Alternative Motorcycle Repair
PO Box 820
Idle Mixture Adjustment Procedure
For the Amal Mk1 Carburetor
While the procedure outlined here is directed
specifically at the Amal Mk1, it WILL apply to virtually any carburetor that
requires a 'clockwise to enrichen, counter-clockwise to lean-out' type idle (or
pilot-air) adjustment. Also, remember that any attempt to achieve a stable and
efficient idle will be seriously hindered if the general condition of the
machine is in any way in question. Things such as engine compression, ignition
timing, spark plug quality, carburetor wear, jet blockages, etc., can make this
an exercise in futility at best, and a reason to sell the bike at its worst. So,
in a nutshell, make absolutely sure that everything else is in good condition
and set correctly before attempting this procedure, unless of course you enjoy
For me, the idle adjustment procedure is performed as the final act of the tuning or rebuilding procedure, and is often repeated during or at the conclusion of a test ride. The bottom line is that the proper procedure is performed only on a thoroughly warmed engine. Any attempt to do otherwise will leave the bike with an overly rich mixture, which while making it easier to start on those cold days, will likely cause spark plug fouling and certain failure on an emissions analyzer.
The first maxim of the engine idle world is, 'if it idles smoothly when started for the first time of the day, it's probably too rich." A cold engine requires more fuel because of the inefficiency of the combustion chamber as it absorbs the heat of combustion. Once the engine arrives at its nominal operation temperature, it operates at its most efficient level, thereby requiring less fuel. This is the reason for the 'choke' or air valve as Amal describes it. This partial restriction of the incoming air to the carburetor alters the fuel-air mixture to an overly rich condition; thereby compensating for the 'warm condition state of tune' that will eventually be achieved.
When starting 'from scratch', the following adjustments may be of help: 1 1/2 turns out (counter-clockwise) from bottom on the idle air screw (the one that screws into the carb horizontally), and 1-1/2 to 2 turns in (clockwise) on the idle stop screw (the one that screws in at an angle) from the point at which the slide or 'throttle valve' starts to move up from the bottom of the chamber.
You can usually feel the contact between the adjustment screw and the slide when approaching it from a very low screw position. The above settings should give some semblance of an idle with a near standard machine in a temperate climate (70's). The other important adjustment at this point would be the carburetor balance, that is, the even response of the slides on dual carburetor models when the throttle is opened. With the above primary adjustments made, simply adjust the slack in each throttle cable with the carburetor top or in line adjusters until the slides rise at exactly the same time. On single carb models this adjustment will serve only to take excessive slack from the cable.
With all other factors being correct, you should now be ready to start the bike. Once running, a stable idle speed (around 1000 rpm) should be established the use of the idle stop screw, the one that goes up at an angle. Where two carburetors are employed, it will be necessary to listen to the exhaust note of each cylinder to determine the balance between the two. Sometimes it is helpful to feel the pulses by putting your hand near the exhaust outlet of the pipe or muffler. It may take several attempts at each side before a stable idle is achieved, that is, both cylinders firing evenly.
In some cases, it may be necessary to adjust the pilot air mixture to help achieve a steady idle. This will be evident when the engine fails to idle smoothly, regardless of the idle stop screw adjustment. Turning the mixture screw in or out a 1/2 turn or so should bring the engine (or that cylinder, when two carbs are used) up to a regular rhythm at which point the idle speed can be adjusted as previously described. Finally, you are at a starting point. Using 1/4-turn increments, adjust the air mixture screw out (counter-clockwise), waiting a few seconds between adjustments for the engine speed to stabilize. Depending on the machine and the carburetors, the idle speed should increase with the first few mixture adjustments. As the idle speed increases, it should be adjusted back down to the original setting by use of the idle stop screw. At some point, the mixture adjustments will cease to alter the idle speed. This will indicate closure on the correct mixture adjustment. Within the next few adjustments, the engine or cylinder on which the carb adjustment is being made, will falter or stumble. Turning the mixture screw back in (clockwise), usually 1/4 to 1/2 turn will restore the regular idle and will give the correct idle mixture adjustment.
Once again, the above procedure should be performed with the engine at normal running temperature for best results. Also, it should be noted that with all things in good condition and correctly adjusted, the engine will likely display a reluctance to idle smoothly and may spit back through the carbs when started from cold, requiring use of the choke or patience while allowing the bike to warm up.
|Engine fails to respond to any air mixture adjustment||Clean out blocked pilot jet and/or bleed holes in carb venturi.|
|Correct Adjustment achieved with idle mixture screw nearly bottomed out.||Air mixture screw too short - replace.|
|Lean condition not obtainable even with air screw turned out beyond four turns.||Screw-in pilot jet too large - replace with proper size or: press-in pilot jet reamed out oversize - drill out and replace with screw-in type.|
|Engine refuses to idle down to 1000 rpm regardless of idle stop screw adjustment.||Throttle cable(s) too short - replace.
Air leak at carb to manifold joint - reseal.
Worn out slide and body - rebore or sleeve to original specification.
|Engine idles correctly, but falters or stumbles at throttle openings just above idle.||Incorrect balance on dual carb models - adjust cables.
Blocked cross drilling in needle jet - clean out.
Worn needle jet - replace.