightly negative, considering restrictions in air filter etc), so even if there is a restriction in the line from the manifold to the ALDA, there should be no compensation for additional fuel based on boost. Cleaning out the banjo fitting, line and switchover valve is a good idea regardless. Thanks again