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I am no expert either but I would check these. Because bad cylinder fires up when you increase throttle I would not think it is piston or rings. But I would check compression anyway. My gut says fuel or spark.
Pull sparks and look at colour and put new ones in.
See if you have spark on the bad cylinder by removing both and grounding tips against engine (bare metal) and turning over engine.
I would also check battery voltage when engine off (12.4v ish) and then check voltage with engine at high idle 1800rpm (14v +). Might not have enough electrical juice at idle, but spark test will show that.
If you have no spark then test coils. An easy check just swap them over (literally swap them not just leads to spark plug) and see if fault follows coil. If it does it could just be the lead or cap You can often solve that by removing cap from lead, chopping a 1cm off lead and replacing cap on lead.
Doubt it is rectifier as one cylinder is good.
If spark good then I would look to fuel.
Doubt it is fuel lines or pump as fires up fine with throttle. Pilot jet on bad cylinder or float blocked. Just give it a good clean.
A split or cracked intake can also mess up idle, spray carb cleaner (combustable type) on inlet rubbers when idling and see if that makes her fire.
After that I would start scratching my head. Hopefully Marcel sees your post and can solve this one.
Good luck and tell us how you get on.