Needle Jets are best replaced as a matter of course especially if the age of the jet is unknown. They are the most vulnerable of the jets to wear with only .001” between sizes and a worn jet will cause very rich running between 1/4 and 3/4 throttle opening.
Replace the needle if there are signs of wear or corrosion, particularly on the parallel section. Check that the needle clip grips the needle securely and is not distorted in any way. The split in the clip is part of the design.
Main jets are resilient to wear but old jets showing signs of oxidisation or any etched deposits should be replaced as their ability to pass fuel may be affected.
In 1966 it was decided to standardise the 376/100 type jet for use in Monobloc, Concentric, GP and TT carburetters and changes were made to the jet to ensure compatability. The groove machined across the hexagon of the jet on the right indicates that it incorporates the modifications. New jets without the groove are not genuine, and are often extremely inaccurate.
Always use new genuine Amal gaskets and O rings to prevent leaks.
Checking the Specification
Manufacturer's original standard carburetter settings are available in the Amal Catalogue and on the Amal Website. However unless a bike has retained absolute originality any changes to the engine, air filtration or exhaust arrangements (including modern replicas of original exhausts) may require adjustments to the carburetter.
If you are not sure that you have a suitable carburetter the numbers on the raised pad on the side of the carburetter can be used to identify it. “R” or “L” stamped on the pad indicates a left or right handed carburetter. A “B” originally indicated that the carburetter contained the upgraded needle and needle jet parts.
A three digit number beginning with a 6 or a 9 indicates the Series and bore size of the carburetter e.g. 624 indicates a 24mm 600 Series carburetter, 928 indicates a 28mm 900 Series carburetter.
The Specification Number stamped below the three digit number identifies the jetting and the original model of bike the carburetter was fitted to, e.g. 62 below 930, read as 930/62 identifies a 30mm 900 Series carburetter for a 1971-72 BSA B50SS.
It is also important to ensure that all the parts in your carburetter are correct for the engine type. The original Concentric carburetters used the same components for 4 Stroke and 2 Stroke and carburetters are often found to contain a mismatch of components.
4 Stroke 2 Stroke Norton Triumph
As a general rule the most noticeable difference between a 2 Stroke and 4 Stroke carburetter is the spray tube, the brass tube visible as you look through the carburetter bore. 4 Stroke spray tubes are cut off straight, 2 Stroke spray tubes are cut off at an angle sloping towards the mounting flange. Some later 850cc Norton Commandos use a spray tube with a square cutaway step while Triumph Triples use an angled spray tube cutaway from the mid point of the spray tube rather than right across as in the 2 stroke version. 2 Stroke and 4 Stroke bodies are not interchangeable and will not run correctly if used on the wrong type of engine.
When first introduced all carburetters were fitted with a detachable pilot jet screwed into a threaded hole in the bottom of the carburetter body. This arrangement is retained for 2 strokes but from 1968 was replaced for 4 strokes by a bush pressed into the gallery behind the pilot air screw.
The detachable pilot jet caused problems with some 4 Stroke engines. The remoteness of the jet from the two pilot circuit outlet holes caused starting and idling problems on bikes with lower manifold vacuums, and could cause stalling during deceleration due to a weak mixture. The introduction of the pilot bush moved the pilot jet nearer to the outlet holes to overcome these problems. The bush has a flow rate of equivalent to 20cc/min. Carburetters fitted with a bush retain the threads originally used to fit the removable pilot jet.
The removable two stroke pilot jet is stamped with a number from 15 to 200 indicating its flow rate in cc/min.
In 1969 a new needle jet, needle jet holder and needle designed specifically for 4 stroke carburetters was introduced to improve low to mid-range performance. The differences between the parts are;
|Needle||2 21/32" (6.74cm) long - 2 identifying rings||2 9/32" (5.79cm) long - 1 identifying ring|
|Needle Jet||13/16" (2.06cm) long cross-drilled||11/16" (1.74cm) long|
|Jet Holder||7/8" (2.2cm) long||3/4" (1.9cm) long|
Check to ensure that the parts in your carburetter are a matched set as above. A conversion kit (622/235) is available to update to the new parts.
2 Stroke and 4 Stroke needle jets are stamped to show their sizes in inches from 0.105 to 0.125. Four stroke needle jets have an air bleed hole drilled through the hexagon section. Two stroke jets have no cross-drilling.
