Converting the Old OMC Dual Fuel Line to
the New Single Line

Fuel Tank Problems :  OMC engines before 1959 used a dual line fuel tank hose & no fuel pump.   They were a Siamese dual hose with pressure one line coming from the engine into the tank, pressurizing it & then pushing the fuel from the tank to the motor.

These old tanks are getting hard to come by & expensive for one & for two they were prone to, if there was any slight leak, the motor would then not perform, if it would even start.  The solution is to convert this old fuel system to the newer the single line. 

First you must understand the functioning of a 2 cycle engine.  The crankcase of 2 cycle engines are sealed chambers of which alternating cycles of pressure and vacuum occur as the piston moves forward & back inside the cylinder. As the piston travels forward, the crankcase is under a vacuum and the fuel/air mixture from the carburetor is drawn into the crankcase. Then when the piston travels rearward it develops pressure inside the crankcase, the fuel/air mixture is then forced out of the crankcase and through the transfer ports on the side of the cylinder into the combustion chamber above the piston. The ports are placed in a position so that as the piston moves forward the piston covers the port allowing compression to build at firing near top dead center.  On a multi-cylinder engine, each cylinder’s crankcase is sealed from the other cylinders. 

The secret to making a fuel pump work in this application is to convert the source of pressure for the original pressure tank into a source of pressure and alternating vacuum to operate the fuel pump. The source of power for the old pressure tank is usually supplied by a hose coming off on the intake manifold, which is usually directly under the carburetor.

Illustrated below is how I did it on a 1958 18hp Johnson.   This conversion could also be utilized by making slight alterations on most of the older motors no matter what size.

I happened to find a sideplate with a fuel pump attached off a slightly newer, but possibly a 10hp motor.  Apparently these motors later than 1959 but up to the model change in 1963 used just this conversion.   All I had to do to utilize it was to drill the mounting screw holes out to match the larger size #10 of the 18hp.   The clear, (amber) colored line goes to the fuel filter & then on to the carburetor, while the larger black line is the suction line from the coupler on the other side of the motor.

This conversion could also utilize the later fuel pump of the 9.9 motors if the mounting base was altered to the newer pump.  By this what I mean is that if you are not as lucky as I was in finding this unit, then take the original sideplate, build it up with J_B Weld epoxy, file a flat to mount the new pump.  Then drill & tap mounting holes for the new fuel pump, & drill a vacuum hole to match the center hole of the fuel pump in this altered sideplate.   The vacuum hole does not need to be in any exact location, just so it goes thru the sideplate.

Another & easier method would be to use the later 9.9 (from about 1988 on) remote mounted fuel pump that does not mount by bolting it to a fixed base.  These have a hose barb that can allow the fuel pump to be mounted about anywhere & connected to the source by a line.

One thing to remember is that the cowlings will need to be able to be re-installed, so this can not protrude excessively.

Right Hand view of this 18hp Johnson after the new fuel line conversion

The fuel line coupler mounting holes in the left hand side of the motor are the same as the older dual line.   Remove it & replace with the newer single connector.  In the picture below the #1 blue arrow points to where the small black line from the filter/sediment bowl connects to the carburetor.   The amber fuel line comes from the fuel pump.  The #2 arrow is where the original copper pressure line went to the old fuel connector.   This old fitting was just silver soldered to plug it & reinserted back into the threaded hole it came out of.   You will note that the new fuel line connector is in place just under the tail of the #2 arrow.  

Left Hand view of this 18hp Johnson after the new fuel line conversion

The fuel filter/sediment bowel does not really need to be there, but since it was already mounted, I just left it & re-routed the fuel lines to & from it.

Originated 09-23-06 & last updated 01-01-2007
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