Impco BRC is good stuff so you have either 1. a sticking injector or 2. weak ignition as ozscott says. I won't say "Lucas Ignition module partial failure" is the most common cause of weak ignition as you have already convinced yourself it is all OK. A Bosch ignition module conversion will alleviate a weak spark issue. I do dozens of these as it is the most common failure problem with D1s on gas.
Lucas ignition amplifier replacement by Bosch 024
A sticking injector would be traceable by shorting out individual plugs until you find the cylinder that doesn't slow down the idle speed. Carby cleaner can be forced backwards through the injectors by removing both the inlet and outlet hoses and blowing some carby cleaner back through the injector with compressed air. Not commonly a problem with BRC injectors but not unheard of.
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