Technical Articles

Amal

This is under construction – more as time allows!

AMAL Concentric Carburetors

To better understand the Amal carburetor it is best to look at it as two carburetors in one body: An Pilot (also known as idle) carburetor and a Main carburetor.

At idle the two pilot circuit transfer ports serve different purposes. The smaller, just behind the slide delivers fuel into the intake port. The larger, just under the back edge of the slide, delivers air into the mixing chamber just under the two transfer chamber.

At idle the two pilot circuit transfer ports serve different purposes. The smaller, just behind the slide delivers fuel into the intake port. The larger, just under the back edge of the slide, delivers air into the mixing chamber just under the two transfer chamber. Idle Carburetor (also known as idle carburetor)
The pilot circuit consists of an adjustable air supply (pilot air screw), a fixed pilot jet (replaceable on the Premier series) and a shared fuel supply.

The fuel delivery by the pilot circuit is through two orifices either side of the back edge of the slide. The smaller. primary orifice, is just to the rear of the slide. The primary orifice’s single function is to deliver which it does at all throttle openings. The larger, secondary orifice, is just under the back edge of the slide. The secondary has two functions: At idle it draws additional air into the fuel/air mixing chamber just under the to orifices. This air mixes with the air from the adjustable pilot air screw blending with the incoming fuel from the pilot jet.







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As the slide is lifted both of the two transfer ports deliver fuel into the port. This helps making the transfer from the idle carburetor and the main carburetor without a hesitation.

As the slide is lifted both of the two transfer ports deliver fuel into the port. This helps making the transfer from the idle carburetor and the main carburetor without a hesitation.

As the slide is lifted, and the venturi vacuum under the slide starts to balance with the intake manifold vacuum, the secondary orifice turns from delivering air into the mixing chamber to delivering fuel into the venturi.  This aids the transition from idle carburetor to the main carburetor.















 

17-CurvedFloatBowl-1web If the the top surface of the float bowl is warped the gasket will not be able to seal the pilot circuit fuel transfer port. The bike will be hard to start and will not idle properly.For the idle carburetor work none of the fuel passages, air passages, primary and secondary transfer ports or pilot jet can be blocked.  To be one with these carburetors you must come to understand that jets are not just another hole in the body.  It is just as important as having a clean flowing pilot jet as it is with a main jet.   More on that later.

A warped float bowl from over tightening the float bowl screws. The main pilot fuel supply passage passes fuel from the float bowl to the main body. It  is located in a place vulnerable to air leakage. The problem is the fuel passes through the float bowl gasket. If the bowl is warped the gasket will leak air into the pilot fuel passage. No fuel will be delivered to the pilot circuit. If the bowl is warped it must be flattened, or replaced.

pilot track color The pilot jet is pressed into the body. The arrow is pointing to the location of the .018″ pilot jet’s orifice. You can see the two air passages. One is in the center and leads to the needle jet. The other is at the top and leads to the pilot air screw and the fuel supplied by the pilot jet. The two transfer orifices are located the slot between the pilot jet and the tip of the pilot air screw.

Problems: There are several areas in the pilot circuit that can cause problems, especially starting. If the bike will not start, and stay running, even though you have the choke on and tickled the carburetor or the idle is erratic the pilot jet is blocked or the float bowl is warped and drawing in air into the pilot circuit fuel passage, or both.


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 The chart above is not drawn in stone. It is a representation of the different fuel circuits and at which throttle opening  they are working. A version of this chart is often found on web sites. The usual problem hem is it is a copy of the one in the Mikuni manual for their VM model. One of the main differences between the Amal and the Mikuni VM is with the VM is the tip of the needle never leaves the needle jet. The needle is moving up and down and never leaves the needle jet’s orifice all the way through wide open throttle. With the Amal, depending upon which groove the needle clip is in, the needle leaves the needle jet at about 3/4 throttle. While starting and ending throttle positions for each fuel circuit represented in this drawing are general representations and the actual ones can, and will, vary.
 


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The tapered needle seldom wears to a point where it needs replacing. It can be easily measured on the part that will wear the most the major diameter. You will find the diameter to be .0925″ (2.5cm). The needle grooves are numbered from the top down, with #1 being nearest the top. If the needle is bent it is a candidate to be replaced.



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The needle jet is the only jet that isn’t identified by how much fuel it will flow. It’s orifice is measured in inches. The most common for four stroke engines is a .106″. The smallest needle jet is .105″. Below that the carburetor will not work. A few 4 stroke engines require a .107″ but they are few. Two strokes use larger as does carburetors set up for alcohol. An engine that runs well with a .106″needle jet will run lean with a .105″ and rich with a .107″. The needle jet is the only jet that gets larger in normal use and can  wear to .107″, and beyond, in as little as 10,000 miles. It should be on the list for replacement at annual service intervals.

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