Tech Tip: How To Adjust The Float Height On Amal Concentric Carburetors
(approx 13min reading time)
If you stay up-to-date with some of the popular British motorcycle forums such as BritBike.com I’m sure most of you have ran across many threads in reference to float height on Amal Concentric carbs.
Today I will make my case and opinion which will be very clear with advice, documents and my own experience of how to properly adjust the float height on new and original Amal Concentric carburetors.
The methods as described below work for me..
I will go over a few tech tips that will aide you with your Amal Concetric carb.
Points covered in this post
- What the Amal Concentric carburetor is
- What float height means
- Why you should adjust float height
- How to identify float related parts (Amal Concentric)
- How to adjust float height (Amal Concentric)
- How to check fuel levels
- Amal Monobloc carbs
- Replacement parts
Let’s get right to it
The Amal Concentric Carburetor
Amal factory during the 1950's (source: Amal website)
The Amal Concentric carburetor was designed by the Amal carburetor company in England, in 1967.
The Amal Concentric carburetor was designed to replace the Amal Monobloc carburetor which was then expensive and time consuming to manufacture and also “bulky” in size.
Today, the Amal Concentric is by far the most popular and reliable carburetor one can install on any vintage Triumph, BSA or Norton motorcycle.
The simple design of the Amal Concentric makes it easy to tune, install and retrofit with a very friendly retail price.
Click here for more information on the Amal Concentric carburetor.
What is float height?
The Amal "Stay-Up" float with metal tabs (adjustable)
Every carburetor (except some racing carbs) has some method of metering and shutting or preventing excess fuel from going inside the the float chamber.
A float is a simple yet effective design which is typically manufactured from plastic or a similar material that is ethanol or fuel resistant.
A "float" pivots on a pin which then fuel lifts the float and at a certain threshold it uses a float needle to shut the fuel off (or on) into the float chamber.
Although one might not think of this, a float is very similar to the way a toilet works. Water enters the chamber, fills and lifts the float, and at certain timed event, the float shuts the water off.
Why you should adjust your float level
Float level should be adjusted and optimized for a couple reasons.
- To properly shut off fuel entering the fuel chamber
On an Amal Concentric carburetor, if the float level is too high it will cause the float to interfere with the bottom of the carburetor and surprise you with a leaky carb.
When this happens, the float will no longer be able to close the needle in the seat orifice to prevent fuel from entering the carburetor.
- Adjust the float height to encourage a lean or a rich condition
When working with a new or used carburetor, it is best to start with the float height before you do any type of jetting.
A high float height can cause a very rich condition while a lower float height can cause a lean condition.
Since the Amal Concentric main jet is located in the bowl, metering the fuel level will alter the running condition at a given point.... this also includes the pilot circuit.
Each manufacture (for the most part) will give you a suggested float height specification on a given carburetor. Often times, you can adjust the float height to suit your specific application.
- Meter how much fuel enters and retained in the float chamber
Under racing applications it is not uncommon to have your fuel level "go low" or "starve".
This typically happens when fuel is being used faster than the fuel entering the float chamber.
To tackle this, you will need to alter your fuel levels and test accordingly.
Identifying float related parts
Before we launch our tutorial on how to adjust the float height on your Amal Concentric Carburetor lets go over the basic parts and there specific functions.
The parts inside an Amal Concentric float bowl
- Float - designed to shut-off / meter fuel coming into the float chamber
- Spindle - designed to allow the float to freely "pivot" up and down
- Needle - float needle is designed to shut off fuel that is coming through the needle seat orifice
- Bowl - also know as the float chamber. Designed to hold and retain fuel inside the carburetor
Adjusting the float height
Float height specs
Now that you know the basic parts and functions of an Amal Concentric carburetor we are ready to adjust the float height.
The first step is knowing where to set the float height.
Click on the Triumph Service Bulletin above to enlarge / download
Above we have an original Triumph Service Bulletin dated (7-18-1973) that suggest that the Amal Concentric float height should be .080" below the float bowl surface.
There are documents online that suggest the float height should be set at .080”, .060” and .040”.
For this tutorial I will set the float height at .040”.
Performing the adjustment
The only tool needed for this adjustment is a caliper and a vice.
Make sure you have the new Amal "stay-up" float as the metal tabs can be maneuvered to change the float height.
Using the original white plastic floats will not work with this tutorial as the tabs have "memory"... as you bend the tabs, they will bend back to there original state.
In the old days, to adjust the float height you would have to raise / lower the seat in the bowl.
No need to do that as we have the new “stay-up” float.
Click here should you need the adjustable Amal "stay-up" float for Amal Concentric's.
Coat your Amal Concentric float bowl with red Dykem to showcase the scribed line which is @.040” below the gasket surface.
With our figures at hand (.040") we will scribe the bottom of the float bowl to indicate where the maximum float height will be. Use the back end of your caliper to do so.
Now that we know where .040" is in relation to the float bowl, we will install the float, pin and needle together as an assembly to test where our current float height is.
To test the float height @ .040" first assemble the bowl with all parts installed (off the carb) and flip the unit upside down.
While the unit is upside down hold the float bowl with one your left hand and use your thumbnail to hold down on the center of the spindle.
Do not press or apply pressure on the float or tabs as this will alter your reading.
When you have this step correct, you will notice that gravity is pushing down on the float and closing the float needle in the seat... this is exactly what you want.
Take notice of the back edge of the float in relation to the scribe on the bowl.
If adjusted correctly, the float should be parallel with the scribe mark we scribed inside the bowl as shown earlier in this tutorial.
That is your float height..
