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CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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Hi Guys. On a stock A12 running a DCG 306 carb, does one need the air cleaner to suck in cold air or warm air. Which will get the carb working best?

Posted on: 2017/8/15 21:34
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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Proper carb tuning will get the carburetor to perform at its best.

A colder air temp will always be much better for performance. Colder air temp is denser than a warm.

Posted on: 2017/8/15 23:23
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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So true

In most areas, Datsun 1200/120Y were sold with non-automatic air cleaner. These should always be put on cold side unless temperature drops down (winter use). In warm climate areas they were sold with fixed air cleaner (no heated air whatsoever)

Warm Region
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Normal Region
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Automatic Temperture for maximum fuel economy (Japan/USA). Always uses temp-regulated air, except at full throttle when only cold air is used
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Posted on: 2017/8/16 3:24
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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Thanks DD. So how exactly does the ATC system work? It draws in warm air from the exhaust manifold into the carb? How does it "regulate" that temperature?

Posted on: 2017/8/16 6:32
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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It has a bi-metallic spring on the inside of the air cleaner. When it's cold it opens vacuum to the "can" on the snorkel, which pull the flap drawing in hot air. As it warms, it cuts the vacuum progressively, letting in more cold air. It maintains a high temp, higher than a normal summer day, so year-round the temp -- and thus the jetting -- stays the same (about 40C/120F). Yet when you floor the accelerator, cold air only enters the engine so you will get maximum power. USA engines are jetted leaner to take advantage of this constancy. The power valve richens it up at full throttle

Posted on: 2017/8/16 8:00
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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DD does that therefore mean the carb requires warm air to run at low speeds but cold air to run at high speed? Why is it necessary to maintain a high temperature in such regions and not others? Is it because outside temperatures are much lower?

Posted on: 2017/8/16 8:39
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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The engine doesn't require cold air nor does it require hot air. However, if the air temperature drastically changes (for example summer vs. winter if living outside the tropics) the carburetor should be retuned. With a regular summer/winter air cleaner, you should re-adjust the idle mixture when the cold season arrives. Automatic Temp air cleaner (ATC) removes this need

The engine will still run OK if you don't retune for cold vs hot, but it runs betters when tuned for the air temperature. This is not an issue in Hawaii (summer is 30C, winter is 26C) but is an issue in say Northern California (summer is 40c, winter is 5C)

The Automatic Temp air cleaner has nothing to do with high speed. It is full-throttle or non-full-throttle. This is because the engine makes the most power with cold (dense) air. But makes best fuel economy will a stable temp. ATC gives it both. Max power when needed (when you floor the accelerator), max economy otherwise

There is another consideration of freezing temperature (the air must be heated slightly to prevent carburetor icing) but that's another story

Posted on: 2017/8/16 9:18
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Re: CARBURETOR AIR INTAKE (WARM VS COLD AIR)
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Great explanation DD... Thanks

Posted on: 2017/8/16 21:16
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