ANATOMY OF THE AMAL GP.

1

Cable operated ‘choke’ is an ‘enrichment device’, adjusting the slot which admits the ‘primary air’ to perform preliminary atomisation of the petrol from the needle jet.

2

Lowering the slide while riding lets you try a richer mix at any throttle opening for testing. Cable must be adjusted so that the slot is fully clear for normal riding.

3

Tapered throttle spring compresses flat at full throttle to minimise body height. A hole in the carb top takes the bit of the needle that sticks out of the slide.

4

Instead of the throttle cable nipple clipping into a keyhole slot, it fits into a brass plug that screws into the slide for added security against it vibrating loose.

5

The slide of a 1½in GP is nearly 2in diameter; it allows the offset needle to operate alongside the carb choke (bore) rather than causing an obstruction to air flow.

6

The needle and main jets are similarly offset and the partly atomised fuel is conveyed via an angled spray tube into the venturi where it’s picked up by air stream.

7

Float height is important; the separate float chamber is adjustable. Too high causes flooding, but if it’s too low the air stream will be insufficient to draw the fuel from the needle jet.

8

Float chamber height needs to be adjusted so fuel level inside matches the bottom of this circle mark, which represents just about the top of jet as shown.

9

Some people remove the plug and raise the float until fuel drips out, but as the hole inside is smaller than the circle I find that too high, making starting difficult.

10

The original race carb needle jets were made from steel to resist wear; modern replacements are brass, in varying sizes – this is a .109.

11

The needle should be GP3 but the steel one here is stamped 3GP6 for use with a racing ‘megaphone’. It will make a standard bike run weak.

12

GP uses standard type Amal main jets as on Monoblocs/concentrics. My bike needed a 370; gentle riding gives 70mpg, but can drop to 30 ridden hard.

13 Jet block screws must be kept tight or fuel could creep beneath and up the sides of the jet block into the airstream, richening the mixture.

14

Pilot system is unusual in that the tapered needle controls the petrol not air supply, so it has a finer taper. Adjustment can be very sensitive.

15

Screwing the needle down hard is bad, a step, bend or wear ridge on the taper can stretch the jet – it’s not the replaceable threaded brass collar...

16 ... but this tiny hole in the casting. Visibly larger on the needle side, mine’s been stretched, which will make it more sensitive. I’ll have to watch that...