It’s been said many times that nothing wakes up an old British motorcycle like a new carburetor. Amal carb fitment shouldn’t be that hard. It would be so nice if you could just open the box of your newly purchased Amal Concentric carburetor, remove the wrapping and bolt it up to your manifold. Well, you can, but I am here to save you some time and aggravation by sharing some tips I have learned over the years about getting an Amal Concentric carburetor to work right from the get-go.
Table of Contents
1. The Preparation
The first thing I suggest doing after removing the carburetor from the packing paper is to lay out a clean white towel on your workbench and to proceed to completely disassemble the carburetor. You are looking for little specks of manufacturing swarf, particularly in the float bowl. Remove the pilot jet (Premier models) to make sure all of the orifices are clear. Check the needle jet and main jet to make sure they are tightened in the jet holder. Verify the specifications of the jets to ensure they match the requirements for your application.
Once this step is completed, take a look at the float bowl gasket to make sure it cannot hinder the movement of the float on it’s pivot pin. If it needs to be trimmed slightly, this is the time to do it. Check the float height before reassembling the float bowl to the body.
The old specification for the pre-Premier carburetors was a gap of .080” between the top of the float and the rim of the bowl, with the tang of the float being held down by your finger. The newer Premier models equipped with Stay-Up floats now are recommended to have the top of the float flush with the rim of the bowl, particularly when the carburetor is fitted with the RJ17 pilot jet (two annular rings on the silver tip of the jet).
Once you are satisfied the carburetor is ready for service, reassemble it taking care to not overtighten the Pozidrive screws retaining the float bowl. You can leave the slide and choke parts out until you are ready to install the throttle and choke cables. Leave the top cap off for now because you may need to prepare it for use with specific Triumph cables.
2. Top Cap Drilling
While many of the new Amal Premier Concentric carburetors set up for fitment to Triumphs already have the threaded holes pre-drilled, you may have a carburetor that has the original threaded cable adjusters fitted, which will need to be drilled out to accommodate Triumph throttle and choke cables with “top hat” shouldered ferrules and inline adjusters.
In most cases, when fitting a new Amal Concentric to a BSA or Norton, you will be dealing with control cables that have plain ferrules and no inline cable adjusters, so no drilling is required as the threaded cable adjusters are left in place and used for cable lash adjustments. For the Triumph applications, a 15/64” drill bit works for drilling out the threaded ports for the adjusters. When I was doing this kind of work regularly in the service shop, I used a homemade wooden fixture to secure the top cap to when using a drill press.
3. Fitting the Throttle and Choke Cables
The next step in Amal carb fitment is working on the cables. Installing the cables to the throttle valve, or slide is best done prior to installing the carburetor(s) to the manifold(s). This is undeniably a fiddly process that requires patience and dexterity. When chokes are employed, I prefer fitting the choke cable to the choke block first, then installing the throttle cable to the slide. Make sure the inline adjusters on both cables are fully collapsed, to allow for maximum free-length of the inner wires. With the choke cable engaged with the handlebar-mounted choke lever, insert the carburetor end of the cable through the top cap and down into the brass guide sleeve and spring.
Next, you will need to pull the cable through the circular opening of the slot (towards the top of the choke block) while fitting the spring and guide sleeve in the choke block. Compress the spring enough to allow sufficient free-length of the wire to locate at the bottom of the choke block. Needle-nose pliers may be necessary to grip the nipple on the end of the cable to guide it into place.
Once you succeed in getting the choke cable fitted to the choke assembly, turn your attention to the slide. You will need to have the throttle cable engaged at the twist-grip first to fit the cable to the slide.
Lift the needle and needle clip assembly out of the slide and set aside for the moment. Thread the cable through the top cap and through the slide return spring. You will notice the cable is not long enough to grip with your fingertips, so you will again be using your needle-nose pliers to hold the cable just above the nipple. Use the jaws of the pliers to bear against the end of the spring, compressing it as far as you can while holding the cable nipple with your other fingers.
