Technical Tips

British Motorcycle Carburettors



Carbs are another source of pain. Typically the classic era Brits use Monobloc, concentric, and Mk 2, with the change to concentric happening in 1968, and the change to Mk 2 in 1979. Monoblocs were the next evolutionary step from separate bowls with the bowl cast in one with the carb, hence mono-block. The concentric has the bowl moved under or concentrically placed below the carb. Both are available as new manufacture. They can also both be resleeved: that is when the worn body is bored to accept a new slide that has had a sleeve of either steel or brass pressed on to it. This results in a tighter fit in the bore and longer life. The original design was a no-no. Similar materials don't wear well against each other and only a few do - definitely not the pot metal used in Amal carbs. It has a tendency to gall and stick. Later MK 2 Amals had teflon coating on the slides and lasted a lot longer. The concentrics come as 926 series with 26-27mm bores. The 930 series have 28-30-32 mm choices and 936 series have 34-36-38 mm choices. Mk 2 have 30-32-34mm choices in a series and 36-38-40 mm choices in the bigger 10 series.

Keeping your carbs clean is the key. Where possible turn off the fuel at end of the street and let the fuel use up before turning off the bike. Or drain the fuel if you are not going to ride for a while. The varnish in unleaded fuel will be left as a gummy residue blocking all the air and fuel orifices, once the fuel evaporates. Only a full strip and clean will get you back on the road. The biggest problem is the idle jet in the concentric: in the monobloc and Mk2 they are removable. In the Mk1 concentric, compressed air and possibly a fine wire will be required to clean this jet which is located behind the horizontal low speed air mixture screw.

A blocked pilot jet makes starting and idling difficult. The pilot jet and its air control screw basically take care of the engines idle. When transitioning to more throttle, the cutaway in the slide has a lot of say as to how the fuel gets mixed: typically a 2-2.5 or even a 3 slide for multi carbs and 3-3.5-4 cutaway for single carbs. As the throttle is opened more, the tapered needle attached to the slide passes fuel either too much, too little or just right depending on the position of the needle clip. This fuel has to come through the main jet. As the throttle gets to 3/4 the engine is fully on the main jet as the needle has cleared its own needle jet.

Once cleaned and refitted with a new O ring take care not to over tighten and warp the flange, which causes the slide to stick. Set the idle screws from 1 1/4 to 1 1/2 turns out from dead bottom and set the slides with the angled idle screws to about 1/16" open. Fire it up and if it doesn't idle then increase a bit at time til you get something. Wind the mixtures all the way in one at a time and the engine should slow and possibly stall as the air is reduced. If there is no difference or it picks up a bit then the idle jet is still blocked. If all is good then the engine will pick up speed as you turn the mixture screw out. Find a happy beat in the middle somewhere. Go back to the idle screws and take one at a time all the way out, noting what the other cylinder does. Then screw it back in till you hear the cylinder chime in. Do the same with the other, adding a bit or taking away a bit from either to get the required even idle. Standing behind and listening helps distinguish the fast and slow cylinders. Then set synchronization by adjusting freeplay on the outer cables, either watching or feeling the slides. At least 1/8" freeplay is needed so as not to hang up the slides and produce a fast idle. If the engine fails to settle to idle quickly then check for freeplay, bound or jammed cables, cables tied too tight to the frame or air leaks at the manifold. Air leaks can be tested with carb cleaner sprayed on the manifold between the carb and head while the engine is running. On a Triple falling back to idle slowly is normal due to the heavy stock crank.

Twisting carb pull rod rack may also cause the engine to drop back slowly, if so fit a light spring opposite to cable end. Typical main jet sizes for 650 twins are 190 with Mk1 and up to 260 on Monobloc. 750 Triumph is 190 or 200 and Triples use 150. You can put 160 in the middle. 750 Norton uses 220 and the 850 as much as a 260. A 500 twin uses a 150 or 160. When determining single carb jetting, I halve one of the Twins jets and add to it. Say a Bonneville has 200 mains and I have a Tiger single carb, 1/2 of 200 is 100 + 200 is 300, a good starting point. Ethanol can be a problem nowadays mixed at 10 or 15 %. This requires more fuel to get an optimum fuel air ratio.

I have found especially at idle you may need to increase idle jetting. It's easier on carbs with removable jets. On the Mk 1 you would need to drill out the cast in the pilot jet slightly and fit a removable jet in the threaded hole of the idle circuit. 26 and 27mm Mark 1 carbs don't have this option. Remember that a carburettor fitted to a normally aspirated engine relies on air speed. This is the speed of the air rushing into the cylinder as the inlet valve opens and the piston is travelling down. This leaves a vacuum or negative air pressure inside the cylinder compared to a positive pressure outside so it is just filling a void that appeared. This will change with altitude. The oxygen content, the part that mixes with the fuel to burn, gets less the higher you go. Not unlike your lungs where the muscles pull apart the chest and air rushes in. So, as the air rushes in, it will pull any fuel available through any orifice available. This means through jets which are usually a set size, except the needle attached to the slide, which moves through a needle jet. This allows varying amounts of fuel to be pulled into the engine depending on its position. As the air pulls fuel it mixes together or atomises. If you use another brand of carb on your classic British you may need to be careful where it came from. A 2 stroke carb in a previous life will be way too rich for 4 strokes. In the case of a Mikuni, say a 32mm, you will need to fit the correct 6DH3 needle jet and a 159 P2 needle or close to it to get a starting point. You will need a smaller main and, as they correspond to Amal mains, then go by the figures I gave and at least a 30-35 pilot for twin carbs and 35-40 for a single carb. Also remove the small air jet on the needle air bleed circuit. Look at the front of the carb and into the holes to locate this air jet. PWK, recently on the market, is a modified Keihin and is a good bolt and go, available in 27, 30, 32 and 34mm with flange or spigot mount. Mark 2's are good but check for 2 stroke use. I like Dellorto but they are expensive and it is best to remove the pumper. Flat slides are good for modified bigger engines but also are expensive. Be careful to stay with about the same size carb as fitted from the factory as there is no real gain on a stock engine, and may even cause it lose power.