Tech Torque: The Amal MK 1 and 11 Carburettor
The Amal is one of the most fitted carburettors over the last 50 years. It is not as popular as it once was, but is still used today
One of the most fitted carburettors over the last 50 years is the Amal. Not as popular as it once was, but still used today Stu Owen gives us the full tech specs
Back in the 1980s, virtually every tuned Lambretta had an Amal Carburettor fitted. At one point they were so fashionable it seemed no engine could do without one. Like anything in great demand, there comes a time when that something eventually falls out of favour. This is exactly what happened to the Amal, most probably down to the fact that better carburettors were being introduced. Though they have their drawbacks compared to their modern-day counterparts, they can still offer very good performance.
Originally Amal was part of IMI industries based in Witton. A popular choice, they were fitted to virtually every British motorcycle made at one point in time. Despite the company being sold off to Grosvenor Works Ltd in the 1970s, production still remained in Birmingham. This finally ceased in 1993 when, for a short period, they were only available from Spain. The company was once again sold off in the early 2000s to Burlen Fuel Systems Ltd, who is still the current owner and manufacturer. Use of them on the Lambretta goes all the way back to the 1960s with Amal’s first series of concentric Carburettor.
Amal MK 1
The Amal MK 1 concentric carburettor was first introduced in 1967. Though intended primarily for motorcycle use, it soon became apparent that it would be the ideal choice for the Lambretta. Scooter tuning was still in its infancy back then, but the few shops that catered for this growing side of the market soon latched on to the fact that the Amal could offer a great performance enhancement, and certainly on a 200cc cylinder. Dellorto didn’t really offer much of a usable choice until a couple of years later, and the Wal Phillips fuel injector wasn’t the easiest or most
practical option. The first problem encountered for the Amal was the manifold, of which one would need to be made if it was ever going to fit on a Lambretta engine. One problem it did have was it was better suited to sitting upright compared to other makes of carburettor that would happily perform well at an angle. This was down to the fact that the Amal had rather a small float chamber and if sat at an angle, the fuel couldn’t fill the chamber up fast enough at full throttle. Luckily, there was enough space for the main body to sit upright and not fowl on the fan cowl or engine mount. What was required was the correct choice of either left-handed or right-handed versions. This referred to where the air screw and tickler was located. If a left-handed example was fitted, then the engine mount stops access to the air screw. The simple choice was to get a right-handed version which has plenty of room for access to the air screw. This point should be remembered if you are going to purchase one present day.
The manifold was produced by making a sand casting mould, and several shops all offered similar versions at the time. You can still pick them up and quite often the name will be on the manifold such as Wildcat, Royspeed or Scooter Mobile. The carburettor bolts directly on to two 8mm studs, which had the benefit of it never coming loose, as opposed to a rubbermounted one where the hose can split. Once the manifold had been made, most tuning-based shops were offering the Amal MK 1 as the only real choice of carburettor, and with a constant supply being manufactured in the UK, there was no problem of shortage. It remained popular, even though by the early 1970s the Dellorto VHB ‘square block’ became available, with dealers offering both makes. It now came down to what the tuner or customer preferred to use.
Technical
The MK1 is a simplistic carburettor, so much so that anyone even with only a little knowledge can set one up. Fuel control consists of a pilot jet and the main jet, that is all. With no choke jet, this enables the body to be very slim and why it is so easy to fit in the space where the original Lambretta carburettor sits. Even easier is the needle, as there is only one made for the two-stroke. This has three clip settings, which again means it's not got too many complicated adjustments. The only real choice is for the needle jet, which does have a lot of different sizes. Luckily, the Lambretta sits in a range where only about two or three work as a happy medium. The final part that allows adjustment is the slide itself, which has the option of several different cutaways.
When it comes to the choke system, that has two different options of how to go about setting it up. There is no choke jet and choking works off the principal of blocking off the inlet, similar to if you were going to put your hand over the mouth off the intake. The mechanical way of doing it is by using a gate slide that moves up and down within the circular slide. This requires a separate cable and control on the handlebars to operate it. It works by the leaver pushing the gate down for cold starting and then pulling back to the top once the engine is warm. Problems arise from the spring not having enough tension to put a load on the gate, causing it to get stuck, and also the aluminium of the gate and slide wearing on each other.