Needle Markings. (600 and 900 Series Concentrics)
Needles have inscribed rings or letters stamped at the head of the needle above the clip grooves. All needles use a constant diameter down to the head of the tapered section. The taper helps to manage the transition from the Needle jet to the Main Jet and varies according to the type of engine or fuel.
|Markings||Where used||Part No:|
|1 inscribed ring||2 Stoke, 600 Series||622/063|
|2 inscribed rings or 'U1'||4 Stroke, 600/900 Series||622/124|
|3 inscribed rings or 'X'||2 Stroke, 900 Series||928/063|
|4 inscribed rings||Norton 850, Triumph 250cc||928/104|
|5 inscribed rings||Triumph T160||622/278|
|Y||Alcohol, 600 Series||622/099|
|Z||Alcohol, 900 Series||928/099|
Throttles Slides are stamped with their identifying size on the bottom surface. Typical markings will be either 622 for a 600 Series slide, 928 for a 900 Series slide, an MB number which is a foundry mark, and a stamped number from 2 to 5 indicating the height of the cutaway on the inlet side of the slide.
The angled cutaway on the slide manages the transition from the pilot circuit to the needle jet when the bike is running. Smaller cutaways will richen the mixture, larger cutaways weaken it.
|600 Series P/N||900 Series P/N||Stamped as||Height of cutaway|
Main Jets are calibrated and numbered according to the rate of flow in cc/min which is stamped on the side to identify them. Jets cannot be reamed but must be replaced if a larger size is required
Many Amal parts are easy to copy but difficult to replicate. The simplicity of tuning and proper operation of the carburetter reflects the subtleties of the design. There are many crude and poor quality copies of Amal parts available, made from incorrect, rapid wearing materials, which will cause difficulties in the tuning and operation of the carburetter. All genuine Amal parts are now individually packaged and sealed for easier identification and are carry a 12 month warranty from the date of purchase.
Re-assembling the Carburetter
(a) Fit a new O ring to the throttle stop screw and install the screw.
(b) Fit a new O ring to the Pilot Air Screw. Screw the screw fully home then back it out between 1 ¼ and 1 ½ turns for an initial setting.
(c) Using the Amal Jet Key install the Needle Jet in top of the Needle Jet holder and tighten it securely. Take care not to over-tighten brass jets as they can break.
(d) Using the Amal Jet Key install the Main Jet into the bottom of the Needle Jet Holder and tighten it securely.
(e) Install the jet assembly into the bottom of the Mixing Chamber and tighten to a torque of 10lb/ft.
(f) For Two Strokes carefully install the Pilot Jet into the bottom of the Mixing Chamber.
(a) Fit the spindle and Needle Valve to the Float and lower the assembly into the float chamber.
(b) Fit a new gasket to the float chamber, slide the securing screws through the holes and gasket to align the gasket and check that no part of the gasket interrupts the movement of the float.
(c) Lower the Mixing Chamber onto the Float Chamber, ensuring the float chamber is the correct way round and the pilot jet (if fitted) is aligned with the recess in the Float Chamber. Ensure the float assembly does not become dislodged.
(d) Tighten the Float Chamber screws securely.
(e) Install the Float Chamber Drain Plug with a new fibre washer.
Throttle Slide and Top
(a) Insert the choke cable into the Mixing Chamber Top and assemble onto it the brass choke tube, the spring and the alloy air slide. Compress the spring and hook the cable nipple into its recess in the end of the air slide. Fit the Air Slide into the Throttle Slide.
(b) Insert the throttle cable into the Mixing Chamber Top and slide the Throttle Spring over it. Compressing the Throttle Spring, hook the cable nipple into its recess in the Throttle Slide.
(c) Assemble the Needle with the Needle Clip in the required groove.
(d) Compress the Throttle Spring, drawing it out of the Throttle Slide and to one side. Drop the needle into the slide, engaging the end with the central hole and drawing it down so that the needle clip sits in the circular recess. Lower the spring carefully so that it does not disturb the needle.
(e) Fit the Throttle Slide into the Mixing Chamber body. Watch to ensure that the end of the needle enters the needle jet and does not become snagged on the side, dislodging the needle clip. Engage the lug on the slide with the slot in the body and lower it home.
(f) Ensure the lid is fitted correctly with the pip on the side of the lid pointing to the rear of the carburetter and securely tighten the screws.
(g) Adjust the Throttle Stop Screw so that it just begins to engage against the slide when the slide is closed.
(h) Fit a new O ring into the recess in the manifold flange.
(i) Open and close the throttle to ensure that the slide moves smoothly in the body and returns sharply when the throttle is released.