The trio of rocker arms on the McIntosh & Seymour engine. Left is on the intake, right is on the exhaust, and the center arm opens the injector to spray fuel into the cylinders at the correct time. The canister above the center rocker arm is used to adjust a big spring that changes the pressure for the fuel. The need for pressure adjustment can be determined by putting a pressure gauge in line, or else by listening to how the engine is running.
Photo Courtesy Dave Preuhs