The symptoms you describe don’t really make sense, so I want to be sure I understand them….You are saying that the engine idles fine until it warms up, then it won’t idle? But, it runs fine if you raise the throttle up? OK, well have you tried leaning the idle mixture adjustment a bit? Perhaps it is adjusted rich so it would idle well when cold, but then it would be too rich once it warms up…How about the carb syncronization? The engine won’t idle very well if the throttle does not close completely at idle…But, this would probably be more evident when the engine was cold, not warmed up…Nonetheless, these early engines had some strange linkages that tied into the carb butterfly that could be holding it open at idle… Is the engine getting up to temperature? They won’t idle worth a darn if they don’t warm up…And yeah, try new plugs, just cuz they might be new, doesn’t mean they are any good…The latest recommendation for plugs are the chamion L82C plugs gapped at .030″. Don’t have any experience with the NGKs, but guys have reported success with them… These engines are known for fouling plugs frequently.
I guess I would move on to ignition troubleshooting after checking the basics…Perhaps the ignition is breaking down once heated up. I would get a couple of those inline neon testers and put one on each plug (be very careful not to short out the plug leads on the block/engine pan). Probably best to run the engine early in the morning or late at night to make it easier to monitor the neon testers while running….Start the engine with the testers cold while it runs OK, then see what happens once it warms up and misfires… You will have to dig into the ignition if you can verify an ignition issue…
Please know that these engines had several updates that were supposed to be performed in an effort to alleviate plug fouling and ignition issues, hard to know what has been done to your engine. Let us know what, if anything, the neon testers reveal…
