There has been some debate about how many gallons/Liters the tank is on the AK 550. Prior to 2021, and prior to the Euro 5 models, the tank is reported to be 3.9 gal/15 L.
Mine is a 2022 AK 550. The debate partly stems from the changes to the Euro 5 models which have a 3.25 gal/12.3 L tank. And now the AK 550 ST for Europe is reported to have a very slightly larger tank, maybe 12.4 L (almost 3.3 gal).
So I decided to attempt to measure the volume of the tank on my AK 550. The manual says 3.9 gal/15 L. But due to controversy I figured the best way is the siphon the tank empty, or as empty as possible, and measure how much gas I put back in for a full tank.
I have a 1/4 inch vinyl tube that I attached a small wooden dowel to the last 8 inches to keep it straight. The tank on the AK is almost a cube with rounded corners, so I was expecting to get almost all the gas out I was able to get the siphon running without sucking the end. (the tube fit snug into a funnel I was using so I primed the tube using that). I have an extra 5 gal gas can so that's where the gas went into, but I only really had less than 2 gallons in the tank to start with.
When the siphon stopped I used my flashlight to find that there was still some gas left. After feeling around the bottom of the tank with my tube and dowel setup I found that there is a slight drop in the tank bottom just under the fuel pump. I was unable to get the siphon going again to get that gas out. I am guessing there was somewhere around .75 to 1 L of gas left in that well under the pump that I wasn't able to get out, maybe a little more. So I started to measure and refill the tank.
I ended up putting 3.63/13.75 L of gas back into the tank. I was using the Liters measurements on the 2 Liter bucket I had with 250 mL markings on the side. With the gas that I could not remove I would guess that my tank is indeed what the manual spec says it is... 3.9 gal/15 L. I put the AK on the side stand before I filled it all the way since that is how I always have it to fill the tank at the station.
Since I was not able to get all the gas out I did run the engine for a while to see if I could burn off what remained but didn't want to do that for very long so as not to damage the fuel pump. I intend to try this again but next time I want to get a siphon bulb with a one-way valve to get more of the gas out.