It was a 1969 "G" plate bike, bought from a small ad in the local paper off an old chap who used it soley to get to and from work. He had just retired & didn't need it any more. I paid £55 for it IIRC in 1977 at a time I was earning £27 a week gross & half my take home pay was claimed by my mother.
I have very fond memories of it - it was great fun to ride & sounded great. I don't recall having any spark plug issues with mine - as I was rather impecunious as a first year apprentice, I'm sure I would have remembered if I had had to buy a lot of plugs. First job was to have a neighbour braze up the perforated exhaust down pipes to quieten it down somewhat, followed by a good clean up and "rattle can" respray.
Sadly it didn't survive long in my less-than-tender ownership - it met it's end buried deep into the back of a nearly new Mini one winter evening on my way home from college. I hit the thing dead straight - the bolt-on downtube on the frame creased the front mudguard down the centre & I ended up spread-eagled across the car roof, kicking out one of the car tail lights on my way up there.
Turned out it was a young girl driving, who had only passed her test the week before & Daddy had bought her the new Mini. She had panicked when a truck had turned across her about 400 yards ahead & stamped on the brakes. A new Mini stopped a darn sight quicker than an old YL1 - even if I had been expecting her to stop, which I hadn't. Her father did kindly take me home in his Merc, though, and my father collected my bent remains of the bike in his company Transit later thae same evening.
I ended up buying the wrong second hand front end - I got an FS1E fork & should have got a YG1F set. They kind-of fitted, but needed spacers knocking up to centre the wheel. I ended up selling it to some "yoofs" as a field bike.
Like most ride-to-work bikes, a rare sight these days - ridden into the ground & disposed of usually.