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B175 Amal Set Up.
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Mannanan
Leigh On Sea, Essex. Uk.
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April 18, 2020 - 6:18 pm
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Honest, I have googled it first but can’t find a definitive answer. What was the reason for BSA specifying four stroke parts in the carb for the B175? The D14 I understand was set up with two stroke parts so what was so very different about the B175 that required this somewhat odd change? Do we know of any other two strokes of that era that had four stroke Amal carb internals specified by the manufacturers.? 

Ed.

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nickjaxe
Runcorn Cheshire UK
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April 18, 2020 - 8:00 pm
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Cant answer your question...but my B175 when I bought it had a brand new B175 4 stroke spec carb on it with all the correct 4 stroke bits...think there are only 4.

I didn't like the way it ran so after lots of tweaking I converted to carb to D14 full 2 stroke spec....never looked back goes great.

I just did not understand why put 4 stroke bits in a 2 stroke care to go on a 2 stroke engine.

Nick.

My Bantam video              https://www.you.....jpOFmzRZRI

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sunny
Norfolk
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April 19, 2020 - 8:20 am
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your  ancer   is  as  folows      B S A   also    fited  a  4 strock  carb  to  the  Bantam  D 5 ,7   & some D 10  modles       the  carbs  nedle   is  set  high  at  4th    from  the  top  on  some     carbs        as  a  staring  point        all   sizes &  setings   are  a starting  point  as each motor   runs  or  wears  out  in diferant  parts  &  places     it  taks  up  to  much  mouny    to be  spot on  so  left   the   dificult    stuf   to  us   puters   to  do  the  paying  out  once   we  wer  hocked  

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cocorico
Central France
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April 19, 2020 - 9:54 am
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As you say, you haven't found a definitive answer, like many questions. Surely if AMAL are supplying them that way, they must have that specification? dunno I think the nearest to definitive is Sunny's opinion - BSA decided it was the way to go - maybe it worked well on a brand new machine fresh from the factory?

1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine now running.

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BASIL
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April 19, 2020 - 11:32 am
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Ok here we go again, the carb for a B175 should be an Amal 626/17 it has a fixed pilot jet a 3 and a half throttle slide a 4 stroke needle and jet holder with a 180 main jet and a slash cut jet tube, the only thing I have found is maybe the 180 main jet is a bit too rich and have cut this down to 170 on my B175s. Regards Basil.

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cocorico
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April 19, 2020 - 1:31 pm
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Well, Basil has been giving us this information since at least 2014 and I think you can accept his word! I also found the following on 'Tapatalk' from 2012:

"If you look in original BSA parts books and you will see the Amal part numbers for the 4T 107 needle jet and 2 ring needle.
As I own 2 B175s with this set up I asked Wilf Harrisson, latter day BSA export manger he told me this was to crisp up the mid range with bigger main jet to improve mid range acceleration."

1956 D3 running, lights to sort. 7 other bikes in the Barn. 1950 D1 engine now running.

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Mannanan
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April 19, 2020 - 2:01 pm
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Yep! My carb is and always has been set up as per the book and Basil is confirming. Not disputing anything that Basil has said, the question was not how but why. That last paragraph Coco is a decent explanation and sounds very plausible. 

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Blue Heeler
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April 19, 2020 - 2:46 pm
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Don`t forget that the previous D14 model, of which I have a brace, is a bit peaky/over-tuned for a basic Bantam engine. High compression ratio with a pretty carp combustion shape, angled spark plug & not very meaty `head. Surprised it goes as well as it does.

From Bob`s find, it seems that when they built the detuned B175 motor, it was a little gutless, so they had to patch things up?

As Sunny points out, the Monobloc 375s on the earlier 175s are 4-stroke carbs.

How about after the quarantine, booking one of Lincolnshire`s many RAF runways so Basil, Nick & other B175 owners can line up for an acceleration comparison 🙂 Oh hang on, I haven`t thought that through, some have big-bore engines.

What`s the weight difference between a standard B175 piston & a big-bore 185cc one? I`m guessing they`re the same, to retain balance.

Blue

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Sponge
Lancashire (A chip shop somewhere near Preston)
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April 20, 2020 - 10:01 am
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I like the idea of booking a runway to do some fine tuning. I have several bikes that need a good fettling. Plug chopping, stop start, brake burnishing, running in, acceleration tests etc........ its difficult to do on modern roads.  Keep me in the loop for that one please. 

Perhaps a club day out ?

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nickjaxe
Runcorn Cheshire UK
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April 20, 2020 - 12:39 pm
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Hi Sponge hope your well mate.

Question...whats brake burnishing....sounds interesting.

Nick.

My Bantam video              https://www.you.....jpOFmzRZRI

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Sponge
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April 20, 2020 - 1:16 pm
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Hi Mate, hows it going ?

 

BSA terminology for bedding in your brakes. Reference BSA service sheet 8/71 BRAKE BURNISHING. ( aimed at unit singles but equally relevant to all models quotes " brakes should be bedded in progressively over the first 300 miles. This is done by gradually increasing brake lever pressure during the period, and braking from progressively increasing speeds." 

Each application at 1/4 mile intervals etc

It goes on to state " correct burnishing will give an approximate minimum lining contact area of 50% which qualifies the published brake performance figures"

Obviously not recommended on modern roads but really useful on a track/runway

 

Sponge

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nickjaxe
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April 20, 2020 - 2:36 pm
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Interesting thanks sponge...I will send you a PM.

My Bantam video              https://www.you.....jpOFmzRZRI

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