My '87 740 UK spec. has started to misbehave.
It can run on both LPG and petrol, and on both fuels it behaves well at low load, but under moderate acceleration there is a slight loss of top end power. Under heavy load (eg. accelerating hard or pulling up a steep hill) this is more noticeable, and often accompanied by knocking when on petrol, and backfiring when on LPG (LPG has a much higher octane rating than petrol, and provoking knock is very difficult).
I have checked the ignition timing; the idle setting is right, it advances when revved, and the vacuum advance works when I suck on it. The valve timing is right. I have changed coil, leads, distributor cap, rotor arm and plugs. I have checked thoroughly for induction leaks.
The engine idles perfectly and revs well in neutral without any hesitation or roughness.
I've never had a failing ignition module before, so don't know if these are typical symptoms, but I can't think of anything else to check. Anybody any clues? Is there a way to test the module? How much is a module in the UK (I'm sitting down with a large glass of whisky)?