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Friday, May 9, 2025

Setting AMAL Fuel Level

Most of these instructions were taken from the Amal web site. I've re-written parts and kept only those parts specific to the Concentric MK1 carb using the RKC/561 4-Stroke Major StayUp Kit from Amal to rebuild the carb.

The concentric carb will function across a wide range of fuel levels. However, an accurate fuel level is the foundation of the overall jetting of the carb and makes a significant contribution to the smoothness and performance of an engine. Setting the float height is hard with the old plastic float because the float needle seat had to be raised or lowered by hammering. The new Amal StayUp float has stainless steel tangs which can be bent to alter the fuel level.

If your float bowl 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 carb takes a long time to tickle, hesitates on pickup and does not idle reliably. A Viton tipped aluminum needle valve is included with the rebuild kits, and that needle overcomes the problem.

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These instructions are written for setting the fuel level with the float chamber (bowl) off the carb. It can also be done with a modified drain plug with clear plastic tube so the fuel level can be seen with the carb on the bike and the bowl on the carb. If using the second method, the height must be checked at the middle of the carb (in line with the main jet.)

Per Amal, the correct fuel level for all Mark 1 Concentric carbs 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 0.24" (4.33mm to 6.35mm) from the top of the bowl. The fuel level can be checked by removing the float bowl 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 its 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 bowl.

It is difficult to see the fuel in the float so I set my caliber at .21' and use the depth indicator to make a fine scratch on the inside of the bowl. Then I use a fine sharpie to make a mark that I can see while adjusting.

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Then I use a single banjo (or dual banjo with one side sealed), and a piece of tubing, and slowly fill the float bowl with water through the tubing while holding the bowl level. The float will stop the flow and, using a flashlight, I can see the level of the water in relation to the sharpie mark. It is important to hold one side of the pivot spindle down while filling and checking so the pivot doesn’t float up and give you a false level reading. Generally, it shouldn’t take much bending to get the level right. If it seems like it’s taking too much bending to get the level high enough, you can carefully tap the float needle seat a little farther into the bowl - just don’t damage it in the process!

Note that water and gasoline have different densities and some say that using water is inaccurate. I've tested both ways and the difference is not measurable. If concerned, use gasoline. I just like the convenience of water and a sink.

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Once I have adjusted the level using the tangs on the float, I double check by checking that the top of the float is even with the far the edge of the bowl with the pivot is held in its slot by a thumbnail and the bowl turned upside down. If the float is higher than the edge, especially on a Norton, this can cause flooding. New Aluminum Premier carbs are normally adjusted correctly form the factory.
Finally, check the gasket cannot interfere with the float.>

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