Tech Tip: "Updated" Amal Concentric Carburetor Jetting (4-Stroke)
If you have an old box of Amal Concentric parts lying around and you plan on using them on your upcoming project, chances are you might be digging your self into a hole before you even picked up the shovel.
On today's CBS series of "Tech Tips" I wanted to dive into the changes that where made to the jetting inside the Amal Concentric carb range starting in 1969.
These changes that took place in 1969 are still standard practice today and should not be overlooked.
If you are not understanding what I'm referring, make sure you have an Amal Concentric carb and a 4-stroke motor and the rest will fall into place.
Lets dive in carb first
The Amak MKI series Concentric
In 1967 Amal released a new carburetor called the "Concentric".
The Concentric carb was deigned to be more compact, affordable, easier to tune, and more economical in terms of fuel economy as compared to the Amal Monobloc.
The Amal Concentric had a new “simple” measuring range offered in metric sizes (26, 28, 30, 32, etc)
By 1968 most (if not all) Triumph, BSA and Norton motorcycles where fitted with this new type of carburetor from the factory. This included both 2-strokes and 4-stroke applications.
Although an Amal Concentric carburetor from 1967 and an Amal Concentric carb today (2018) do look the same, they are internally different in regards to the standardized jetting.
Lets touch base on the differences..
1967- 1968 jetting
From 1967 - 1968 the standard jetting for a typical 2-stroke and 4-stroke Amal Concentric carb where installed with the following jets:
| Needle | 922/063 |
| Needle Jet | 622/079 |
| Needle Jet Holder | 622/080 |
In 1967-1968 all Amal Concentric carbs (2 and 4 stroke) features a pilot jet which could be found if the float bowl where removed.
That type of pilot jet was changed to a pressed in jet on 4-stroke models in 1969... 2 stroke applications retained the removable pilot jet throughout the years even till today.
However, the jetting as shown above is refereed to as the "old jetting” and also known as the "2-stroke" jetting that was only used from '67-'68.
Jetting could vary based on application...
1969 & later jetting (current)
Shown is an original BSA service bulletin dated 7-20-1970 describing the new “updated” jetting for Amal Concentric carbs (also includes twins & triples) (click to enlarge)
Starting in mid-1969 the Amal Concentric carb received a jetting “update" to help with starting and smoother running for Triumph and BSA 4-stroke models (singles, twins and triples)
This update consisted of a new needle jet, needle and a needle jet holder - below are the specs and part numbers of the “updated” 4-stroke parts.
Although we refer to this change as the "updated" 4-stroke jetting, 2-stroke applications still retained the “old jetting” from 1967 and on.
It was only the 4-strokes in which this change took place for.
Old (1967-1968) vs New (1969- current)
I'm sure you are curious to know how and what the differences are between the early and later jetting.
I have compiled some photos for viewing and also how to identify all 3 parts that make up the jetting.
Please use the photos at your discretion to figure out which type of jetting and parts you currently have in your Amal Concentric carb.
*4-stroke applications - later "updated" jetting is better!
Needles
Amal throttle needle with identifying marks at the top
Throttle needles come in many shapes, tapers and lengths to accommodate either a lean or a rich mixture.
The 4-stroke throttle needle features 2 identifying grooves at the top while retaining the 3 needle clip (adjusting) grooves right below..
Although there are many throttle needles on the market today, the 2 groove throttle needle is described above as the “standard” 4-stroke needle.
Also the most popular needle for 4-stroke applications.
Needle Jet
Here are 2 types of Amal Concentric needle jets.
One should not confuse a 2-stroke needle jet with a 4-stroke needle jet.
They are interchangeable in terms of fitting but not interchangeable in terms of tuning.
The 4-stroke Amal needle jet measures .812" long while the 2-stroke needle jet comes in at 0.717".
Needle Jet Holders
Needle jet holders are fairly simple to identify.
The 4-stroke needle jet allows the main jet to sit further in the bowl while the 2stroke needle jet hold does not.
This is also a part that you do not want to get mixed up.
