With your electric fuel pump you need a filter between the tank outlet and the fuel pump, and the fuel pump and this line should be as close to the same height as you can get it. The output line from the electric fuel pump should connect directly to the carburetor. Electric fuel pumps are self regulating, so they don't need the 3 port bypass filter that was part of the original fuel system. The return fuel line that came from this 3 port bypass filter should be removed and the inlet to the fuel tank plugged, as it is no longer needed.
Some newer replacement fuel tank fill caps are designed with pressure controlled vents. The Cushman fuel system does not work with these, so it is necessary to remove the spring, etc. that prevents continuous cap venting.
The early Cushman brake systems use DOT 3 brake fluid which is alcohol based and the master brake cylinder has a cap vent. The vent is necessary, but it allows moisture in the outside air to unite with the alcohol in the brake fluid. Alcohol and water combine easily. When sufficient moisture has combined with the brake fluid, rusting of the internal brake lines and components occurs. It's best to completely replace the brake fluid every several years to minimize this problem.
In the inlet to the Keihin carburetors there is a tiny screen filter that plugs easily and is no longer available. It's best to just pull this filter out, if there is one, and discard it.
Valve lash is quite important to make these engines run well. These Cushman vehicles were grossly under powered and need to be at their peak to perform well.
An 18 hp OMC can be upgraded to a 22hp by replacing the cylinders. Higher compression achieves the additional horsepower. When last checked, these are still available from
www.directparts.com and
www.denniscarpentercushman.com. These cylinders come with valves and springs installed. There is no difference between the right and left cylinders. They are both the same and both valves are identical in each so it doesn't matter which is intake and which is exhaust.
There is a manual available that contains the complete OMC motor information on
www.sillylittlecars.com website. Go to "Manuals" and then click on the text "service manuals" and a list of available manuals will appear. You want to download the "supplement" of the manual. The main manual contains only body part diagrams and part lists for parts that are no longer available. The "supplement" contains the motor information and electrical diagrams for many models of Cushman vehicles of the 70's and 80's
Charley