I would focus on the scuttle drains then, I don't think you can't see them from below as the gearbox might be in the way.
Like I said before Hoover out all the big stuff or if you have a compressor maybe blow all the crap out from under the plenum etc.
Then boil the kettle and pour that into the scuttle area, when I first did mine there was a dribble to start with then the rubber most have softened with the hot water and it all came gushing out, repeated a few times to make sure I flushed as much organic debris out as possible.
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