The other, much easier option, is to use the tickler on the side of the float chamber. By pushing it down, this allows fuel into the float chamber and floods it in a way similar to what a choke jet would
do, but in a much cruder fashion, thus enabling easy starting. If you are going to use this method, which is far better, then the slide must have the blanked-off gate. You must also fit a blanking screw in the top of the body where the gate cable enters, to cut off the air that would suck through it.
Baseline set up
Presumably, you would use the MK 1 with a cast iron barrel or a conventional cylinder layout that allows the manifold to fit. The 900 Mk 1 series is used, which has a bore size of 28, 30 or 32mm. Normally you would use the 30mm version with a 200cc and a 32mm with a 225cc. Both would use a similar baseline set up, except you may go slightly higher on the main jet on a 225cc. Pilot jet – 25 Main jet – 200-220 Needle jet - 107-108 Needle – X2 (the only one available) Float – Shallow type (standard) Clip setting – Top Clip Slide – 3.5- 4 Air screw adjustment 1 .25 turns. Please note these are only baseline settings and may be subject to alteration to get them perfect. You must check that the engine is not running too lean. These settings are a starting point guide only.
Problems
The first notable problem is the position of the mouth of the body against the side panel. Because of how it is positioned, it sits virtually flush against the panel with only a slight gap of around 4mm. You will need to cut a hole in the side panel to allow the carburettor to breathe properly.
Due to the small float chamber, it is essential to make sure that you fit a fast flow fuel tap. This will allow the chamber to fill up as quickly as possible. Using a tap with a reduced fuel flow wll almost certainly mean the chamber will not have sufficient fuel, certainly at high revs. If so, then there is far more chance of the mixture becoming weak, which can result in overheating and a possible piston seizure. The MK 1 comes with the option of a shallow or deep float. It is best to use the shallow float, as this will allow more space for fuel as opposed to the deep one.
The velocity stack fixes on to the mouth of the carburettor with the idea to aid smooth air flow. The problem is because the inlet manifold is long, adding the velocity stack makes the whole inlet length too long. More than often not, enough air can get in and this will cause the engine to choke up. Though a velocity stack may look good, there is no real advantage to using one and it will throw all the settings out, certainly at low revs.
What type of engine should it be used on?
The MK 1 ideally should be used on a conventional piston port cylinder. It is possible to fit an adapter so it can be rubber mounted. By doing so it can then be fitted to any kit, but the reality is its small float chamber isn't capable of allowing enough fuel through. Also, it is quite heavy so if rubber mounted, they can fall off quite easily. Finally, running one at an angle as already stated is not a good option, again because of fuel flow.
The Mk 1 is best suited to those doing old-school tuning and retro-type street racers from the 1960s. If set up correctly, they can kick out a good performance on a cast iron cylinder and as a bonus, the induction roar is a pleasure to behold.
Amal MK 11
The Amal Mk 11 carburettor was first introduced in 1974, making its way on to scooters slowly through the rest of the decade. For a while, the Dellorto had dominated as the Amal MK 1 was by this time more outdated compared to its rival. The MK11 boosted many more refined features, and none more so than a proper choke jet system, making starting a much easier affair.
Also, because the body and intake were far better designed, throttle was response was far better. A MK 11 that was set up correctly could be a proven race winner.
Slowly but surely, the MK 11 started to appear on race scooters and not long after tuners started fitting them on road machines. Again, the big advantages were the carburettors themselves and all the spares which were readily available. For those shops wishing to stock them, having everything to hand was a big advantage. The manifold required was made from adapting a Dellorto inlet manifold and the simple process of machining it down to fit. The body would then be rubber mounted and sat at a 45-degree angle. This didn't affect performance as the float chamber was far bigger than on the MK 1 and it would run adequately in any position. This also allowed ample room for fitting next to the Lambretta petrol tank. Because it sat at an elevated position, access was far easier so it didn't matter if either a left- or right-handed version was used.
