Thanks to this thread, and helpful posts from Mudnut and Dom, I've got Dad's ST30 Patrol running sweetly again.
The sensors inside the air cleaner control the warm air intake, and had been blocked off at some stage, probably in a misguided attempt to disable emission controls. This would explain why the vehicle ran poorly in cold, wet weather. I've restored these sensors and the warm air intake now operates correctly. The image below shows how the vacuum hoses are run.
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New plugs and leads. The old leads were the cause of the engine misfiring. They were the original leads, dated 1992...
The new plugs and leads are all genuine NGK.
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Thanks to Mudnut for the post about using a piece of hose to thread the plugs in. This really is an easy and safe method to ensure the plugs are correctly installed and not cross-threaded. I was concerned about damaging the alloy head, this method eliminates that potential issue.
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After a good spray with carby cleaner, the RB30 is revving easier as well. That carby cleaner is like magic.
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I'm very impressed with how smoothly this engine runs. It sounds like a well oiled sewing machine.
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