This is the Air Intake Pre-heat assembly fitted to a stock RB30S.
When the engine is cold, vacuum is applied to the diaphragm inside the silver unit on top of the air intake.
This unit has a small arm which operates a flap that closes the normal air intake and opens the "Stove Pipe".
The Stove pipe is attached to the metal shroud which collects hot air from around the exhaust manifold.
Vacuum pressure to the diaphragm is halted as the engine heats up, so the flap closes the stove pipe and opens the normal air intake.
The Pre-heat system must be operational for the vehicle to pass a roadworthy.
To prevent the pre-heat flap from opening during a trip where I am likely to encounter water crossings or dusty conditions, I take the normal hose off and stop up the holes with the two plugged pieces. (2nd picture).
Another simple way to block the hose is with an appropriately sized ball bearing.
As I live in a cooler climate, and have had to remove the heater grid positioned at the base of the carby (It had started to disintegrate and pieces were falling into the engine) For normal driving I keep the pre-heat system operational as it helps the engine warm up quickly to normal running temperature.
stove pipe.jpg
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