This is an original NOS hot idle compensator for Mercury Cougar or Ford Mustang models with V-8 engine and factory A/C. It is installed in the middle of the hose that connects the PCV valve to the rear of the carburetor spacer. It opens when it reaches a certain temperature which makes the engine run faster when at idle and the A/C is on thereby making the engine run more efficiently. This is an original Ford part that came on air conditioned models from the factory. It was manufactured by Carter and has the original "CARbureT ER" logo stamped on the underside of the top arm. This part is mostly brass and is in great shape. It has never been installed. If yours is missing or defective this is an excellent replacement. It is also correct for Fairlane, Galaxie, Torino, and other Ford & Mercury applications. Limited quantity and a very rare and hard to find part in NOS form!
*NOTE* This answer was given at www.ask.com; When the under-hood temps get very hot, the fuel has a tendency to want to boil in the carburetor. When it boils, it percolates with bubbles like a coffee maker and will push fuel out of the carburetor over into the engine, making it idle rich and run rough. The idle compensator opens up when it gets hot, letting air into the vacuum line, basically a vacuum leak. This makes the engine idle faster, which turns the fan faster, which helps cool down the under-hood temperature and basically makes it idle better on a very hot day.
From VMF; "When the ambient under hood temp increases to a certain point the bi-metallic strip on the hot idle compensator opens a rubber needle - brass seat metered orifice, causing what is essentially a vacuum leak of a predetermined amount into the intake system. This extra air will in turn cause the idle speed to increase, moving more air through the radiator and water through the cooling system due to increased fan and water pump speed the result being the motor will cool down to the point where the ambient under-hood temp is lower, and the valve closes and the vacuum leak will stop, returning the engine to a normal idle speed.This cycle repeats itself over and over. Obviously this will only help during periods of engine idle. Remember, this was engineered in the early sixties and in theory is workable. What normally happened is that IF the little rubber needle valve did lift from the brass seat and cause the idle to increase, it sometimes would not seat properly when cooler, and caused an erratic idle. Most were tossed in the garbage back in the day. In the real world, this little device probably caused more problems than it solved! It had a somewhat short life, was kind of a band-aid fix, and was replaced with a bigger and better cooling system as more and more Ford products were sold with factory A/C."
