PREMIER AMAL CARBS

11 Dec

In recent years the Amal Mark 1 Concentric has under gone a series of changes. Seeing that we have not previously written an article on the subject it is long overdue. The Premier Concentric carb (referred to simply as the Premier for the remainder of the article) has been out in some form or another since 2013. Initially it was introduced as a 600 series carb (smaller body with 26mm bore). Several years later it was available in both 900 and 600 series styles. In May of 2021 aluminum replaced the Zamak (zinc alloy) that Amal had used to cast the bodies since the carburetor design was introduced in 1967.

So what is a Premier carburetor?

The Premier carburetor is an improved version of the MK1 Concentric. Characterized by the following improvements: wear resistant aluminum anodized throttle slide, new solid StayUp sink proof float, and an updated pilot circuit with removable/ tunable jet.

Anodized Throttle Slide:

New aluminum slides (right in image below) are precision ground and anodized to reduce friction. These have replaced the zinc “pot metal” slides (left in image below). The result is less wear and a longer lasting carburetor.

StayUp Sink Proof Float:

Originally developed for the British Military. The material specified was used by the S.U Carburetter Company for military installations in the 1980s, where a guaranteed fail safe operation was critical. Due to the close link between S.U and AMAL today, the secret behind the material has been released and upgraded for the manufacture of this float, making it virtually unsinkable. The new ethanol resistant foam floats are solid and impossible to puncture. Holes caused many of the original hollow nylon floats to sink. Setting the float height was hard with the old plastic float because the float needle seat had to be raised or lowered by driving with hammer and punch. The new Amal StayUp float has stainless steel tangs which can be bent to alter the fuel level.

Nylon float after fuel exposure (left), Nylon float new (center), New StayUp float (right)

Removable/ Tunable Pilot Jet:

Opposite the pilot air screw the new pilot jet can be removed, replaced, and resized (change size). Easy removal allows for cleaning of the jet and the carburetor low speed passage. The original pressed in bushing 622/107 was just one size fits all (.016 inch ID) . Factory jet size is #17 for most applications, #19 for Norton specific applications. We have actually found fitting #19 gives better results on most Triumph models. The number of rings tell the size:

1 ring #15, 2 rings #17, 3 rings #19, 4 rings #21, 5 rings #23

Number #15 pictured below. Note the single ring on the far right end of the jet.

Aluminum Bodies:

There had been delays with MK1 Concentric carburetor production for many months in 2020/2021 sales years. The root cause of this was damage to the special tooling used to cast the Zinc bodies. Upon further investigation it was determined that the die was beyond repair and would need to be replaced. Looking into the costs of replacement Amal decided to change to aluminum. Zinc is cast under high pressure in a polished smooth mold. This allows it to capture the finest details in a part. You may recall carburetors made between 2004-2010 having a “crack” above the inlet side. This was actually a crack in the high pressure casting die. The result was a raise defect in the casting which could be found not to be a crack by running a finger nail across it.

Moving to aluminum construction has several advantages:
Weight savings (nearly a pound has been shaved off), improved wear characteristics that the aluminum slides had not fully addressed, and lower tooling costs (allowing for more consistent supply and lowering initial production costs). The choice to move to aluminum has had the benefit of increased production volume. Using investment casting (such as the lost wax technique) many bodies can be cast at one time from a single casting tree.

Changes made to the MK1 Amal Concentric have brought it up to date to meet the demands of vintage motorbike rider. Ensuring that it will be around for years to come.