Originally Posted by tgreese
Gasoline tends to condense on the inside of a cold intake manifold. Admitting heated air when the engine is cold helps to keep the engine running without using a wildly rich mixture. A good feature - benign and simple - keep it if it works. I'd guess it has a slight benefit to engine life for an engine that's run in cold weather a lot, by reducing the washdown that results from a bunch of extra liquid gas in the cylinders. More about cold weather driveability than emissions, but it does have a secondary positive effect on emissions.
Closing the air cleaner snout seals the air horn when the engine is off. This improves emissions by preventing bowl vapors from leaving other than through the vapor recovery system. Likely also reduces evaporation from the float bowl, reducing the likelihood of "dry bowl syndrome." Also a benign and beneficial system. Keep it if it works.
Thank you. I'll just add a few things to that.
There is this radical idea that engines that run better produce fewer emissions too. It doesn't have to be one at the cost of the other. They happen together, hand in hand.
Carbureted engines HATE cold intake air until the carburetor and intake manifold become warm. Cold air doesn't vaporize the fuel very well, so liquid fuel just dribbles into the intake manifold and runs by gravity into the nearest cylinders, flooding those cylinders while starving the others. This runs like crap. Another problem with cold intake air is the air becomes colder as it passes through the closed throttle plate by Bernoulli's Principle. This can condense and freeze the moisture in the air onto the throttle plate and cause the engine to stall. Heating the intake air until the carburetor body and intake manifold warm up sufficiently eliminates all of these problems. The air cleaner has a thermostat that switches to cooler air after everything has reached a sufficiently warm temperature. Heated intake systems have existed since the very dawn of internal combustion engines. In fact, the very first internal combustion automobile (Benz Patent-Motorwagen) has a heated air intake for fuel vaporization, and I assure you emission control was not on Carl Benz's mind in 1885. Many cars used heated intakes in the 1920's through 1950's.
The inner door (called the trap door) is purely an emission control device. The heat soak from shutting off a hot engine causes the fuel in the carburetor bowl to boil and vaporize, releasing a lot of vapor into the atmosphere. This is obviously bad for emissions, but it also makes your garage STINK like gasoline....really bad. I developed the habit of raising the hoods of my cars after I park them in the garage to alleviate the heat soak so they would not smell up my garage. The inner door simply shuts a few seconds after the engine is shut off to completely seal the carburetor off from the atmosphere. Any fuel vapor that is boiled out of the carburetor is captured and stored in the charcoal canister instead of vented to the atmosphere. The result is cleaner air, and a garage that doesn't stink like gas after you shut the engine off. I LOVE having this feature.
If either door fails shut it will absolutely ruin performance, fuel economy, and everything else. Ruin. That is why you maintain them.