Fuel Level
Although the Concentric carburetter will function across a wide range of fuel levels, an accurate fuel level is the foundation of the overall jetting of the carburetter and makes a significant contribution to the smoothness and performance of an engine
The fuel level is determined during manufacture by the position of the float needle valve seat and should not require adjustment under normal circumstances. For performance tuning or where a carburetter runs consistently rich or weak despite being fitted with the correct jetting, it may be desirable to check the fuel level to establish a calibrated basis for tuning, or re-establish the carburetter within the correct parameters.
Because there have been changes to the float chamber since the Concentric carburetter was introduced, and because there is no way of knowing what alterations may have been made by a previous owner, measuring the fuel level is the best way of setting up the float chamber..
The correct fuel level for all Mark 1 Concentric carburetters is 0.21” plus or minus 0.040” below the top edge of the float bowl. Thus when the needle valve is being held shut by the tangs of the float, the level of the fuel will be between 0.17” to 0.25” (4.33mm to 6.35mm) from the top of the bowl.
The fuel level can be checked by removing the float chamber and observing fuel running into it. The fuel flow should be sufficient to hold the needle valve open until closed by the action of the float. Insufficient fuel flow will cause the needle valve to seal under it's own weight before the float rises far enough to press the valve shut. The level of the fuel can then be measured down from the top surface of the float chamber.
The fuel level can be checked by removing the float chamber and observing fuel running into it. The fuel flow should be sufficient to hold the needle valve open until closed by the action of the float. Insufficient fuel flow will cause the needle valve to seal under it's own weight before the float rises far enough to press the valve shut. The level of the fuel can then be measured down from the top surface of the float chamber.
The fuel level can also be checked by attaching a piece of clear tubing to the bottom of the float chamber. A plastic float chamber drain plug can be modified to mount a suitable spigot, such as an old jet, to attach the tubing. Route the tubing in a vertical position alongside the float chamber. Open the petrol tap and fill the float chamber with fuel. If the fuel level is in the correct range the fuel will rise in the tube to a point between 0.170” and 0.240” below the top edge of the float bowl. Start the engine and ensure the fuel level remains within the correct parameters.
The new Amal “Stay Up” float has stainless steel tangs which can be bent to alter the fuel level. The tangs can be easily adjusted by clamping the float in a vice up to the spindle hooks and tapping the body of the float gently in the direction required until the required level is achieved.
Needle Valve
If your float chamber is fitted with a brass needle valve you may find the valve sealing under its own weight before the float has risen far enough to press it shut. Symptoms of this problem can be that the carburetter takes a long time to tickle, hesitates on pickup, and does not idle reliably. A Viton-tipped aluminium needle valve is now available that overcomes this problem. It is being fitted as standard equipment to all new Mark 1 Concentric carburetters.
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