Unlike a two stroke, you do not want to tap a pulse hole in the crankcase. While this will work, at least on the Honda four strokes, it will eventually be problematic due to oil getting into the pulse line.

What we are doing is tapping directly into the intake manifold between the engine and the carb. This provides a vacuum pulse when the intake valve opens, but you need a small spring in the fuel pump chamber to "return" the diaphragm after it receives the pulse.

This manner does not provide a "+/-" pulse like a two stroke setup, hence the need for the spring in the pump.

AV8TOR