On after market (RB) set ups, drill 2 holes right under the carb into the manifold. (with either the manifold off or foam stuffed down under the drilling holes, then vacuum out drill debris when finished). Tap the 2 brass oil tubes from the old stock carb into the new holes and attach oil lines. This set up keeps the carb clean and drops raw oil down into the engine under the carb.

 

no need to drill the manifold...my weber comes with two little tubes for feeding oil into the bowls, it premixes the gas in the bowl before atomizing the whole mixture.

this was a lake city mod back in the 80s...I don't know if it still done.

 

That's why you drill the manifold. The later model RX-7's inject raw oil into the rotor housing. (lubricates better) and you don't get dirty/clogged carbs...