Author Topic: A7 ST tank - taps 1 or 2?  (Read 818 times)

Offline Richard A

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A7 ST tank - taps 1 or 2?
« on: 18.07. 2017 13:22 »
Hi,

Another query. On my 52 ST plunger the fuel tank has two taps. Is one the reserve, or do you open both taps. If one is reserve I assume it makes no difference which one it is.

Hit problems on seeming flooding from carb float chamber on test run after rebuild with both taps open. Contacted Amal and they suggested it can take a while for the the tickle button to settle.

Thanks again

Richard

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Re: A7 ST tank - taps 1 or 2?
« Reply #1 on: 18.07. 2017 13:29 »
The taps are different; the main tap has a vertical pipe above it so that it cannot drain the tank. When the tank level gets to the top of that pipe the engine will falter and stop unless you turn on the reserve tap which allows more petrol to get to the carb. So, do not open both taps, (unless you are racing and need more flow of juice!)


Cannot answer your second question but it's not tap related; carb needs sorting.

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Re: A7 ST tank - taps 1 or 2?
« Reply #2 on: 18.07. 2017 14:35 »
My Star Twin has 2 identical brass push-pull taps. I use the right one as main, the left as reserve. I believe my ST tank setup is entirely original. Off the top of my head, the underside of the tank is not symmetrical and the left side of the tank is greater capacity than the right, giving a better reserve fuel quantity. If I get a moment, I'll have a peer underneath to remind myself. 
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Offline RoyC

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Re: A7 ST tank - taps 1 or 2?
« Reply #3 on: 18.07. 2017 14:40 »
The taps are different; the main tap has a vertical pipe above it so that it cannot drain the tank. When the tank level gets to the top of that pipe the engine will falter and stop unless you turn on the reserve tap which allows more petrol to get to the carb. So, do not open both taps, (unless you are racing and need more flow of juice!)
Well, I learn something new every day.
I always thought that if you only opened one tap that you had the other half (below the saddle) as a reserve.
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Online JulianS

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Re: A7 ST tank - taps 1 or 2?
« Reply #4 on: 18.07. 2017 16:47 »
Lever taps usually come in a choice of main - with a tube inside filter and reserve - with no tube.

The plunger type taps on the A7 A10 are usually without the tube. The right hand tap providing the main supply and the left hand tap the reserve, due to the differing shape of the underside. Dont put a tap with a tube in the left side of the tank for obvious reasons.

Usually you dont need both taps on.

You can see the difference in the underside of the swinging arm tap in the photo.

Offline Richard A

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Re: A7 ST tank - taps 1 or 2?
« Reply #5 on: 18.07. 2017 16:52 »
Gents,

As always, a font of knowledge which is just great.

Many thanks

Richard

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Re: A7 ST tank - taps 1 or 2?
« Reply #6 on: 18.07. 2017 16:53 »
The taps are different; the main tap has a vertical pipe above it so that it cannot drain the tank. When the tank level gets to the top of that pipe the engine will falter and stop unless you turn on the reserve tap which allows more petrol to get to the carb. So, do not open both taps, (unless you are racing and need more flow of juice!)
Well, I learn something new every day.
I always thought that if you only opened one tap that you had the other half (below the saddle) as a reserve.

Well, I suppose the tank tunnel does seperate the fuel to some extent. I don't know how much fuel would be left on the reserve side though after bumpy road surfaces and hard riding would tend to slosh the fuel about. Some motorcycles have a balance pipe between the two halves of the tank so the tap-with-a-pipe would be more appropriate.

Offline Butch (cb)

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Re: A7 ST tank - taps 1 or 2?
« Reply #7 on: 18.07. 2017 17:13 »
My lever type taps were identical. I would presume to only use the RH one ordinarily as additional to the stuff on the left of the tunnel I'd expected that use of the prop stand kind of ensures you get a bit extra on that side for reserve.

More recently I'd been having to use both for good running which, with my usual good hindsight, I now see I should have taken as an early indicator of pending trouble before taking to the rope.
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Offline Sluggo

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Re: A7 ST tank - taps 1 or 2?
« Reply #8 on: 18.07. 2017 21:28 »
I cannot say with authority what they were originally on these models (Hence joining to ask & learn) but in my experience with a wide range of vintage bikes they are in this instance nearly 70 years old and any number of petrol taps and fittings could have been installed.

I can take some photos of some I have in stock, but BSA tended to use a different type than the typical Triumph.  Many are push pull, but I have some that are also lever action and some, have 2 positions on the tap, one for regular and one for reserve.  The tap has 2 feed pipes at differing lengths.  BSA tended to use a larger bung fitting in their tanks for both single taps and dual taps as well and I try to keep in stock brass adapters as for a rider, I use a repop ewarts style tap instead for anything thats a rider and not a show bike. (Rivet counter angst). I have had challenges getting some of the original taps to work properly in useage without leaks or mishaps.  A static display bike does not have these issues.

