wash out molasses with hot water, tip out the water then straight away spray the complete inside surface with WD40, then tip out the WD40, blow any droplets of water out with compressed air and then let it dry out somewhere warm and dry. When in use after that, keep it full of petrol (to avoid condensation forming on the inside). If you don't ride it much, change the fuel for fresh fuel every few months.
If it is a museum bike, just do the WD40 treatment and dry it out, or do the WD40 treatment and then keep it full of oil.