A Look at the Amal Concentric Floats
In this blog I will cover some information regarding the Amal Concentric MK1 carburetor floats and float needles. Preliminary checks of fuel level in the petrol tank, adequate flow of fuel through the petcocks (and inline filters, if fitted) , and fuel flow through the inlet banjo and filter into the float bowl will need to be carried out before turning attention to the float. Given that float levels can affect carburetion across the entire range of the instrument, it is important to consider the float and fuel levels before setting out to making any other adjustments to the carburetor and fuel system.
The original specification for float height in the first-generation 600 and 900-series Concentrics was to have the rounded top edge of the 622/069 hollow float between .060”-.080” below the top edge of the bowl, with the float’s plastic tang depressed. This was the starting point for float adjustment with yesteryear’s nylon floats and float needles.
We now have some upgraded components to consider with modern “Stay-Up” floats, Viton-tipped brass and aluminum float needles, and extended float “tickler” assemblies. With a thriving aftermarket of reproduction parts for the Amal Concentric instruments, production variances in float needle lengths, floats, jets, and even float bowl gaskets can affect the calibration of the fuel levels in the float chamber of an Amal Concentric or Wassell Evolution carburetor.
Let’s take a look at some of the design changes over the years:
Float Bowls
The original 622/050 Concentric float bowl was cast out of Mazak alloy and equipped with a .100” float needle seat orifice and no drain plug. Many early Concentrics still in service have had these float bowls replaced with later versions. The next float bowl supplied for 4-stroke use was the 622/055, which retained the .100” float needle orifice, but featured a removeable drain plug, from 1971-onward.
The latest version, 622/055AL retains the .100” orifice and drain plug but is now cast in aluminum alloy and is standard equipment on the Amal Premier Concentric models. Note: be careful when swapping around old parts bin float chambers, as there are some 622/057 bowls meant for use with fuel pumps equipped with smaller .062 float needle orifices. Accidently fit one of these to a twin-carburetor T120 or A65L and you will go nuts trying to figure out why the engine won’t run properly.
Float Needle Valves
Like the float bowls, there have been changes made to the float needle valves over the years. The original 622/068 float needles were made of nylon and were similar to the 376/089 Monobloc style, but with a “mushroom” head to retain the tangs on the Amal Concentric floats. These little troublemakers were lightweight enough to correctly operate but were prone to wear on the tip of the needle, where an adequate seal against the seat was not possible. They have been replaced with 622/197 Viton-tipped brass float needles, which have a rubber tip to enhance sealing.
The Viton-tipped needles can also wear but offer a longer service life and are recommended for replacement of the plastic needles when encountered in early original Concentric carburetors. The latest iteration of the float needle is the Viton-tipped aluminum 622/197AL, which is now supplied as original equipment in the Amal Premier Concentric carburetors. They are recommended for use with the current Stay-Up floats but are also compatible with the original hollow floats as well. Modern fuels may necessitate aluminum float needles as the brass needles can be too heavy to lift the needle off the seat under fuel pressure alone.
Amal Concentric Floats
The original 622/069 white hollow nylon floats are still available and while vulnerable to ethanol, cracks, and punctures caused by the rough ends of the tickler roll pins, they provide a buoyancy that works with the earlier float needles and are an economical replacement option. When the fuel level requires alterations in the float bowl with the original hollow float, the float height can be changed by warming the float bowl and lightly drifting the brass needle seat up or down as needed.
A more recent iteration of the Amal Concentric floats, the 622/069A “Stay-up” type is similar in dimensions, but not exactly the same, which is the reason for the revised float height setting and the suggested use of the aluminum float needle. The Stay-up float is easier to adjust with its metal float arm that can be slightly bent to achieve the correct float level. The composition of the Stay-up float is also resistant to the effects of ethanol and has a military-specification closed cell construction, making it puncture proof.
Norton BSA Triumph etc. Genuine Amal Concentric Float(s) PN# 622/069
Price range: $26.10 through $46.72
Ticklers
The “tickler” is a plunger that is used to depress the float, which in turn lifts the float needle to allow the ingress of fuel into the float bowl. The assembly was originally comprised of the cylindrical cover, roll pin plunger and return spring. The early ticklers were short and did little to prevent water or dirt from entering the float bowl. A subsequent version features a longer plunger, ventilated sleeve, and an alloy head that allows for easier and cleaner plunging of the tickler. The later iteration is available as a retrofit kit under part number 622/172. The extended ticklers have been supplied on all new Amal Concentric carburetors since the early 1990s.
Float Bowl Gasket
The float bowl gasket is designed to seal the joint between two flat surfaces, the carburetor base and the top of the float bowl rim. There are numerous producers of the 622/073 gasket, so there may be slight variances in the shape from the genuine Amal gaskets that are supplied with new carburetors. It is advisable to check that the gasket fits the float bowl without hindering the movement of the float. Sometimes some slight trimming of the gasket is necessary to optimize the fitment, especially in the area of the float pivot pin.
Amal Concentric Float Bowl Gasket(s) for Triumph BSA Norton etc PN# 622/073
Price range: $7.45 through $11.90
Technical Tips
So now having identified the various components that comprise the float chamber and fuel management system on an Amal Concentric, how are they assembled and calibrated?
- Check the float bowl rim for flatness. These can be warped due to overtightening the retaining screws. The fuel supply passage for the pilot jet passes fuel from the float bowl to the main carburetor through the float bowl gasket. When the bowl is warped, the gasket can leak air into the pilot fuel passage, resulting in a lack of fuel delivery to the pilot circuit. Lapping the top of the bowl on fine emery paper placed on a flat surface will usually correct the warpage to allow for a leak-free joint with a new gasket.
- Verify the integrity and size of the float needle seat orifice. The brass seat should be clean and free of any damage caused by drifting the seat in an effort to correctly adjust the float height. Before adjusting the needle seat, the bowl needs to warmed with a heat gun or dipped in boiling water to ease the adjustment process. The seat should have a uniform countersink to promote a good seal with the tip of the float needle.
- When reviving an older Concentric carburetor with an original hollow float, be sure to use a brass or alloy float needle with the viton rubber tip. Inspect the tip to verify there is no groove worn into the rubber. Check the float by submerging it into a basin of water and make sure it does not fill with water and sink. Sometimes over a long period of service, the rough and unfinished flared end of the tickler’s roll pin can wear a hole in the top of the float, resulting in a sinking float. Because of the closed-cell composition of the Stay-up float, this is not a problem.
- Setting the float level on an original hollow float calls for a gap of .060”-.080” between the top of the float and the rim of the bowl, with the float tangs pressed down to seal the float needle. Using a cigarette lighter to soften and bend the tabs on the float is not a recommended method to adjust the height. The float height must be adjusted by moving the brass seat up or down to find the optimal position, as previously mentioned.
- The Stay-up float, on the other hand, can be adjusted for height by carefully bending the float tangs, being mindful that the slight adjustment at the float arm end of the float will be magnified at the other end of the float. In the case of the Stay-up float, the desired height of the float should be level, or slightly above the rim of the bowl when the float arm is pressed down. Theoretically, when the correct float levels are set, the fuel level should be very close to the desired height. Measuring the fuel level is a good way to ensure the correct amount fuel is present in the float chamber.
- The correct fuel level for all 600 and 900 series Concentric carburetors is .210”, plus or minus .040” below the rim of the float bowl, so when the float needle is being held shut by the tangs of the float, there will be a fuel level of .170”-.250” from the top of the bowl.