The OMC pressure tank system did not have a fuel pump as such. It incorporated an air line off the crankcase that pressurized the fuel tank to 4-6psi forcing the fuel out of the tank and up to the motor through a hose that had a fuel and air line. This system was used (as I noted above) starting in 1949 and ran through 1959-60 – there were some modifications and updates throughout the years.
The tanks all had a provision to pump fuel up to the carb to start the motor via a manual primer pump. Once started, the pressure took over provided that the cap was screwed down and there were no air leaks. All the tanks had a pressure relief valve in case the pressure got too high. And as noted, the early tanks also had a fuel shut off valve so you could leave them pressurized – thereby saving your manicure and not having to prime them. 😆 That feature was eliminated on the 1952 and later tanks, frankly, it just isn’t that hard to prime them!
Pressure fuel and oil systems were quite common on cars, tractors and other machinery for many decades. Many chain saws still use a similar system today for bar oiling. A common reason cited for not using a fuel pump is not having to pay GM royalties on their fuel pump patent. (Something that dates back to Henry Ford and the Model T &A) I don’t know if that is true in the case of OMC, but the pressure system was/is safe and reliable – I’ve been running motors with it for over 40 years and had no more problems than with later siphon systems.
Some of the many styles of OMC pressure tanks:
