Hi Tyler,

Much thanks in advance for the reply & assistance - its very much appreciated. :-)

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Governor should have no affect - provided it isn't surging or rev spiking I would negate it.
Yes isn't doing this, so thank you for eliminating.

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I still reckon its running rich.
My diagnostic process would be:
What spark plug is in it? bp5es or 6es? If 6es, swap to the hotter 5es for cleaner burn
The current plug was an NGK BPR6ES. I've cleaned it regularly but upon removal it had far more carbon on it than I've previously seen. SO a tad unusual.

In trying to adjust the carb last season, it's possible I got it well off where it should be - as mentioned I was using a cheapish RPM monitor and couldn't really find improvements from changing. So unsure if my attempt to get back to 'default' levels have been less than ideal.

I will try and grab a few of the NGK BPR5ES as you have recommended - thank you.

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Confirm with a honda dealer (with the model number) if a snorkel requires a smaller main jet to be fitted (I believe it technically should)
I will see how I go - the airfilter-snorkel assembly is a factory one that i removed from a GXV160 engined Masport that I had (subsequently sold off). I only run the large bottom single airfilter in it rather than the two that one can do. I use it purely to ensure that the air intake is higher up and well away from the dust zone. The top snorkel intake only has a layer of flyscreen inside it - so offers very little to no resistance to the airflow at all.

So valid point - but if I'm only running the single airfilter, even with the snorkel I can't imagine this would really make it much harder for the engine to draw the air in vs just the single bottom filter. I'll note that I've run it like this for 5yrs or so - and not had this problem occuring previously, which makes me think that it's unrelated to the snorkel with single filter. But is a good catch by you.

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Inspect bore/head with an endoscope and confirm carbonisation levels.
Ok will do - I just happen to have a cheap and cheerful endoscope so I will see what I can ascertain but I suspect judging by the sparkplug that it's going to be grubbier than I'd have hoped.

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In any case, the first thing I would do is start running it on 98ron with a Polyether Amine based injector cleaner. This ingredient will help most with decarbing. Most cleaners don't have it, Caltex techron, stp complete fuel system cleaner (silver bottle) or rislone High-Performance Injector Cleaner all have it. As does Yamaha ring free (go to a boat dealer)

Point taken - just curious and I know this is a point of contention for many - but why the 98RON recommendation? I always use 91RON on anything that specifies it and ensure that fuel stabiliser is added to the storage container as soon as I get it home. Fuel stored in a flammables cupboard thats in a cool area.

I've used injector cleaner in it very regularly BUT I suspect it might have been rubbish - which I kind of knew being that it was a Valvoline product.
I will keep a look out for some of those ones you advised. Much thanks for that.