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I use NGK DCPR8E spark plugs in my 900, I think those are the right ones. I also take care (now) to order the suffix type 4339, which are the version with the threaded terminal, so you can unscrew the ally nipple and use them bare threaded.
It looks like you have exhausted all the avenues possible to avoid a carb strip and if the cooler and oil lines are spoiling the fun then it makes it even more justified. ( but could the lines be removed from the carbs and be safe to run with the tap turned off?)
Given what you say about an earlier conversion to the 900 engine makes me wonder if the carbs are right for the engine or maybe they have been played with?
I think it's well worth checking all the jet sizes and so forth... Blueprinting the carbs for want of a better expression.
The problems you are having in the lower rev range and lower throttle settings seem to indicate pilot jet problems. Unfortunately there is no pilot jet as such, it's just a drilling which blocks very easily with corrosion from standing and is somewhat harder to unblock! Ultra-sound cleaning is a good option.
You've been very patient and methodical so far. I think you stand a better chance of success with know clean carbs set to the base settings.
Since you have got the electrical side sorted and new plugs fitted, I would suggest checking the valve clearances while you have the carbs and cooler and other gubbins out of the way. The valves and sparks should always be in perfect order before attempting tuning the carbs.
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