first of all, since this is my first post on this forum, let me say that I have made great use of the DIY articles and collective wisdom on this site since I aquired my first Goldwing in July. So a collective thank you to you all.
So I bought a 1994 GL1500 Aspencade in July, an American import in the Netherlands with 43000 miles on the clock. Hadn't been ridden much if at all the past years, and the PO obviously wasn't the most meticulous type (how else to explain the presence of what seems to be blue mouse poison in the airbox to accompany the newspaper bits, i.e. mouse nest, under the intake manifold?

I have already done quite a bit of overhaul, including the following:
- New air filters (main, sub, cc)
- New coolant, oil and brake/clutch fluid
- Carbs cleaned (except for the pilot screws since I lacked the d-shaped tool

- Replaced carb boots and many vac lines
- Timing belts replaced
- New spark plugs
- New tires
- New brake pads
- New front wheel bearings
- And other small niggles
The reason for getting in the carbs was a surging idle, one leaner and one richer cylinder bank based on spark plug color, and an obvious vacuum leak. Turned out to be one of the carb boots being loose. So replaced those and like 20 vac lines, all the ones in blue here:
Reinstalled the cleaned carbs and airbox with new filter, synched the carbs and the surging idle is gone

However, and here is where we come to the problem, the bike runs fine at high rpm's but below 3000-3500 it still has a form of the dreaded hesitation/flat spot. Sometimes a stumble, sometimes a weird sort of low/no-torque fluttering depending on rpm's and, I guess, its current mood. Mind you, this is only under load, it does not hesitate when in neutral on the center stand! Here is what the hesitation looks like:
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I have scoured the interwebz for the right explanation, but I haven't found it yet. I have already done some of the often proposed solutions: timing belts, plugs, cleaned carbs, vacuum lines,... so I thought I'd do some more telling diagnostics.
- I checked the operation of the slide valves and accelarator pump, and they seem to be working as intended. Correct me if I'm wrong here:
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You can also hear that the bike has no problems revving in neutral.
- I thought it might be an ignition problem, so I checked to see if all spark plugs sparked by slowly removing the leads and all of them arced, and the further removal caused irregular running of the engine.
- I checked all the plugs again. They now look similar on both sides (so that left side vacuum leak no longer leans out one bank over the other), but they seem to indicate an overall lean mixture (white tips). They all look something like this: Keep in mind that these plugs are about 150 miles old.
- I tested whether the mixture was lean or rich by blocking off the air intake at the airbox. rpm's increased when I blocked 80-85% of the intake opening. Bike immediately stalled when covered fully. See here:
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This seems to indicate a) that the bike is running lean, and b) that there are no other huge air leaks.
However, I'm at a bit of a loss as to what to conclude from all of this, and what my next steps should be. And so I turn to you, Goldwing guru's. Lend me your collective wits and insights, and we'll get it back to proper health (I hope)!