1960 BSA A10 Super Rocket carb dilemma

[dstahnke]

Hey ya'll,

I'm in the process of building a 1960 super rocket. On to the carbs... I've got two carbs that were in a parts lot that came with the BSA. The two carbs are a 389/202 and a 389/203 which I believe are off a 1963-65 A65 Rocket and a 1964-66 T120R respectively.

Here's what I found:                                                  Bore         Pilot   cutout  Main      Jet needle     position
389/202   BSA 650cc 1963-65; A65 Rocket (USA)   1 1/8"             25        3.5        310    0.11               3
389/203   Triumph 650cc 1964-66; T120R (USA)        1 1/8"          20      3              260   0.11              3

I believe I need this:
389/046   BSA 650cc 1960-63; A10 Super Road Rocket 1 5/32"   25   3      420   0.11                2

or is it this:
389/047   BSA 650cc 1959-63; Road Rocket & Spitfire (Export)   1 5/32"   25   3    290     0.11     2

I guess I don't understand why the big jet difference between the super rocket from England and the export model main jet size. I'm guessing one is with filter and the other is without?

So, can I use a modified 389/202 to make roughly a 389/046 by taking the cutout of the 203, using the 25 pilot I have, buy a 420 main, and move to position 2?

How much effect with the 1/32" in bore difference have?

Is there an upgraded jetting due to new style gas (USA) or a better setup than what's listed?

Thanks in advance.
Dave

[duTch]

 Hiya Dave-- I know enough to be dangerous about carbs and forgotten what the 202/203 might relate to,  but the usual pilot jets for A10's is a 25 and if the  0.11 refers to the needle jet , I think bit big- factory starting point for A10 is usually106 but I'm running a 105.... not sure about mainjet block ...otherwise seems more or less normal.... that's about all i can suggest

[muskrat]

G'day Dave.
I agree with duTch, 106 needle jet. You are correct about the larger mains for no air filter. So that depends on what you use. The 1/32" increase in size was for more top end power so unless you want to race I wouldn't worry. Main thing is the match of the carb bore to the inlet manifold. Sml carb on a lge manifold OK but a lge carb on a sml manifold not so good.
Your conversion from 389/202 is a good place to start. You will be tuning once on the road for altitude and fuel quality.
Cheers