If your float level is below or above the .040” scribe mark then you must bend the tab (very gently) on your Amal “stay-up” float.
Keep checking your work until you get your float to meet with your proper scribe mark.
Always double check your work!
Alternative checking - fuel level
Another alternative to checking, measuring and adjusting float height is to make a jig or a system to physically see the fuel level.
You can do the steps below on or off your motorcycle.
This method I will go over below is an accurate way of double checking your work but is not mandatory.
Instead of setting you float height and staticly checking, you can check via fuel levels as an alternate way..
Before you begin testing fuel levels you will need some of the following tools / parts to execute the task...
- Drain plug
- Auxiliary tank
- Float bowl (with a drain plug access hole)
- 1/4" I.D. clear fuel line
Complete internals for float bowl (spindle, needle, etc)
- (1) Main jet (largest size - drill if necessary)
- 2BA Tap
- Drill bit for 2BA tap
- Zip ties
- Rubbing alcohol
On later Amal Concentric carbs the float chamber was introduced with a drain plug. This allowed easy access to change the main jet and drain the bowl rather than taking the entire chamber off the carburetor.
For this tutorial, you will need a bowl with a bottom drain plug if you do not already have one.
If you do, having a spare drain plug to do this mod is recommended as we will be drilling and threading the plug.
Drilling and taping
Using a hand drill or drill press (or lathe) you will have to find the center of the plug.
From that point you must drill the plug, then cut the threads using your tap.
Amal main jets are "2BA"
Once the plug is ready, install and tighten your Amal main jet.
If you have pipe thread tape in your shop, apply it now.
When done, your plug should look (better) than mine shown above.
Don't laugh! - I did this many years ago with little tools and no skill!
Now that your plug is ready, we must measure the fuel level.
Insert the float, needle, and spindle as an assembly as if you where assembling the carb for the final time.
Use your 1/4” I.D. clear hose and slip it over the main jet that’s threaded into the bottom of your plug and use zip ties to fasten the fuel line as close as you can next to the bowl.
Install your fuel banjo (single or dual) and connect another section of 1/4" I.D. hose to an auxiliary tank or testing tank.
If you do not have a separate tank for testing, you will have to feed the line manually.
Use zip-ties to secure both lines to the float bowl.
Make sure both hoses are as vertical as possible
Add alcohol through the feed line (or tank) and watch the float rise until the needle is fully seated (needle must be fully closed).
As the alcohol is filling the float chamber, you will see the fuel line reflect the level of the bowl.
Keep adding alcohol into the bowl until the needle "shuts" the fuel off.
If you notice that the fuel level in the bowl is not correctly reflected in the clear fuel line, you may have an "air bubble" trapped in the line. Remove the air bubble by installing or drilling a larger hole in the main jet, or let the bowl "sit" and the air bubble may clear itself out.
Make sure the float does not "lift" the spindle as it will throw-off your readings - use a wight as shown.
Monitor the alcohol inside the clear hose.
Since we set the float level at .040” below the gasket surface, the fuel level in the gas line and bowl should sit anywhere .170" - .240" (according to Amal) below the gasket surface.
Remember, we are measuring fuel levels at this point, not the float height as described earlier.
You can choose to mark the float height on the outside of the carb bowl when using this method for checking so you can compare the fuel levels more accurately.
If either to high or low, make adjustments to the float by bending the tab as described earlier in the blog post.
When happy with the results, assemble your carb.
Now that you have read the above steps on how to adjust the float level in an Amal Concentric carburetor, now is a great time to put your carb to the test.
Although I do recommend checking float height, I do not recommend removing your Amal carb or taking your carb apart (that is in good working order) from your machine to perform this test or adjustment.
This tutorial is great for new carbs, original carbs, or carbs in which you are starting the tuning process from the beginning.
It is not uncommon to try different float heights. As stated before, if you require an overly rich condition raise the float, lean, lower the float. Be sure to take minor adjustments then check your work.... repeat when necessary.
Although this Tech Post was intended for Amal Concentric carbs you can perform a similar test on Amal Monobloc carbs.
Unfortunately I will not be covering the Amal Monoblocs in this post but in the near future I will dive into checking and adjusting.
In the mean time you can enjoy an awesome photo of my Amal Monobloc on a testing jig I made to check the fuel levels.
Plexiglass was cut to see the fuel level in "real time".
When performing the float height adjustment, it is vital that all your parts are in good working order.
If the condition of any parts are questionable, please replace them immediately.
Here is a compiled list and links of the basic parts inside the Amal float bowl
|Part Type (Name)||Part Number||Link|
|Float Bowl (Concentric)||622/055||Click Here|
|Float Spindle||622/071||Click Here|
|Float Needle||622/197||Click Here|
|Bowl Gasket (Concentric)||622/073||Click Here|
|Stay-Up Float||RKC/550||Click Here|
Thanks for reading
If you enjoyed today’s Tech Tip post on how to adjust Amal Concentric float height, then we recommend checking out our full range of related Tech Tips posts and blog / news.
Click here to view our archive of literature.
Should you have any questions or would like to chime in, please feel free to comment in the section below and we will follow up with you at our earliest convenience.
Thanks for reading, ride safe!
By far the best I’ve seen!
By far the best I’ve seen!
By far the best I’ve seen!
I have just realised that I have been confusing float level and fuel level! (doh).
The Amal document refers to fuel level and the Triumph and other documents refer to float level (which is obviously higher than the fuel level).
Classic British Spares
I personally would try .040 to .060 – I am aware of the different figures but it does not hurt to try different settings prior to tuning the carb