Set up was fairly easy compared to today’s more refined carburettors, so it quickly became a favourite both with dealers and customers. By the 1980s it had become the preferred choice and virtually every tuned scooter had one fitted. Because of its versatility, it could be used on any type of two-stroke, and when the TS1 was introduced with its reed valve induction, the MK 11 worked
perfectly well with it. Later on, a powerjet version was introduced and it became the must-have addition. It was a crude device in some ways and way more suited to track use than the road. The idea was to reduce the main jet size, making it more responsive mid-range. Many people got confused by how it required setting up and though it sounded good, if you had one the power jet soon faded into obscurity.
By 1990 other carburettors such as the Mikuni and Keihin had come to market. With a wide range of settings allowing the throttle response to be super-sensitive, the Amal MK 11 started to fall out of favour. In 1993, with the company sold and availability more difficult, mostly from Spain, that availability dried up. However, with UK manufacture re-starting in 2003, this problem was solved. Though they would never become mainstream again, some people reverted back to using them. Certainly, when it comes to old-school tuning they are a good choice. Also, with the resurgence in 1980s customisation, they look better fitted to an engine than a modern-day carburettor. The Amal MK 11, it seems, has its place cemented in scooter history, but also in the retro scene that is alive and well.
Technical
The MK 11 has a much squarer body, even though the throttle valve (slide) is cylindrical. It uses the same theory as the MK 1 from pilot to main jet, and control by throttle cutaway and needle. Again, there is only one needle available – the 2D1, and it only has three clip settings. The slide is available with several cutaways, but the baseline is between 3, 3.5 and 4. Apart from the choke, jet set up is similar to that of the MK 1, but obviously using different jetting sizes.
Baseline set up
Because there are such a wide variety of different Lambretta cylinder kits and combinations the MK 11 can be used on, its impossible to list a baseline set up that would suit all. Therefore, it is best to contact the manufacturer or supplier of the kit you intend to use one on beforehand and ask for their recommendations. The 2900 series is what should be used, which has either 28, 30, 32 or 34mm bore sizes. What is essential is the fitment of a chrome-plated brass slide. This is opposed to an anodised aluminium one, which is far cheaper. The aluminium ones tend to suffer in the wet from foaming, which can, in extreme cases, lead to a sticking slide. By using the brass chrome slide, this problem is solved.
Problems
The MK 11 has a rubber mounted body and only the correct size rubber should be fitted. Make sure the jubilee clips used are good quality and that they are wide enough to get a good grip. Always check the rubber hose for wear and if a split is starting to appear, change it immediately. Failure to do so will mean air can get in and cause the piston to seize. Always carry a spare hose in your toolbox, just in case it needs changing.
One problem the MK 11 has suffered from over the years is the top coming undone off the main body. The problem lies in the fact that it screws into a thread like a bottle top, rather than it being bolted down. If it does come loose, then it will suck in air and will only be noticeable at low revs as the engine will tick over erratically or race. In the past, it was common to see people holding the top in place by securing it with a jubilee clip. Periodically check it is tight and make sure if you have changed the needle position or throttle cable, you have screwed it up securely.
Power jet
The system works around the idea of sucking fuel through a small jet into a channel around the outside of the air intake adapter. The fuel then flows through a small tube into the mouth. The idea is to lower the main jet and raise the power jet so both are equal to the original main jet size. Working on the induction theory, it should give a better balance at certain revs, thus allowing improved acceleration and prevent bogging down mid-range. The problem arises from getting the balance exactly right and understanding its characteristics. Also, the fuel is fed around the outside of the intake through a channel under a black rubber band. This can easily be knocked out of place, stopping the transfer of fuel. In reality, they are not worth bothering with unless you fully understand setting them up. If you have acquired an MK 11 with a power jet system this can easily be removed by taking off the small brass tube that goes into the inlet and putting a blanking screw in place of where the jet fits in the main body.
Availability
Both the MK 1 and 11 are still manufactured in the UK by Burlen Ltd, along with all the jets and spares required to fit one. Manifolds for the MK1 are only available secondhand, but do turn up quite regularly for sale. Manifolds for the MK 11 are widely available. Just make sure you get the correct one for the bore size you are using.