A simply mix-up can have your machine from running rich to lean.
*quick side note - new Amal Premier and non-Premier carbs come with the "updated" 4-stroke needle, jet and holder exactly like the 1969 and later carbs. Should you need different jetting on a new carb, contact us before purchasing.
The mix-up
Although this change in discussion is nothing new, many folks and enthusiasts today are not aware of the change that took place which exposes your self to problems.
It is not recommend to switch the old 2-stroke jetting with the new 4-stroke type jetting.
If you have found that you have a mix in parts installed, we advice you to change out all 3 parts simultaneously to avoid any tuning or running problems.
The "updated" 4-stroke kit - now for sale
If for some reason your Amal Concentric carb has the old or mixed jetting, we stock a new kit offered by Amal in the UK that comes with all 3 parts to "update" any Amal Concentric carb to modern 4-stroke specs.
Using the kit as shown below it comes suited for most Triumph and BSA applications including singles, twins and triples.
Great upgrade for Amal Concentric carbs from 1967-1969 with the early 2-stroke style jets.
Shown is the new Amal 622/235 "update" kit - comes with everything as shown
The 622/35 "updated" kit consists of the following parts:
| Kit (622/235) | Part Number |
| "U1" Throttle Needle (3 Clip Grooves) | 622/124 |
| .106 Needle Jet (4-Stoke) | 622/122 |
| Needle Jet Holder (4-Stroke) | 622/128 |
Click here to purchase your genuine 622/235 update kit by Amal
Parts book comparison
Now that we have covered the nitty-gritty, I wanted to show you 2 Triumph parts books that reflect these changes.
Triumph was clear on the change of parts (for once)...
1968-1969 Triumph parts book
1970 - Current Triumph parts book
Take note of the different part numbers between the years on the needle, needle jet and jet holder.
The different part #'s reflect the jetting changes through the year span.
Recap
In closing, it is vital to check and ensure that your Amal Concentric carburetor has the correct jetting inside for your motorcycle's year, model and application.
Remember, if you are running an Amal Concentric carb on a 4-stroke application whether your ride is a Triumph T120, BSA A65 or a Norton Commando, start with jetting as described above if it has not already been already changed.
If you have old original parts please keep in mind that needle jets and needles do wear and should be replaced accordingly.
The "4-stroke" jetting as discussed above will work on both original and new Amal Premier carburetors.
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Thanks for reading
As always thank you for reading!
Should you like to chime in, please use the comment section below.
Thanks for reading and ride safe!
Donald Johnson
Hi Kyle, It is important to note the new Amal Premiere has different float level settings. The static level is much higher. This is noted in the latest instruction sheet from Amal.
In many cases the static level will actually be above gasket surface of bowl on the newest carbs.
However, actual fuel level in bowl is the same. If you set static float level to the earlier setting of .080" below the surface the actual fuel level will be low causing a lean condition.
I confirmed this on a few premiers using the clear tube method to measure actual fuel level in bowl. Also you can mark bowl inside as you suggested to get double conformation of level. I used gasoline as the test fluid. One gallon low in tank with bowl on jig to simulate actual installed conditions.
I had thought Amal had set level too high so I adjusted it to .080" below, but bike ran lean. Indeed actual level was very low after my adjustment. I went back to the way Amal had set it & was exactly in middle of actual fuel level in bowl range.
The new Premire carbs look like originals, but many changes have been made in castings & other parts. Even though they are very similar, they must be looked at in their own way & not as the original Concentrics.
On a side note regarding the old with plastic floats. They will deteriorate with 10% ethanol fuels sold in USA. It can be a risk to use them as the tangs tend to soften & fuel level can rise such as the float can no longer reliably shut off fuel. This can result in overflowing of fuel from carb & a potential fire hazard. I’ve seen this on several old floats & experienced this on my own bike.
Don
Classic British Spares
@Geoff – thanks for reading today!
Geoff Eardley
excellent reading
Classic British Spares
@ Russ – Good stuff!
Classic British Spares
@Michael – true