Try this website for inspiration -
https://www.andybuysbikes.com/bikes.htm

If you're confident with modern bikes you'll find old British bikes a doddle. Everything is easy to get to and makes sense. My B31 is a bit like a meccano bike. You've just got to look at it and you can see how it works etc.
Sounds like you'd love it. I'd go for a single if you can handle the low top speed - buckets of character and dead easy to work on  .
Also you can have a nice sounding exhaust without breaking the law 

If I was local Id be happy to help but I'm many miles away. It might be an idea to give an idea of model year as there were significant changes to the B31s over the years e.g. from rigid, through plunger to swingarm suspension, different seats, different headlamp arrangements etc. They can look very different. Might be better if the bikes a similar model to the one he remembers etc

My first classic bought around a year ago is a C15T. I wanted it for a bit of green laneing. It's been in bits for most of this time due to me rushing in and buying something with issues (I still love it). In the meantime I've bought a B31 (350) with plunger frame so I have something to ride. It has also needed a lot of tinkering but I'm doing lots of miles and enjoying both the riding and tinkering. It's rewarding to get it running nicely.
These bikes are the least powerful I've owned since passing my test about 25 years ago but the B31 is the favourite out of all the bikes I've owned. It is great fun to ride around backroads. I live in Cornwall and we have plenty of those to enjoy. I plan routes to avoid dual carriageway or motorway. Modern traffic speeds on these roads are too much for the majority of old British bikes (I believe - someone correct me if I'm wrong), the reason being the limited number of gears. My B31 tops out at about 65 but it's revving hard at that so it cruises at 55 comfortably - not good on dual carriageways etc. I think if you wanted to do motorway miles you might be better off with a later bike like a 5 speed Bonneville or perhaps something like a Yamaha XS (think these are 5 speed but could be wrong).
My last reasonably modern bike was a 1995 CB750 - must be the dullest bike on earth!

It would seem sensible that the number of links quoted is for the installed chain so including the split link.

On my B31 i had to compress the spring in the vice and put some cable ties through it so I could get the nut back on. No idea if that helps at all.

Cheers, the carb is fairly new and fitted by the previous owner I guess. Just a bit of fine tuning to get the best out of it hopefully. 🤞

Thanks Neil,
I have the 106 needle jet, 160 main and #3 slide. No air filter.
Amal recommend the same but with a 3.5 slide.
It seems lean up the top but playing with the air slide makes no difference.
It was running fine in cooler temps but now misfires occasionally, particularly when hot and been sitting for a while. I see fuel vapour coming out of the carb after shut off so I think the carb is getting too hot. It's also difficult to get a good idle and has to be adjusted to suit the temp at the time.
I have a new coil on order (it has a thorspark system) and a gasket set for the carb as well as a thicker Tufnell spacer to cool it down a bit. It has new plug, cap and lead also.
I've planned a plug chop route for full throttle so that should answer that.
Fingers crossed.

If so - I'd be interested to know your jet, needle and slide cutaway please.
I've mine in bits at the moment and getting some contradictory info regarding main jet size. Also whether or not you have an air filter.
Thanks

Thanks people. All done a couple of weeks ago on the B31, including some oil tank repairs. The C15T is without its top end for now so no worries on that one 

On my B31 with thorspark the ignition coil is under the top frame tube.

Thanks, bit of a lottery buying without decent pics of the inside (which is hard to do). Just contemplating making one. Not too fussed about originality as long as it's healthy. Anyone else made one?

For context I cleaned mine out with paraffin and a few nuts shaken around inside. Revealed a load of pinholes including in the breather tube. I cut the tower off the top to replace the breather tube as it was full of crud and rust. With the rust brushed off the inside and outside of the tower it's as thin as lace and has plenty of pinholes. I tigged it back on where I could but had to use filler for now to fill the pinholes and fill along the tack welds. Not good but should do until I come up with a better solution. Have considered making a new tower out of tube/hollow steel sphere but it seems like flogging a dead horse.

As mentioned elsewhere I have a thorspark type on my B31. It was on there when I bought it. Faultless so far. Happy to get some pictures etc if you need them.

Anyone know where I might source a B31 plunger oil tank (I know about eBay etc)? I'm doing some repairs on mine but would like to replace it with a less corroded one.

Any news Fraz?