An itch I had to scratch

I gave up biking 2 years ago at 68 partly because I dropped my triton and hurt myself with a hole in my foot, and partly because she who must be obeyed said I was a silly old fool for thinking I could regain my lost youth by doing what young men do !!! so the triton went to a new home, and ever since I have lusted after another but decided firmly on electric start so I am in the process of negotiating on a1975 Mk3 interstate which is running but not very well ! it has been stood for a couple of years and I am hoping it is carburation , it starts easily and runs but on low revs a cylinder is missing once up to 2500 rpm it seems to be running on two but I cannot be sure because it is not under load and from the video I have seen even while missing no smoke anyway I hope to be seeing it in the flesh by the end of the week and if we agree on price I will be reverting to my foolish hobby, apparently it has a new starter fitted with receipt and does start on the button but apparently the switch is hit and miss is this a common problem or simply remove clean and replace, also could someone let me know the correct carbs and jets and if there are known improvements on the market and if so what they are, has anyone had any problems with this new diluted fuel that has been forced upon us and is the Esso premium still free of this because I looked at a premium pump the other day and it had a sticker with a 5 on it which I took to mean ETHANOL Many thanks in advance for your help and advice and if all goes well you will be hearing from me more often Chris   PS I now have it home and will attempt to post a pic