I think Zach is right about gas in the vent line restricting the vent. If there isn't a positive slope from the tank all the way to the vent outlet, you could have fuel trapped in there. Another potential issue is where the vent is located on the tank relative to how the boat is sitting in the water or on the trailer. The fuel fill on my Seafari is on the starboard side a couple feet from the transom. The tank vent is near the front of the tank a couple inches in from the port side. It's designed so that if I'm refueling at a marina the boat the boat will be heeled to starboard and down at the stern with my weight in the back, the vent is at the high spot on the tank.
However when boat is on trailer at a gas station, the front of the tank is lower than the back, so the vent will spit gas before the tank is full. When I get home and raise the front of trailer, it will typically take about another 5 gallons to get the tank completely full.
Regarding your fuel gage, those wiper type sending units can be notoriously inaccurate because if you don't use the boat frequently, oxidation can build up on the resistance coil and wiper and change the resistance and gage reading! I've noticed mine often does not show full until I've burned off enough gas for the float to start bouncing around to clean off the wiper surfaces, and of course by then it's not full anymore, but it usually reads higher than it did when I first started out with a full tank! A Weema sending unit like Gillie has should eliminate that problem.
One thing I did when I first got the boat was to calibrate the fuel gage. I was running premix then, so it was a big help to know about how much it would take so I could add the proper amount of oil if refueling at a marina. The calibrated gage was accurate enough that I could usually predict within a couple of gallons how much it was gonna take! Here's how to calibrate your gage, starting with an empty tank:
1. Put a piece of white vinyl tape across gage. If it's just a simple 90 degree sweep gage, maybe use two pieces of tape, above and below needle, so you can put reference marks at each end of needle. A fine Sharpie marker works good and the ink doesn't fade.
2. Pull the sender, measure the inside height/depth of the tank, and then make sure the float is adjusted so it just touches bottom of tank at one end of it's travel, and that it's also at the other end of it's travel when it touches the top of the tank.
3. Hook up battery charger so battery is at same voltage it'll be at when motor is running.
4. With the sender still out of tank but connected to the gage and ground, turn on power to the gage and mark Empty and Full positions of needle when sender is at the extremes of its travel. Then install sender and recheck your E mark.
5. Put boat on a level surface and use trailer jack to get it level fore & aft.
6. I then filled the tank in 6 gallon increments, since I was running the 50:1, 1 pint/6 gallons oil mix, and I had 3 portable tanks. Wait for gas to quit sloshing around and needle to stabilize and then mark gage after each increment. If not running premix, maybe 5 gal. jugs would be more convenient.
Although the E-TEC's I-Command gage system tracks fuel usage and is usually accurate within a gallon or two, I did notice on one occasion on a long trip that the gallons used number had stopped increasing so I had to reset the gage to get it working again. Since running out of gas can ruin your day, I like to have an independent measurement to keep the electronics honest!
Here's another tip that makes it easy to drain the tank. Most Racor type filter heads have more than one outlet, so just install a male outboard fuel line fitting like those in portable tanks in one of the outlets. I made up a 3/8" drain hose with a female fuel connector in one end, a primer bulb about 6" away, and then about 6' of fuel line at the primer bulb outlet. I use my boat as my emergency fuel supply for my vehicles during hurricane season, so if we have a storm that knocks out power to the gas stations, it's easy to just run that hose out the bilge drain hole into a 5 gal jug to get gas to keep the cars or a generator running without the hassle of trying to find a gas station with power when everyone else is trying to do the same!
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