Do I need an additive or new valve inserts to run my 1956 B31 on unleaded petrol please?
Also, the fuel tap is under the nearside 'half' of the tank. I can't see how fuel gets to that half from the offside half. Does it simply slosh across.
Probably dumb questions I know!!
Thanks
B31's run fine on unleaded as they are. I did many 1000's on mine. Yes it just sloshes a cross, its the reserve!
So plenty of tight turns then?!!!
Thanks for the reply.
Most standard OEM BSA tanks have two petrol tank outlets, one each side of the tank, you may have non-stock tank or one is plugged?
I use my right side (o/s in UK) as my usual on/off and keep the left (n/s) as my reserve because if it coughs and splutters, when I know I am running low, I can simply reach down, 'on the fly' without releasing the twist grip, and switch to reserve. This gives me a few miles to find somewhere to fill up.
Otherwise if you run out you will need to tip the bike on its side with front wheel on the kerb to get fuel to slosh across.
Quote from: neil1964 on 26 Aug, 2026, 07:18 Most standard OEM BSA tanks have two petrol tank outlets, one each side of the tank, you may have non-stock tank or one is plugged?
Although all the s/a frame tanks I've seen have a tap on each side, my 1948 B32 has only one tap an the LH side, as also do all rigid framed B31,and B34 models. In fact all B31 and B33s are listed with only one tap up until 1955.
Although the rare late 1940's Goldie tank has two taps on the off-side of the tank, it seems to be the usual one seen is the tank with single tap (with reserve) on the near-side.
Good info on the early bikes chaps! I've seen the twin adjacent tap comp bikes.
The OP says 1956 and I'm pretty sure all twin tap models one each side.
Hi,
On the singles the carb. sticks out where the tap and pipe would be.
Dave.
Quote from: DAVE BRADY on 28 Aug, 2026, 07:15 Hi,
On the singles the carb. sticks out where the tap and pipe would be.
Dave.
I thought about adding a right hand tap to my M33's tank but as you say, there's no room for it.
Quote from: DAVE BRADY on 28 Aug, 2026, 07:15 Hi,
On the singles the carb. sticks out where the tap and pipe would be.
Dave.
There are taps both sides on the Gold Star tanks and they manage to clear TT and GP carbs.
As Derek says no issue on Goldies. In fact the tanks have offset bosses (right is forward of the left if you see a tank underneath, one of the ways you can tell an RGS vs Goldie.
Interesting replies. Thanks lads.
Hi,
How hard would it be to fit another tap? This would be useful for C15s and B40s as well.
Dave.
Too hard if there's no boss already there. You can always fit a tap with a built in reserve if that's the issue. Unless the engine is struggling for fuel at full power (as some twins do) then there's no benefit of an extra tap and a few disbenefits as its another place to leak and its quite likely not be used often so when it is it will weep.
There are cheap taps with reserve position on ebay. I have one fitted to my Norton Electra and although it's not very substantial it does the job and doesn't look too out of place.
Hi,
It is more about access to the fuel that is trapped there. It could be a few pints and enough to get to the next garage. Laying the bike almost on its side to get to it is probably not an option for many of us these days. Something like a small water butt tap fitting may work but getting the nut on the inside would be fiddly.
Dave.
I never was in the "lay it down" camp, I always went for violent shaking which seemed to work. If its really a concern I think the easiest fix is a balance pipe.
I have (as far as I know) a B31 or B33 tank on my M33. The M33 tank is shown in parts books with a balance pipe, near the front of the tank; probably visible on the inside from the filler hole. The tank I have does not have a balance pipe, but I don't think it would be too difficult to install one as it's possible to drill the tank where the hole can be seen from the filler hole, perhap using something like an adaptor bush (https://www.classicbikeshop.co.uk/fuel-tap-adaptor-3-8-thread.html) on each side to mount the balance pipe into.
I went for an easier fix and bought a tap with a reserve. But be warned, as bought my tap had a reserve range of about six miles; I had to extend the tube out past the filter gauze, and now rely on an inline fuel filter. I checked the range on reserve by measuring how much fuel came out before I went anywhere, and checking again the extended tube gives me about 23 miles on reserve.
It wouldn't be difficult to fit one of these adaptor bushes, screwed into a hole drilled in the tank then soldered into place, to allow a second tap to be fitted.