Super Rocket Carb

[Topdad]

Hi Glen, been following this post and glad you've sorted it . My interest was due to the fact that when I first got the bike back on the road I had mono 389 carb fitted , 420 mainjet as per johns ,it ticked over fine pulled initially well but only accelerated when it felt like it, I got completely fed up with and not then having the wisdom that is this forum available, I fitted a concentric , I've had no problems but if I'd had your perseverence I'd have ended up with a better carb . best wishes Bob

[worntorn]

Hi Bob

I  have done the same as you, fitted a Concentric for now. It's a new one I had on the shelf.   The previous owner had also fitted a Concentric, a smaller one that became very clogged from sitting with ethanol fuel inside.
Long term I would like to purchase a new Monoblock so the bike has the proper Carb on there.

Glen

[trevinoz]

Glen,
              What is your speedo's ratio which should be printed on the face?

[worntorn]

Directly under the needle shaft is the number 1650

Further down on the face there is S.525/3

Glen

[duTch]

Depending on the gearing from standard, wouldn't it normally be about 1490?...Mine is a1620 (I know it's the wrong one), and reads about 10% under true speed. ..

[trevinoz]

There's your problem, Glen. Wrong speedo.
With standard gearing it should be 1420 or 1440, can't remember which. I will have a look later.

[worntorn]

Thanks Trev, will try to find the correct speedo.  According to the numbers from the GPS, the 1650 is showing 60 MPH at 65, so this means a 1440 should show about 68 MPH at 65, which is probably the way BSA wanted things. The odometer should be very close.

Glen

[trevinoz]

Glen,
                    1450 seems to be the ratio, that's that the bloke who renovated mine used.

[worntorn]

I borrowed a 1548 ratio chronometric from another bike here and tried it on the Super Rocket. With the GPS mounted both speedo and GPS gave identical readings to 75 mph, the highest speed I tested for.
Also, the odometer was bang on for an eight mile test.
Could it be that BSA purposely used a lower ratio in order to give optimistic speed readings? I guess the fuel economy would also appear to be about 7 % better than reality, not a bad thing for sales?

Glen

[Greybeard]

Could it be that BSA purposely used a lower ratio in order to give optimistic speed readings? I guess the fuel economy would also appear to be about 7 % better than reality, not a bad thing for sales?

Yup; still going on today; compare your modern speedo with a GPS, which is very accurate, (but only on the flat). Mine over-reads by about 10%.

[bsa-bill]

(but only on the flat).

We've moved on a bit from that scenario, When I retired 2008 the GPS sytem on tractors could take account of angle and adjust accordingly and still be accurate to within 4 inches (width )