VFR (5th gen) hesitation/stalling after applying lots of throttle. Sudden Issue, ran fine before.

A few weeks ago the bike (1999 vfr800, 5th gen) ran fine-ish. I drove around about 100km, washed it, refueled it with aral premium (without ethanol) and then let it sit for 2 weeks. A few days ago I wanted to go for a ride and it stalled on me when i wanted to drive out on the streets and I'm now stuck with the following problems/symptoms:



- Bike ran fine. Then after 2 weeks standing still problems began
- starts only without high idle adjust (usually it should be down (i.e. higher idle) for the first cold start, but it doesn't start then.)
- when I pull the throttle slightly the rpm rises, sometimes dramatically but it's impossible to hold steady. The engine fights against it
- when I pull the throttle a bit more sudden the engine struggles and/or stalls
- when I start the bike and let it run its rpm slowly lowers until it dies
- rough idle/engine (always has been, but maybe a symptom? thought it's normal as it's a v4 and not i4, but maybe it has been rougher as it should be)

what I already did:

- as the battery was old anyway I bought a new one. No difference. I knew it was a shot in the dark, eh.
- I changed the sparkplugs for new iridium ones before winter. the bike drove fine after i changed them
- all the wire connectors on the wiring harness at the back of the bike look fine, no heat dmg or anything that could hint for a electrical problem
- I tried running without the airfilter, no difference in symptoms

I'm kind of at a loss of what it could be, especially since it happened from one ride to the other with a few days only in between. Before I start dismantling the fueltank and pull the fuelpump etc. and pretty much brute force my way through everything maybe some of you have a better, more logical way of figuring this out. Or maybe you already know the problem from the vid/symptoms I described. Any input is much appreciated!

As it's the start of the new season no workshop has time for the machine atm, and I would love to be able to solve it with some help of you.

 

Chance for H2O in the fuel? We get that a LOT around here in the E-free stuff as almost no one buys it.

 

Are there any FI flashing codes? I doubt this is electrical if not.

Has a mouse made a home in your airbox?

No start with the "choke" lever pulled but running with the idle screw turned in makes no sense as the two are doing the same thing; the idle screw opens the starter valves more, as does the lever. When you did rev the bike up it sounded healthy enough.

It seems like a vacuum/airflow issue to me; I would take the airbox and filter out for a look, maybe a vacuum hose has come adrift or split, or a starter valve has seized? If you had the equipment, a starter valve synchronisation might be useful.