Fuel guage

I have a 2011 skeeter ZX225 Yamaha 225 SHO. It has 2 fuel tanks and button on the dash to change from one to the other. I keep the valve in the back in the middle so it will use both tanks equally. When I change the guage from one tank to the other they read different, one tank always has more than the other. Or at least that's what the guage says. I think one of them is not reading correctly. Does anyone know how these guages work? Also when I put gas in it I put equal amounts in each tank. Thanks

The fuel sender readings typically used (float style with rheostat) are notoriously inaccurate. Additionally, the sender often gets "tuned" for the shape of a tank by bending the arm. This could account for differences in the apparent reading even if the tanks held exactly the same amount of fuel.

If the gauges read unequal, and you add the same amount of fuel to each tank, I would EXPECT them to continue to read unequally! (for a working gauge). Just because you add 10 gallons to each tank, for instance, doesn't suddenly mean they should read equally. What if one tank had only 5 gallons and the other had 8 gallons before you added the 10 gallons?

For reasons to be explained, I'll bet if you topped off each tank, you'd find that it DOES take different amounts to fill each one. This could be why the gauges are showing a difference in level, relatively. Topping off the tanks is the easiest way to get an understanding of whether the gauges are not accurate with respect to each tank, or if the tanks in fact are holding different amounts of fuel.

When you select the "BOTH" setting on the fuel valve, you are not guaranteed a perfect world where exactly half of the fuel is drawn from each tank. From that valve to each tank, there are differences in the fuel system can can cause a small variance in how much fuel is drawn from one tank vs. the other. A difference in length of fuel hose to each tank can affect fuel system vacuum pressure (negative inches of mercury on a vacuum gauge). Dips or bends in the hose as it is routed from the valve to the tank can also affect the suction required to draw fuel. The biggest difference of all is in the fuel anti-siphon valve at the tank fitting. These devices are of a crude "go/no-go" nature, but a tiny difference in spring tension can make a difference in how much fuel gets past the ball in the valve.

You're never going to get the two tanks to draw down equally to perfection. One tank will go dry before the other because of the differences in the fuel system beyond the valve. This is one reason why step 1 of the check-list for lost fuel pressure on aircraft is often to try single tank settings.

Do the top off to check, but I bet it will take different amounts. The same tank will always be favored. Over time, you can learn about how much extra to feed one tank vs. the other. Run "BOTH" for convenience, but if you lose fuel suction on the water, simply switch to the tank you think still has fuel; it will be sucking air from the tank it emptied first causing a loss of prime.

Wow !! Very informative. Thank you

The sending units are simple rheostats, usually 0 to 90 ohms. And the gauges read 0-90 ohms. One unit has nothing to do with the other. There are many things that can affect the gauges and sending units. The only "cure" is to replace sending units or gauge.

I'm guessing that by "one unit has nothing to do with the other" you mean they aren't calibrated or synchronized, so to speak.

I will note that even with PERFECT fuel level readings using expensive technology other than the traditional rheostat solution, there is a 99.9% chance the gauges will show different amounts because the levels will begin to differ as soon as the engine starts to draw fuel. The engine fuel vacuum system will favor one tank or the other, pulling UNEQUALLY from the two tanks. No technology in use on boats is going to change that.

In my opinion, fuel gauges in boats are just an estimate at best. They were never designed to be super accurate. Each boater needs to learn how his gauges read and act accordingly. The only way to keep fuel level the same in both tanks would be to put a connection at the bottom of both tanks to allow flow from tank to tank.

My gas gauge works fine above half but as the Lighthouse guy on Lydia Ann channel who towed me out of the way of a barge will attest it stays on half until it runs out of gas.