Page "Rotary engine" Paragraph 16

Cutting of cylinders using ignition switches had the drawback of allowing fuel to continue to pass through the engine, causing the spark plugs to oil up and prevent the engine from restarting.

A raw fuel / oil mix would also collect in the cowling.

As this could cause a serious fire when the switch was released it became common practice for part or all of the bottom of the basically circular cowling fitted to most rotary engines to be cut away, or fitted with drainage slots.