1966 Atlas 389/689 Monoblock carburetors

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Hi All.
My 1966 Atlas has 389 and 689 Amal monoblock, both with the floats.
Atlas fuel tank has only one fuel tap.
Please, i need to know the fuel line from tank to and between the carbs.
When you fit 389 float with 389 chopped you join the two carburetors from the bottom fuel pipe banjo and from tank to 389 float banjio up on the float body.
But the two float carbs have not the bottom banjos but they have on the float body each.
Thank you.
Piero
 

texasSlick

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You will need a double banjo such as Amal Part Number 376/410. This is for 5/16 ID fuel hose.

If I had 389 and 689 carbs, I would fit a balance tube between the float chambers. This could be done by using the banjos at the bottom, or by tapping into each float chamber at the bottom and linking both carbs with a 1/8 inch or 3/16 inch ID tube.

The balance tube will compensate for any float level difference that may exist in the two carbs, and give a more regular idle.

Slick
 
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IIRC The 66 atlas is the year with the 389/689. The concentrics very soon came out. and did away with monoblocs. There are approximatel a little over 150 variants of big series monobloc and of those, about 15 series/variants of 389/689. several for norton and some BSA's and a few others I do not recall.
They should be easy enough to pipe together...
Final point is the 66 atlas has a different air cleaner body, with dimples to accomodate the pair of inlet connections that interferes only once with the chopped monoblocs .

Dave
63-65-66-68 atlas:cool:
 
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You will need a double banjo such as Amal Part Number 376/410. This is for 5/16 ID fuel hose.

If I had 389 and 689 carbs, I would fit a balance tube between the float chambers. This could be done by using the banjos at the bottom, or by tapping into each float chamber at the bottom and linking both carbs with a 1/8 inch or 3/16 inch ID tube.

The balance tube will compensate for any float level difference that may exist in the two carbs, and give a more regular idle.

Slick
Hi.
You suggest to fit the bottom balance tube like 389 float and 389 chopped.
They came to me with pipe from fuel tank to upper left 389 double banjo from that to 689 upper single banjo.
If i join 389/689 with the bottom balance tube, i cannot join the upper 389/689 banjo?
 
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IIRC The 66 atlas is the year with the 389/689. The concentrics very soon came out. and did away with monoblocs. There are approximatel a little over 150 variants of big series monobloc and of those, about 15 series/variants of 389/689. several for norton and some BSA's and a few others I do not recall.
They should be easy enough to pipe together...
Final point is the 66 atlas has a different air cleaner body, with dimples to accomodate the pair of inlet connections that interferes only once with the chopped monoblocs .

Dave
63-65-66-68 atlas:cool:
Hi.
I have the stock air cleaner that is different, as you say, between monoblock and concentrics.
 

texasSlick

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Hi.
You suggest to fit the bottom balance tube like 389 float and 389 chopped.
They came to me with pipe from fuel tank to upper left 389 double banjo from that to 689 upper single banjo.
If i join 389/689 with the bottom balance tube, i cannot join the upper 389/689 banjo?

Piero:

Upper left 389 double banjo to 689 upper is main fuel feed supply.

If you can, add a balance tube at bottom of both float chambers. This is to make same the float levels in both carbs. A small diameter tube will be sufficient.

Slick
 
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Hi.
I have the stock air cleaner that is different, as you say, between monoblock and concentrics.
There are 3 filters:
1- chopped monobloc 1 dimple
2- Left / right 389/689 monobloc 2 dimple
3- concentric 0 dimple needed but will also work OK with either filter above (I don't have one of these for my 68)


If you can, add a balance tube at bottom of both float chambers.
Do you feel this is necessary for monoblocs and not for concentrics too?

Dave
63-65-66-68 atlas
 

texasSlick

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@dynodave Re: Balance tube between carbs.

The chopped Monoblocs are inherently balanced, being supplied off one float.

A balance tube between 389 and 689, or between Concentrics is not necessary, but may be helpful. Setting float levels on Concentrics is rather straight forward, and a technition working carefully is likely to get the floats the same, obviating the need for a balance tube. Setting the floats on Monoblocs is less definite, working with shims to raise/lower the needle seat.

My thinking is most tuners of 389/689 Monoblocs will set both carbs with the same shims and call it good, but a variance in float level may be present. Such a variance will be evened out by a balance tube.

Slick
 
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