Introduction to the Amal Concentric
If you’re in to British bikes, you know the name Amal. Often cursed with the vitriol reserved for Lucas components, the Amal carburetor is much maligned and misunderstood. It’s job is deceptively simple, deliver fuel to the motor. In practice this is much more difficult than it sounds. When you factor in that it was built down to a price, using questionable materials in such a way to be practical for myriad makes, models and purposes, the success of these instruments becomes more and more impressive.
The Amal Concentric carb was a development of the earlier Monobloc. It’s the most common Amal carb, used from 1967 on up. (It was later developed in to the Amal MK2 Concentric, but here were going to focus on the original Concentric.) The name was meant to showcase the fact that the main jet was in the center of the float bowl, as opposed to the somewhat compromised location in the previous Monobloc model with a side mounted circular float chamber. It was ubiquitous on British bikes through the late sixties and seventies. They fell out of favor, along with British motorcycles in general, as they were overshadowed by more modern and practical offerings from Japan. The design of the Concentric was well suited to the old style Brit singles and twins, but they did not work well with the multi-cylinder models being developed at the time. The main issues being the lack of any suitable arrangement for linking them together to a single throttle cable, and the overall height of the assembly. The attempt to rectify this issue on the three cylinder models from Triumph and BSA only highlighted these problems, making the carbs even taller and the throttle pull heavier. The Amal Concentric is still available today from the Amal Carburetter Co. and has received improvements in materials and production along the way.
The main advantage to using the Amal Concentric on bikes to which they were supplied as O.E. is the ease of fitting. The cables, mounting flanges, intake rubbers, air filter, and fuel lines all attach easily with no modifications needed. Fitting an aftermarket carb (such as a Mikuni VM) inevitably involves making cables, fuel lines, and possibly modifications to the air filter set up. There is also a large reserve of information on tuning and set up for the Concentric, allowing the tuner to start off with a base line set up that is close to ideal.
Another advantage is the ease of tuning with these instruments. In one example of testing, a Honda XBR500 single was set up with a 40 mm Amal, and at 6000RPM (wide open throttle) the carb went through seven sizes of main jet between a lean misfire and poor running due to an overly rich mixture. There are only a few needles listed for the Concentric, and tuning consists of changing the slide, main and needle jets along with the needle. In fact, the whole carb assembly, including fuel banjo, cable adjusters and gaskets/seals only carries 35 separate part numbers.
Minuses of the Concentric include rapid wear of the slide and body. The fit of the slide in the bore can become excessively sloppy within 10 or 12 thousand miles on high vibration machines. Without careful fitting, the body can also warp causing the slide to stick, sometimes wide open with exciting results. Any used carbs must be carefully evaluated before use, including measuring the needle jet and needle for wear.
A sticking slide can often be repaired, but specialist tools and skills are required. Never attempt to ride a bike with a carb in this condition, as it can be unsafe to the rider if the throttle hangs up in the wide open position, which it usually does.
Another common problem is the clogging of the pilot circuit, often due to modern fuels and dirty fuel tanks. The pilot circuit controls the fuel/air mixture while idling, and can lead to the bike being hard to start, and reluctant to idle correctly, if blocked. This can be rectified with a very thin wire, or a #78 drill bit mounted to a thin brass tube to gain length. After removing the idle mixture screw (The screw is on the outside of the body, entering horizontally) the passage can be cleaned with the bit/wire and then given a squirt of carb cleaning spray. This will often solve the problem. If not, the carb may require removal and more thorough cleaning.
Much of the bad reputation of the Concentric comes from frustrated owners dealing with worn out carbs on worn out and badly tuned bikes. Ignition problems and worn out, low compression engines also show problems which are often blamed on carburetors. Replacing worn out carbs is often a necessary step in restoring or recommissioning an old motorcycle, and this can usually rectify frustrating issues that persist after careful cleaning and adjustment of old carbs, including poor idling and hard starting.
In spite of its flaws and age, the Amal Concentric remains a great choice for the machines of its time. It’s easy to fit, tune and work on, and is not hard to find and remarkably affordable. The Concentric provides value, performance and originality for your old Brit bike. British biking and Amal carbs go together hand in hand, so don’t be afraid to get your finger wet!