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#829370 11/10/20 2:09 am
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JoejBSA Offline OP
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Hello all,

Just picked up a 1957 D3 bantam. Bike runs but previous owner didn’t know much about it. First question is , fuel can has the 6 1/4 inch or so tube, is it accurate to pour 2 full caps of those into a full tank of fuel on this bike? Seems like a lot but don’t want to blow it up by not adding enough. Also assuming any modern 2 cycle oil is well enough?

Second question is after bike runs for a few moments and warms up , it starts revving up to high rpms . Throttle cable is not getting stuck , it has a 523 AMAL carb on it. Is there a reed valve anywhere on these bikes? What can I adjust or is it just that double ended needle float getting stuck? Hoping someone has experienced and cured this

Thanks in advance

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Read the cap, I might be mistaken in your instance but I believe the measurement refers to caps per British gallon (4.5 litres)

With modern oil you could go leaner, but I always prefer to use what bsa recommended. It still sparks and runs and that’s what matters to me.

No reed valve on any bantam

I’m not familiar with the 523, but have you checked the jetting, timing and spark plug being used?

When I first built mine many moons ago I assumed that the settings in the carb were right, they weren’t. The engine warmed up and started revving it’s lil heart out (dieseling) turning the ignition off wouldn’t stop it, we had to whip the filter off quick and choke it to kill the motor. A change of jetting solved this and it never did it again. (Back then I knew nothing about bikes and a visit to some friends solved it for me) So I am not too sure of the specifics.


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68’ A65 Lightning “clubman”
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Beware of too slow idle.
My brother used to have a Bantam D14, sat at traffic lights engine idling boing-boing-boing-bb-boing-boing. Lights change he put it in gear, grabbed a handful and let the clutch out. The bike shot off BACKWARDS


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Originally Posted by Andy Higham
Beware of too slow idle.
My brother used to have a Bantam D14, sat at traffic lights engine idling boing-boing-boing-bb-boing-boing. Lights change he put it in gear, grabbed a handful and let the clutch out. The bike shot off BACKWARDS

That's canny, I was thinking something similar when I wrote my reply. My friend ended up with my first bike and first bantam. It would run it best when the ignition was more retarded than stock. Intact you could almost time it by ear. With a lightweight rider it would really zip... (For a bantam)

We were in Belgium and this friend was Infront and when we stopped for traffic my other friend rode aside me. Having a natter as you do then I heard the exhaust note of said bantam change. And I remarked to this other friend who was also on an A65 that it didn't sound right. Then when friend on the bantam let the clutch out it went backwards. A real WTF moment. She stopped and eased the clutch out again and come back again. She thought we were pulling her backwards as a wind up. Quite funny at the time. Killed the ignition and kicked it over again and it ran fine after that.


Two real good bantam gurus are Alan in NC and Shane in Oz (I think I got their handles right) I played with D14's for a little while but the knowledge of these gents outstrips mine.


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Originally Posted by Allan G
Two real good bantam gurus are Alan in NC and Shane in Oz (I think I got their handles right) I played with D14's for a little while but the knowledge of these gents outstrips mine.
I was sure I'd posted a reply a couple of days ago, but here goes again.

Thanks for elevating me to the rank of guru, but I thought Alan and Allan were the Bantam men. And, of course, "Villiers" is Mr Two Stroke.

First up, fuel mix.
BSA quoted a 20:1 mix, using SAE 40 oil. Modern synthetic two-stroke oils can be mixed much leaner than that, partly because they lubricate better, and partly because they mix better and stay mixed. I'd be inclined to go with somewhere between 25:1 and 32:1. The reason for 32:1 is that it's 1/4 (Imperial) pint of oil to 1 (Imperial) gallon of petrol. The ratio used comes down to ease of calculation to a very large extent. A US gallon is smaller, so 1/4 pint to a gallon will still be in the general range. If the spark plug whiskers too often, you can lean out the mix.

Second, factory settings for the D3 AMAL 523 carb.
According to Roy Bacon's BSA Singles book (yeah, I know, but the carb appendix seems to be mostly correct):
Main jet: 90
needle jet: .107
needle position: 3
slide: 5

Third, racing after start-up.
The mixture needs to be enriched considerably for a cold start, either by tickling or by closing the strangler.
Once that initial rich mixture has burned off, the mixture is leaning off excessively, which is causing the racing.

If it's only doing it with the throttle largely closed, it might be as simple as the mixture screw.
Otherwise, it's most likely to be an air leak somewhere in the system downstream of the carb. Early Bantams had a stub-mounted carb. These can suck air around the stub, so it might be worth removing it and refitting with a sealant such as Loctite 515.
The crankshaft seals tend to dry out over time and suck air into the crankcase, especially if the engine has been sitting. Replacing them involves at least a partial engine strip, so you don't want to go there until you've confirmed that everything else is alright.

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You will find a PDF of your riders handbook at www.icenicam.org.uk
They are a Cyclemotors, Autocycles, Mopeds group with a lot of BSA material there.
Sorry there is no direct link so you will have to search for it in the downloads section


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Now having dug out a copy of the riders hand book to post the source , one naturally goes to the effort of reading it.
And because information posed on the web lasts forever, one should make sure information that is on the web is accurate.
From the specification page the fuel : oil ratio is 20 : 1 which is 2 caps per UK gallon .
As per Shanes response that was of engine oil, not modern ashless 2 stroke oil.
Also as pre Shane's response you should be able to halve the oil when using modern oil so one cap per gallon will be fine particularly considering the USA gallon is a bit short.
However here is the rub
Thinning the oil means the actual volume of burnable petrol will increse per unit volume of fuel passing through a jet.
So the result is the engine should run rich on a leaner mix thus the jets may need to be reduced for use with modern 2 stroke oil
Because modern oil is designed not to burn it becomes deceptive because the exhaust does not blow massive clouds of blue smoke when the oil is excessive .
It is just as bad for the engine to have too much oil as it is to have not enough.
Excessive oil will cause carbon to build up around thepiston, head & exhaust port and heat will cause this build up to become quite hard
Not only does this restrict the movement of the exhaust gasses but chunks tend to break off & get wedged between the piston & the bore where they cause deep grooves to be cut in the cylinder wall and can get under the ring and jamb them .
Because of the relative low Hp of the engine and because it has a cast iron linner you will not notice it till a lot of damage has been done.

Blown crankcase seals show up by making the engine hard to start when it has not been ridden for extended periods because the oil drains off the seal lip and prevents an effective air tight seal being made till enough air: fuel has leaked out to regain some sort of a seal .
Once the engine has been rinning for a short while, almost full sealing is returned & the engine runs fine .
Hot starts will be easy then a month latter, it will be near impossible to start again .

This is different to hard starting cold Vs easy starting hot which is usually a timing or carb problem .
If you want to test the seals you need a leak down tester .
They are easy to make and you can get some really cheap ones on line ( remember cheap is seldom good ) .

In 2 stroke parliance crankcase compression is termed primary compression & cylinder compression is called secondary compression .


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