Ive noticed my RD350 when stood few days when I start it only runs on one cylinder briefly..then the other cyl starts backfiring and eventually runs. I have previously had a problem with the bike cutting out on uphill run until I cleaned the fuel cap of a lot of crap..then it was ok. I still suspect fuel cap as it closes very tight..this afterrelining the tank. So tried today after she stood awhile to start it with the cap off. Started better on both cylinders. So I suspect the cap too tight causing suction/breather problems into carbs (funny enough the first carb under the tank is the problem child). The cap also makes a significant gas release noise after a ride. Can it be the fuel cap?

If it's making a release noise then I would suspect so, install a replacement and see how it goes

Or get some graphite grease spray and utterly flood the thing out. Flush it several times, alternating between the grease and some compressed air. Work the lock mechanism a few times as well.

Just have some paper towels to hand, as the graphite will drive the crap out and it will get kinda messy...