When you have reached the limit of spring compression, grip the pliers tight on the cable to hold the spring compressed. Now comes the really fun part, where you insert the choke block assembly into the chamber of the slide, while inserting the free-length of the throttle cable into the depth of the slide enough to locate the cable nipple into the hole in the bottom of the slide.
Once you achieve this, you can use your fingers, or a hooked tool, to lift the end of the spring out of the way far enough to drop the needle and clip assembly into the other hole adjacent to the cable nipple hole.
Now you can let the spring relax over the top end of the needle, taking care not to disturb the placement of the needle clip. It may require more than one attempt at this procedure, but once completed, the top cap and slide assembly can now be inserted into the mixing chamber of the carburetor, taking care to guide the needle inro the needle jet. The top cap is tightened into place and the carburetor is ready to install on the manifold.
4. Mounting the Carburetor onto the Manifold
During the evolution of the unit-construction Triumph TR6 and T120 models, there were different methods for carburetor-to-manifold mounting. The earlier manifolds on Amal Monobloc-equipped engines used 5/16” bolts or studs with plain nuts to secure the carburetor to the manifolds, usually with a thick phenolic insulating spacer and paper gasket sandwiched in between the carburetor flange and manifold face.
The Monoblocs used a thick o-ring that fit nearly flush in the machined groove on the flange, while earlier Concentrics were fitted with a thin o-ring until the new “rubber mounting” system was introduced for the 1970 model year. The new system featured revised shouldered manifold studs, cupped washers with o-ring buffers, ¼” Cleveloc nuts, and a thick o-ring for the mounting flange.
It is not uncommon to encounter different variations of mounting hardware, but in any case, it’s important to not over-tighten the carburetor retaining nuts when mounting the carburetor(s), as this can warp the flange, resulting in an out-of-round mixing chamber and a sticking slide. Most of this information applies to the unit 500 and 750 Triumph twin models through 1977 prior to the introduction of the Amal MK2 carburetors. For more details on the different o-rings used for carburetor mounting, refer to the following tech blog: Carburetor Mounting O-rings: 622/101 Thin vs 70-9711 Thick.
5. Synchronizing Dual Carburetors
For Amal carb fitment – twin carburetors on a Bonneville model, it is critical to have the slides synchronized so they rise and fall at an equal rate when the throttle twist-grip is rotated and released. Any disparity in the movement of the sides can result in a misfiring condition, particularly noticeable in the middle range of throttle openings.
I have had good success in synchronizing the slides by using a pair of 13/64” drill bits placed under the cutaways on the slides to detect motion when the slides are raised by the twist-grip. 13/64” will give you a pretty good starting point for the throttle stop screw on the carburetor.
Using the non-spiral shank of each drill bit, you can lower the slide on each carburetor over the drill bit until there is a slight amount of resistance felt when withdrawing the drill bit, as if you were using the bit like a feeler gauge. Once the initial setting of the slide heights is established, adjust the inline cable adjusters so there is a slight amount of cable slack present before the rotation of the twist-grip causes the drill bits to drop.
Keep adjusting the individual cables until you see both drill bits behaving exactly the same, with equal movement at the same precise rate, then tighten the lock nuts on the adjusters. You now have the “cable lash” set. Typically, the tapered air screw on the carburetor is adjusted to 1-1/2 turns out from bottoming out in its bore. This screw controls the fuel and air mixture by metering the amount of air with fuel supplied through the pilot jet. It’s not uncommon to deviate from this initial setting by up to ¼ turn, plus or minus.
Once you have gone through this procedure you are ready to plumb the fuel lines, fit the air cleaners, turn on the petcocks, and light it up! The tuning guide supplied with the carburetor(s) from Amal will come in handy for final adjustments and is worth reading through before the shakedown rides commence. Amal carb fitment should be a little easier now that you’ve reviewed the steps. Best of luck and thanks for reading this.