The original taps in many cases used a cork seal internally and of course these are prone to leak when they dry out. The repops use Brass and viton and much more durable.  I have seen a few that were simply well machined brass fittings and refurb was lapping them in with grinding paste.    As noted, some bikes use a double banjo on the carb(S) or a T fitting so when using double taps you get fuel from both sides and in dual carbs, to both carbs.  But not all bikes do that.  The challenge when doing restorations is also getting leak proof joints and many of the originals used a crimped metal cup on the fuel lines.  IMHO these are a PIA and very prone to leaking once installed and then disturbed at a later date.  I use often times modern fuel injection type full width stainless worm clamps that look very similar to original fittings.

Modern fuel as well is hard on fuel lines and bits. Avoid alcohol-sugar water fuels if you can,, but I have been using a German car fuel and oil line with a cloth covered weave exterior that seems to hold up well...  I think we got the last batch from a NAPA warehouse and was BALKAMP brand, but need to source more of this and looking into it.  The clear tubing just does not seem to hold up anymore to modern fuels and is a source of constant problems.  I have built a few very early machines and customs using Copper tubing. The coils and finish is very cool and steam punkish, but in reality, copper work hardens and challenging to work with and in application with vibration the same result, metal fatigue, cracking and leaks.  But it DOES look super cool.

I have a wide selection of fiber sealing washers and some work well, but not always.  Triumph/BSA in early 1970s switched to a Stainless AN washer and a bi-metal sealing washer with a Nitral O-Ring on the ID.  Any time I can, I upgrade to these types.  They work really well.

When fitting a reserve type tap to a bike, I made it a practice of the following,. Right side is reserve. R=Reserve (easier to remember) (Assuming the tank is symmetrical )  On some taps the reserve has a R on them but not common,  Sometimes I will also drill a small hole in the lever that actuates reserve. This serves as a physical and visual cue as well its reserve.  This is my own little quirk, but the reality is own a lot of machines as well as access to many others or past shop work.  My memory was poor before, but now I have memory issues all the time so LABEL LABEL LABEL.  Sure beats struggling to reach a lever and try to remember which one when sputtering along and a large 18 wheeler or SUV bearing down on you out on the road.

Ill dig out my petrol tap collection this week..and post up a topic on this. Some of them I would be very interested in group feedback on what they are and proper applications.  If it can be used as a reference for others that would be cool.

That exhausts my knowledge on this topic
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Offline chotus52

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Re: A7 ST tank - taps 1 or 2?
« Reply #9 on: 19.07. 2017 08:00 »
There's no crossover pipes in these tanks so if you only use one tap, there will be fuel left in the other side as a reserve.
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Re: A7 ST tank - taps 1 or 2?
« Reply #10 on: 19.07. 2017 08:57 »
So, my setup, with the tap with a pipe may be a mistake as it will leave petrol in that side of the tank that could only be used if I lay the bike on its side.  *sad2*

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Re: A7 ST tank - taps 1 or 2?
« Reply #11 on: 19.07. 2017 13:14 »
Hi All,
Quote
I don't know how much fuel would be left on the reserve side though after bumpy road surfaces and hard riding would tend to slosh the fuel about.

Quote
So, my setup, with the tap with a pipe may be a mistake as it will leave petrol in that side of the tank that could only be used if I lay the bike on its side

With two taps without pipes the reserve amount on the second side of the tank  will only get you a couple of miles
DAHIK *sad2*

GB do you have two taps?
The tap with pipe is "normal" and without is reserve
Of course there are taps with two positions or levers that give main and reserve ??

John
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Re: A7 ST tank - taps 1 or 2?
« Reply #12 on: 19.07. 2017 18:47 »
Here is the original Star Twin setup - two brass push-pull taps with a copper joining pipe.
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Offline Sluggo

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Re: A7 ST tank - taps 1 or 2?
« Reply #13 on: 19.07. 2017 18:58 »
Here is the original Star Twin setup - two brass push-pull taps with a copper joining pipe.
That is an incredibly helpful photo. Bravo!  *yeah*
I have been amassing pictures similar to that of Original fitments for tech files and very hard to find pictures like that. (plus you cant see those details on complete bikes)
I have nearly a dozen photos of the twin carbs, chopped monoblocs and accy floats as well as bracketry for 1959 Bonnies as they changed them several times and a hard detail to document and replicate. (None of them worked very well, but still, a important detail).
BSA stuff like this is truly obscure so thank you for that.
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Re: A7 ST tank - taps 1 or 2?
« Reply #14 on: 20.07. 2017 06:17 »
Delighted to be of service. Years ago I was fortunate enough to get home from working overseas to find that my wife had bought a one-owner, low mileage, untouched and original Star Twin. It's not quite so untouched and original now but has a lot more miles under its belt.   
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