It's been a while since I've messed with my carbs, but the circled (black circle) item looks like the accelerator pump plunger. Not likely to be your problem. Mine doesn't have the vacuum solenoid, but I'm pretty sure you've circled a leak with your white circle. There's an excellent chance that's your problem.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.