how to fix warm air flaper

There is a copper cylinder inside the air cleaner snorkel at the end of a push rod that connects to the flapper valve inside the snorkel. That cylinder is the temperature sensor. It is supposed to start opening the flapper valve at 100 degrees Fahrenheit and have the flapper fully open by 130 degrees Fahrenheit.

That copper cylinder sensor is actually filled with pink wax. As it heats up, the wax melts and expands. This pushes on the rod to push open the flapper valve in the air cleaner against spring tension to let in fresh cool air from the end of the snorkel. 

The problem is the seal goes bad inside the sensor and allows the pink wax to leak out. This prevents the sensor from opening the flapper valve all the way, if at all. Without the sensor to push the flapper valve open, the spring holds the flapper valve shut and only hot air from the flexible hose on the underside of the snorkel is fed into the air cleaner. This hot air causes a serious drop in engine performance. 

The sensor is threaded into a bracket inside the snorkel. You could replace it if you found one from another air cleaner. I believe some Fords used this same air cleaner temperature sensor starting around 1967. 

Or you can do what I did to my 1977 six cylinder: find an air cleaner to fit your carburetor that uses a vacuum operated air cleaner flapper valve. They use a vacuum valve and vacuum motor instead of a wax pellet to operate the flapper. 

Or you can jam the flapper valve open. It'll make the engine run poorly as it warms up though. Not exactly ideal.  

The 105 stamped on the thermostat means it should start to open the flapper valve to begin letting in cool air at 105 degrees and have the flapper valve fully open at 130 degrees. It works the same way backwards too. The valve should remain fully open so long as the air remains hotter than 130 degrees but the valve will start to close if the air cools to below 130 degrees and be fully shut if the air cools below 105 degrees. 

That information is from the 1977 AMC technical service manual.

I used to test mine by running hot tap water over the thermostat. My hot water heater was set to 130 degrees so it should just about be able to make the flapper valve fully open. 

Actually 100-130 degrees is not super hot for carburetor intake air. Cold air is denser air and makes more horsepower theoretically....but only if the fuel in that air is vaporizing. The fuel won't vaporize if the air is too cold. Carburetors have a happy medium that they like to run at, and it's right around 100 degrees. Warm air entering the carburetor has special benefits while the engine is warming up. Firstly, because cold carburetors with cold, damp air flowing though them tend to form frost inside which plugs them up and makes the engine bog or stall. Warming the intake air eliminates this. Secondly, because warmed intake air does not require as much carburetor choke, so the choke can be released sooner. This pours less raw gas into the engine, improving both fuel economy and engine life. Raw gas pouring into the engine washes the oil off the cylinder walls and accelerates piston ring wear. Warming that intake air and releasing the choke sooner improves this greatly. 

It is better to have the valve open (cold air) all the time than to have it closed (hot air) all the time. A valve stuck shut (hot) will absolutely murder your power and performance on a warmed-up engine, especially in hot weather. A valve stuck open (cold) can make the engine run poorly during warm-up because the choke, heat riser, etc. are all calibrated for warm air. Without the warm air, it'll run lean or ice up. Stall, bog, hesitate, etc. until it warms up. Some engines do ok, some don't. 

I've experienced both situations on my 1977 232 I6. Stuck closed (hot) all the time completely murdered my power and performance. The intake air was easily over 200 degrees in the summer and it's also restrictive of airflow. (The engineers did not expect the engine to need lots of airflow as it was warming up). Stuck open (cold) made my engine totally miserable to drive for the first 5 minutes. It would bog, hesitate, and backfire because it was too lean. Except in damp cool weather, then it would ice up, flood rich, and stall. It has none of those problems with a properly working air cleaner.