I redid 2 of these 12s last summer. I put new condensers , plug wires, plug caps in both engines. I had checked both old sets of condensers with my Stevens Ignition tester and they didn’t pass the leakage test. The old coils tested good in both engines. Both engines idled very well I will include videos.
You might consider putting new wires and condensers in it that’s not terribly expensive. I lost my notes from last summer when my hard drive crashed or I could tell you exactly what condensers I used when I replaced them but I think I used 1970ish chrysler in one engine and standard omc in the other engine. Both sets were almost exact to the old ones I measured with Stevens meter. I wish I did still have those notes I had a pile of info on them some of my posts are still here on these forums like the water impeller used etc just do a search if you would like to read them.
It is possible your condensers are breaking down at idle speed thus causing poor idling. If you got some spare condesers you might try them first.
Their is also a post on here somewhere showing how to check the top seal for leakage. You put like wd 40 on the top seal then turn the crankshaft and look for air leaking on the seal. I will look for it and try and post it.
Hope that helps,
Joe
Videos of the 12s I worked on last summer
https://www.youtube.com/watch?v=kqTx56TadjQ
https://www.youtube.com/watch?v=fwIxjs1Cq-o
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