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The Amal Concentric MKI

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The purpose of this homepage is to explain the function of the most widely used carburetor in classic biking. A carburetor has been thoroughly modeled in Solid Works, a professional parametric solid modeling program, and the model has then been sectioned and rendered in PhotoWorks.

The idea is to make this site a dynamic source of information for rebuilding and troubleshooting of this simple and rugged carburetor that has received a somewhat bad -and unjust- reputation over the years.

I would therefore encourage anyone that would like to contribute to contact me with photos, info, tips and tricks. Full credit will be given to all contributors.

Richard Trygg

Short history of the Concentric

The Amal Concentric was introduced in 1966 and was a long awaited improvement of the Monobloc, then in production for 30 years.

There were a number of reasons for the introduction of a new carburettor, one of the most important being that the Monobloc was expensive to produce.It was also to tall for a number of new bikes and hard to set up twin carburettors due to difficult acces to inside adjusters.

Also, the Monobloc had some problems when set up on two-strokes-although it worked well on four-strokes- and since there was a renaissance in two-strokes in the sixties, Amal needed a more two-stroke friendly carburettor to offer new clients.

The list of specifications for the new carburettor looked liked this:

The first major manufacturer to use the new Concentric- or series 900 as the senior managers at Amal preferred to call it- was BSA who installed it on their top of the line model A65 Spifire in 1966 and thus set the ball rolling.

A few alterations were made after the introduction that are important for serious enhtusiast to know about, namely:

1. The pilot jet was removed on all four-stroke carburettors and replaced with a fixed bush close to the outlet.

2. A drain plug was fitted to the bottom of the float bowl, providing access to the main jet and enabling to clean out the bowl without removing it.

3. The needle jet/main jet asssembly for four-stroke carburettor was altered to give easier fuel lift at cranking speed:

So if you have problem setting up that swap-meet bargain on your four-stroke watch-out, it may be set up for a two-stroke!

Cut away view

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Split view

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How it works

The Concentric has four steps of adjusting that all need to be set up correctly to give a smooth running engine.The amount of opening of the throttle valve is used to identify the different steps.

The pilot jet system

Up to 1/8 open...

...the pilot jet system is responsible for metering the right amount of fuel to the engine.
This is done by means of a fixed orifice, either a jet submerged in the bowl as in this illustration, or on a newer carburettor a bushed pilot jet closer to the outlet.

Note that the screw regulates the amount of air being mixed with the fuel.

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Throttle cut away

From 1/8 to 1/4 open...

... the amount of cut-away of the throttle valve controls the amount of air let into the carburettor.

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Needle position

From 1/4 to 3/4 open...

...the needle controls the amount of fuel let into the carburettor.
The needle also controls how the carburettor works during acceleration.

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Main jet size

From 3/4 to full open...

...the main jets controls the fuel supply.
Although the supply of fuel goes through the main jet even at smaller throttle openings, the amount is diminished by the metering effect of the needle in the needle jet.

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Tuning the carburettor

Since the carburettor is best tuned while the engine is pulling, it is important to find a deserted stretch of road preferably with a slight up-gradient.
Also, it is important to check that all parts in the carburettor is in top condition as worn needles and needle jets can cause problem as well as worn throttle valves.

It may be wise to clean the carburettor thoroughly, meaning removing the blind plugs to get access to all channels in the carburettor.
Even in a new carburettor one can find burrs and dirt left there from production as were the case with the two new carburettors installed on the BSA Firebird as shown on this site.

Tune in the following order

1st Main jet

If at full throttle the engine runs heavily, the main jet is to large.
If at full throttle, the engine seems to have better power when the throttle is eased of or the air valve is slightly closed, the main jet is too small.

When you are satisfied with the main jet size, check the color of the spark plug after a full throttle run. Turn the engine off quickly.

2nd Pilot jet

With the throttle between closed and 1/8 open.
Screw out the throttle stop screw until the engine runs slower and begins to falter, then then screw pilot air adjusting screw in or out to make the engine run regularly and faster.

Now lower the throttle stop screw until the engine runs slower and begins to falter, adjust the pilot air adjusting screw to get best slow running. If this this second adjustment leaves the engine running to fast, go over the procedure one more time.

3rd Throttle cut-away

If, as you take off from idling there is a spitting from the carburettor, slightly richen the pilot mixture by screwing in the air screw.
If this is not effective, screw it back again and fit a throttle with smaller cut-away.
If the engine jerks under load at this throttle position and there is no spitting, either the jet needle is much to high or a lower cut-away is required to cure richness.

4th Needle

The needle controls a wide range of throttle openings and also the acceleration. Try at first the needle in as low position as possible, i.e with the clip in a groove as close to the top as possible.
If acceleration is poor and with the air control partially closed, the results are better, raise the needle by two grooves.
If there is a great improvement try lower the needle one groove and leave it where it is best.

Finally, check the idling once more.
Tuning twin carburettors

When tuning twin carburettors it is important to keep a few things in mind:

When setting the pilot system, detach one sparking plug lead wire and set the pilot jet on the other cylinder, as a single unit and then reverse the process to the other cylinder

When both sparking plug leads are connected the engine might idle slightly faster than desired in which case a readjustment of the throttle stop screws are necessary.

It is of utmost importance that the idle speed is exactly the same on both cylinders as many low speed problems are caused by difference in idle speed on the cylinders.

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