So, my 510 had sat for a while and I was having some issues with keeping it running, thought maybe there was crap in the tank, also wanted to check if any rust had built up in the tank (side note: it was my carbs untuned). So I removed the fuel tank sender unit to look around, put it back on in the correct direction. Filled it with some fuel and it read 3/4 of a tank. Drove it for a while and noticed it was still at 3/4 of a tank and finally it felt like it was running out of fuel, sputtering. I opened it up again, checked the position, checked if I had damaged anything and checked the cable. All looked fine. Put it back on filled it with about 5 gallons of fuel and now it reads 1/4 of a tank... I'm pretty sure it should read more than 1/4.
I'm at a bit of a loss. It has been working fine for years with out issue. When the car is off it reads empty, when turned on it goes up to 1/4 of a tank and stops.
I don't want to blindly replace parts, any ideas on what could be going on?