Intruders also have problem with the fuel cap as they rattle due to poor seals. I solved the problem, once and for all, by lining the underside of the cap with a thick rubber sheet. Now a little down force on the cap is necessary to lock and unlock the cap. Perhaps you can try this method.
Just drained off half the petrol, and top up again. If it starts and runs, just give it a good ride to clear the system. I don't think it's necessary to do carb wash, and I haven't even heard of "wash the whole engine system"