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Wait, vacuum leak? That shouldn't make your engine run lean, the O2 sensors will detect it and compensate.
You need a wideband O2 and narrowband emulator to trick your engine into running lean, and then redo the fuel map so it's running rich under load so your engine doesn't blow. You probably want to add some extra premix to the fuel to help lubricate the engine.
I would not get too greedy, and try to get it working at something modest like 1.1 lambda. At 17 AFR, my FR-S engine was not running too great, and that thing has 12.5 compression ratio and port + direct injectors. The 22:1 Honda lean burn engines had a lot more tricks up their sleeve to make it work.
I think your idea about the spark plug is interesting though, it had crossed my mind before. Hope you get it working.
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