Alternative Motorcycle Repair

PO Box 820
Sahuarita, AZ  85629


Concentric Carburetor Facts

The following is a list of facts relevant to the Amal Mk 1 Concentric carburetor that may be of assistance to those of you who are undertaking the task of rebuilding or reconditioning such an instrument. The information herein has been derived from years of personal hands-on shop experience, as well as road riding and racetrack applications. This fact sheet is intended as a supplement to your shop manuals and your current working knowledge of carburetion principles.

1. Needle Jets - The most wear prone item in the carburetor besides the throttle valve (Slide, to most of us) is the needle jet. Because of the relatively small internal surface area of the brass jet in contact with the stainless steel throttle needle which is constantly moving up and down with the slide, a high degree of wear is inevitable. While opinions may vary, it has been our experience that most needle jets are worn beyond specification in 10,000 miles or less. Remember, a mere .001 of wear changes your #106 jet to a #107.
    The most obvious indication of this condition will be a gradual decrease in gas mileage regardless of the quality of the engines' state of tune. The other, less obvious indication will be a tendency for the engine to hesitate or 'stumble' as the throttle is raised gradually from idle, once again regardless of engine tune, leanness of the pilot mixture, or precision of carb balance (in the case of dual carb systems). This will be most obvious when a worn needle jet is installed in a new, re-bored, or sleeved carb body. The excess fuel being metered through the worn needle jet creates an over-rich mixture as carburetion passes from the idle circuit to the needle/needle jet and slide cutaway stage. The exception to this latter indication may occur in the case of a thoroughly worn out carb/slide combination. Under this condition, the excess air being drawn past the worn slide/body may 'mask' the worn needle jet by compensation for the excess fuel. The long and the short of this all is simply; when in doubt - replace it.

2. Slide and body wear - The most common weakness of the Amal Mk 1 concentric carb is the quality (or lack thereof) of the material used in its' construction. Aggravating the situation is the fact that both the slide and body are made from the same 'pot metal' like material. As any engineer can tell you, the rubbing of two like metals together especially without adequate lubrication creates a condition known as galling which is loss of metal due to friction. This destructive wearing of metal rapidly increases the clearance between the slide and body and allows for an excess of air to enter the carburetor throat or venturi at idle thereby upsetting the mixture adjustment. Being able to withdraw the idle stop screws while still maintaining an undesirably high idle manifests the worst case scenario of this situation. At this point, the amount of air being drawn past the slide is so great that the correct idle circuit adjustment is impossible.
    While in some cases the replacement of the slide with a new item may correct the problem temporarily, the wear in the body usually precludes this as a viable repair. The options at this point are usually to either replace the carb with a new or optional item, or rebore or sleeve the existing unit. At AMR we have had considerable success with reboring the carb body and replacing the slide with a chrome-plated brass item that exhibits long term wear resistance, while bringing original specification performance back to an otherwise 'junk' carburetor. At this time we have carburetors with over 20,000 miles of reliable service in use with no indication of excessive wear.

3. Body Warpage - The aforementioned wear between the carb body and slide has often been misinterpreted by some as an air leak between the carb and manifold. In a misguided effort to correct this situation, several atrocities have been performed to the suspect carb. Installation of an oversize flange o-ring and over tightening of the flange mounting nuts are two of the most common. Both result in a 'bowing' of the carb flange which ultimately translates to a warpage of the slide bore. This reduces the slide to carb bore clearance at higher openings and produces slide 'sticking' at high engine rpm's, a good case for handlebar mounted kill buttons!
Once again, the proper fix is to sleeve, rebore or replace the defective unit, and to observe the proper mounting procedures, I.e. never over torque carb mounting hardware, and use self-locking fasteners whenever possible. Particulars on this subject are available from AMR upon request.

4. Early model upgrades - Many 1968 and all late 1967 model Amal Mk1 concentrics used a series of components that have since been replaced with more suitable parts. If you have such a carb that has not been upgraded, you will need the following parts: Throttle needle #622/124 (identified by two rings above the clip groves), needle jet #622/122 (identified by a small cross drilling at its' base), and jet holder #622/128 which is approximately 1/8" longer than its' predecessor thereby allowing the main jet to extend that much further down into the float bowl. These components are standard equipment on all 1969 and later carbs and have proven to be the best setup for all model applications.

