I have a sonic 35ss, 725's super charged, bravos. The boat has 2 70 gal aluminum tanks which were replaced in 1997. I have only owned the boat for 2 years and bought it as a complete project.
The port side tank uses much more fuel than the starboard . Each engine runs independent on its own tank. There is no fuel valves or crossover that I can see. In fact it is a very simple fuel system . 1/2" line with AN fittings straight from tank to mech fuel pump then to fuel filter and then to carb. Port tank drains 1/3 faster than other.
Here is a list of the tests I have done so far:
Filled both tanks to the top, ran the boat then topped both tanks again, results 45 gal used port . 30 gal used startboard.
I did that test several times with always the save results.

Next I swapped the fuel lines so the port engine now draws from the starboard tank. The problem does not follow the engine it stays with the port tank. Again I filled both tanks to the top, ran the boat for a while and topped the tanks both off again . Port tank took 1/3 more fuel .

There is NO smell of gas at any time and my bilge has no signs of fuel.

Any ideas??????

Billy, are you 100% positive the tanks are not connected to one another ? I have 2 -100 gallon tanks and I always notice one empties a little faster then the other. Not as fast as yours but noticable by time I run low on fuel. I always "assumed" that they are connected somewhere and one just gets pulled from a little more then the other.......

That is a lot of difference. Have you noticed a difference in RPM, thorrtle response, power, etc. on the starboard engine? Have you changed both engine mounted fuel filters? Did you run rhe starboard motor out of fuel while still running the port motor? If so, you may have clogged the starboard fuel filter. Check your fuel pressure on both engines. Previous owner may have replaced the starboard fuel pump with the wrong pump.

Ed
'99 Sonic Prowler 260
502 MAG/380 Hours/B1 Drive/B1 24P Prop
Dart PRO-1 Aluminum Heads/SS Intakes/Inconel Exhausts
Comp Cam Roller Rockers/Push Rods (.375")
Cometic SS Head Gaskets
ARP Studs
IMCO Aluminum Exhaust
Hardine Polished SS Risers
Complete MSD Ignition/Cooling Fan
Denso Iridium Plugs/MSD Plug Wires
K&N/Hardine Marine/PWR Air Intake
ECM Upgrade
San Juan FWC
RACOR Water Separator

Thanks for your response Ed, if you read my post you will see that I switched tank pick ups from one tank to the other so that port engine now pulls from starboard tank and vice versa . That eliminates any variables of engines or fuel system. It's is tank related

The more I read the more I feel there connected....... I'm no expert though.....

Weird problem....., you don't have the fuel tank valves under the rear seat marked right/left?
I think someone asked if the vent lines were tee'd together ?
Can you see the entire fuel line from the outlet of the tank all the way to the pumps.

The vent lines are not tee'd together and the fuel lines go directly from tank to pump. I made the lines and I stalled them myself. I didn't use the fuel selector switch because it only has 3/8" fittings which are not enough for 725hp. I might just install a transfer pump between the 2 tanks to manually keep them even . But the big question is , where is the fuel going?

OK, let's review. The fuel systems are not connected in any way. The fuel is not being pumped into the boat. That would be a lot of BOOM!. So I believe the fuel is some how being syphoned out of the port tank while you are running. That is the only way it can get out. Question: are the tank vent lines located in the same place on both tanks? I don't see how this could happen but it is the only way you could be loosing the fuel. You are not burning it, it is not leaking into the boat and the systems are totally isolated. I'm still scratching my head on this one!

Vent line is pinched or clogged causing the tank to pressurize and push more fuel to the motor?

Can't push more fuel to engine, I have fuel pressure regulators