Hey silly, does your boat sit level on trailer once hooked up to car or bow down once hooked up. It can be a problem if boat is bow down as usually breather is a front of tank.

My little traily has the standard 120ltr tank and will flow out of breather once close to full if l dont click it of to let the fuel settle. Then l can slowly fill to the brim. I have had problems at stations if there is a slopping driveway and boats not sitting level.

Can you get more than 80ltrs once it overflows by slowing down flow after that or is that it.

If you say its 200ltrs and you can only get 80ltrs in it theres something not right.

Is this tank custom made?

Sorry cant help with fuel sender. Doesnt sound like factory fitted.

Any photos of the tank and breather?
Weird question but how did you know the tank was empty when you got 80L in it? Is it possible the guage is wrong and it still had 120L in the tank prior to filling.

CCC

 The only way to ensure you have correct measurements to your gauge , suck it all out and measure what goes back in, in stages ie. 40 lts.  Then 80 lts and watch and record what's on your fuel gauge for future reference , my trailly had the 240 lt tank but indicated full when only 165. Lts where pumped in

 If you use a little thought. I had no fuel gauge or way of measuring contents in  last boat with supposed 170 litre tank under floor. So I fitted a Lowrance fuel flow gauge/totaliser. But you need a starting point, obviously, an empty tank. I blocked one breather by the simple method of removing it from the skin fitting and sicking a 1/2" bolt up it. I then disconnected the line from the filter and stuck a bit on so it would reach a jerrycan. I then inserted a tapered air nozzle into the other breather and GENTLY added air pressure, just enough to get the fuel flowing. I would do this once a year from then on as the fuel flow meter was optimistic and you always had more in the tank than you thought.

 Cheers guys, tried to put up pics in the first post... The boat is fairly level on the car and the servos I goto don't have sloping driveways either. Is usually wait til it overflows then maybe put in another 5 litres. I don't always get a chance for a click before it overflows.

i have filled it up after a days fishing and left it in the level driveway and the next day once the sun heated up the tank a bit fuel started coming out of the breather.. No idea whAt causes it but could it be my breather hose is partially blocked or something? 

I just used the gauge when it goes past empty then I fill up.. It could very well be that I have 120L and I fill it up 80l to overflow? It's not too bad tho as I have nmea connection which I automatically calculates fuel but if I forget to reset it I'll be back to using manual gauge. 

I think it will be a worthwhile investment for a decent fuel sender unit.

 Take the gauge out and manually check looks like old school to me it I was you I would replace with a weama fuel gauge they are very accurate and maint free is the pick showing fuel lines aft of boat or forward you could even adapt a second breather as set up might not be breathing quick enough

 first problem likely because it appears that the breather (and filler) goes downhill from tank. 

Fuel sits in the low point and spits.

 being out of whack. You are probably nowhere near empty when it says you are. But Rob H's observations are probably right, as well-- badly installed fillers and brerathers will give you no end of grief. Try my idea above next time it shows empty, (make sure you have plenty of empty containers on hand)or just take the sender out and have a look with a match 

In the photos this appears to be a veethree style sender where the aluminium channel from the mounting plate can be cut to suit different depth tanks and uses a float on the end of a hinged arm to determine depth. These can have issues, but the WEMA sender as mentioned above uses the same 5 hole mounting plate and has a captive float on a shaft, you just buy the right length sender for the tank.

The part number is S5-A(Depth in millimetres) for American 240-33ohms or S5-E(Depth in millimetres) for European 0-180ohms. EG S5-A200 for 200mm Depending on your gauge you need the correct sender or it will read backwards. Yamaha Gauges can be programmed for either sender. They can be found in chandleries for around 100-120$ for a 200mm WEMA unit.
Bryan

 Yep looks like the fuel sender is shot! It was reading empty and I just did the broomstick test and the tank looks to be 40% full.. I had a peek inside and there's a partition dividing the tank? Is this normal? You can see some diagonal welds on the top of the tank in the last photo of the original post. There's 2 partitions o think judging by the welds. 

Im off to buy the wema fuel sender and new hoses throghout

 Ya going to pull it all out for a good clean? Judging by what's on top reckon you might have some filthy sediment in the tank. 

 They are put there to minimise free surface effect, ie, fuel sloshing around and upsetting balance, they slow down the flow as you roll and pitch.  They will also have a stiffening/reinforcing effect.

Hey mate after going through something similar Rob H is likely right. If the breather allows fuel to 'pool' it will create an air lock which can only be cleared by pushing the fuel out of the breathers as you fill up.

Another thing to note is that the clear hose that you have there is NOT suitable for its use (apparently insurance companies wont cover it). I discovered that it was quite difficult to track down the right hose as it needed to be rated to have fuel sit in it, not just pass through it. I ended up sourcing all my fuel hose from henderson hose who were quite good at telling me what i needed after explaining what i was using it for. They also do the filler hose too. Aaaand they have quality stainless hose clamps which are not perforated like the ones you have there. 

Also, if your fuel sender screw holes are all the way through the ali and use self tappers you will need to seal the screw threads with something. I used Stag (available from Repco, but i have a full tube i could squeeze some out of if you're up near Joondalup) as it is apparently 'fuelproof', before i did this i had fuel leaking out of the top of the sender as it was getting past the threads of the screws, the rubber gasket DOES NOT stop this. Not good considering the only way you discover this is by having the floor up and having a FULL tank of fuel. 

Have fun

Steve

 Would be interested in your thoughts about what all the mess is on top if your tank and surrounds. Saw Randells pics of his tank in his thread and it looks the same. Ive had my tank out of my traily afew times and for a 11 year old boat the tank and underneath where as clean as the day they where put in.

 I just pulled the floor up in my trailcraft last week to replace the fuel sender and found the same muck around and on the tank it's just dirt and fish blood from the deck as the hatch was not fully sealed easy to wash off just a lite scub and hose 

  I've had a look and its just the seal in the lid wasn't 100% tight hence the muck. 10yrs of dirt, grime built up causing the it to look like that. I've checked Inside the fuel tank and its immaculate tho. Since the fuel tank is fully sealed theres no dirt to get in. I haven't been having any fuel problems (touchwood) just doing this as a part of preventative maintenance. Main reason was to check the fuel tank size. I originally thought it was around 120L as documented in early model trailcrafts so I'm stoked it's 200L. Won't have the feeling of running out of fuel after a big day. 

Cheers everyone for their help too. Looks like problem solved. 

 Hi silly

Did you find out what was causing your problem with the breather as I'm having the same issue with my trailcraft

When I fill it up it doesn't click off at the bowser it just flows out the breather and also when I leave the boat sitting there with a full tank it flows out the breather too

Cheers

I think it's just the tank design with the breather below the filler as Rob H was saying.. Once the tank is full the fuel will flow out of the breather as there is no place for the fuel to go, the fuel filler won't click as the nozzle won't be low enough before it comes out of the breather.

change the fuel sender if you get a chance, I plan on filling using the gauge so hopefully won't have fuel coming out of the breather again..

 Yeah thanks silly

Ive been testing mine and it doesn't seem to leak unless it's hooked up to my car, when it's off and jockey wheel is wound all the way out lifting the bow it doesn't seem to leak out the breather