5. Pilot Jets - Aside from the typical slide/body wear problem, the lowly pilot jet is probably the next greatest source of frustration for the average do-it-yourself tuner. Since 1969, all Mk1 concentrics have had the pilot jet pressed into the carb body at a point deep in the passage in which the pilot air adjusting screw is housed. Earlier models employed a screw-in pilot jet located in the float chamber area on the bottom of the carb body. Since the threaded hole still exists in the carb, some have been fooled into thinking that the jet is missing on their later instrument. Careful inspection of the pilot air adjusting screw passageway with a focused light source will reveal the presence of the tiny brass jet with its' even tinier orifice. The major difficulty with this jet is blockage due to residue left by evaporated fuel (common on infrequently ridden or improperly store machines) or foreign material entering from the float chamber. Since the blocked jet will not allow any fuel to pass into the idle circuit, no amount of air screw adjustment will rectify the situation. The condition is readily identified when the cylinders, fed by the suspect carb will suddenly not hold an idle, but seems to function properly at higher throttle settings.
    Visually checking this condition can be trying at best, so we recommend the following procedure. With the float bowl and air screw removed (note: you may want to count the number of turns out from bottom on the air screw so that you can return to the original setting when finished), place the small plastic tube of an aerosol container of carb cleaner into the air screw passageway as far as it will go. This should place it nearly against the pilot jet. A small squirt of fluid should pass through the jet and be seen exiting from the threaded hole in the carb body where the old screw-in pilot jet used to be fitted. Failure of fluid to come out there indicates a blocked pilot jet, and a mechanical cleaning will be required. Compressed air might be of use here, but only if channeled through a tube no greater than the inside diameter of the passage at the pilot jet. Otherwise, the air will simply escape out through the air intake passage from the carb mouth.
    CAUTION - Carburetor cleaners, while efficient at what they do, are highly flammable and toxic to the respiratory system. Use only in a well-ventilated area and away from open flame or other ignition sources. Always wear proper eye protection and avoid prolonged contact with skin.
    Mechanical cleaning of the pilot jet can be affected in a couple of ways. A #78 drill bit which is the same as the standard inside diameter of the pilot jet (.016"), can be secured from an industrial tool or hardware supply and with a suitable extension can be forced through the blocked jet. We prefer a simpler option which involves using one strand of the common copper wire found on most British motorcycles. A single strand of 14-16 gauge wire generally measures out to around .014" in diameter, and when forced through the pilot jet will be less likely to damage or oversize the jet in the process.
    This brings us to another occasional problem with pilot jets. In some cases, an over zealous 'mechanic' may have increased the size of the jet orifice by using an improper cleaning tool (welding tip cleaner, steel wire larger that .0161, etc.). If the jet orifice has been enlarged beyond a certain point, it will become impossible to supply enough air to the pilot circuit to lean out the mixture to a viable specification. When such a case has occurred, we recommend drilling out the damaged pilot jet and installing a screw-in version in the old position.
    One last oddity concerning the pilot circuit involves the air screw itself. On a few occasions, we have discovered pilot air adjusting screws that were too short to bottom out in their seating. This condition is manifested when the air screw is turned all of the way in and yet the idle mixture appears to be too lean (this is assuming that other aspects of the carb are within specification). To verify this condition, simply place a finger over the idle circuit air intake hole at the carb mouth (the one that is off center). With the air screw bottomed out, no air should be drawing through this passage. Any feeling of a vacuum accompanied by a sudden change in the engine speed indicates that the air screw it too short for this application. Simply replace it with one of proper length.

6. Float Level - Probably more attention has been given in various publications to the significance of float level than it really deserves. The 'official' Amal specification is .080" from the float bowl gasket surface down to the top edge of the float measured on the opposite side from the needle. In practice, we generally have set the top edge of the float flush with the float bowl gasket surface with no adverse effects. It should be noted however, that allowing the float to rise much above the float bowl gasket surface could create an overflow problem as the float may contact the carb body thereby preventing the float needle from closing. Conversely, setting the level too low may aggravate a potential fuel starvation situation at sustained high speeds, and at the least will make flooding of the carb with the tickler during cold starts a challenge.
    Whatever your float level choice, the following is the proper adjustment procedure. To raise the level, tap the brass seating in the float bowl downward with a suitable drift (larger in diameter than inside of the seating). To lower the level, invert the float bowl on the bench, insert a 1/8" drift through the bowl casting and carefully tap on the bottom of the seating, moving it upward in the bowl.
    In a related note, it has been found that on some occasions the float bowl gasket may foul the float causing it to stick open. To prevent this, we recommend trimming the area of the gasket that lays over the float spindle directly adjacent to the float at a 30-degree angle, as shown in the accompanying picture. Care should be taken not to take off too much material so as to allow the float spindle to rise out of its' slot.

Over the years, we have found the Amal Mk1 carburetor, when kept up to original specifications, to be an adequate and efficient instrument for use on most British motorcycles. With the right knowledge and a reasonable amount of maintenance, you should be able to enjoy many thousands of reliable miles with this unit.