Clean Air Tube - Small Block

[Ed S.]

Can anyone explain the purpose or function of the "clean air tube" found on the right side of small blocks (at least my 64 L76). One end of the tube goes into a small hole on the bottom of the right side exhaust manifold. The hole on the bottom side of the manifold is just to the right of the heat riser and it can only be seen from underneath the car. The metal tube runs from the manifold towards the back of the block a few inches and comes up in the space between the exhaust manifold and block - there a rubber tube slides over the metal tube - the other end of the rubber tube connects to the bottom of the air cleaner assembly.

This device is referred to as a "clean air tube" - I don't understand its function. Does warm air move from the manifold to the air filter assembly or does the exhaust flow pull clean air down into the manifold? The inside diameter of the tube is so small, about an eight of an inch, I can't imagine that enough "air" is able to pass through it, regardless of the direction, to perform any use or benefit.

BTW - I am not talking about the tube that runs from the top side of the manifold to the choke assembly on the carburetor.

I've attached 2 pics, one from the bottom showing the steel tube going into the exhaust manifold, the other from the top showing the tube coming up between the manifold and block.

[Jim D.]

Re: Clean Air Tube - Small Block

The choke housing has a vacuum supply that allows it to draw warmed air into it and allowing the bi-metallic spring to operate the choke plate and fast idle cam. The air is warmed in the exhaust manifold by the hot exhaust heating a metal tube that is connected on one end to the choke housing (your 2nd pic.) and on the other end to the air cleaner or carb. (your 1st pic.) there by supplying "clean air" to the whole system.

[Mark E.]

Re: Clean Air Tube - Small Block

It's the source of air for the choke's hot air system. Air from the air cleaner is piped through this "clean air" hose/tube, routes through a tube inside the exhaust manifold to heat up, then exits the manifold and goes to the choke via the "hot air tube" (the one you said you're not talking about).

[Ed S.]

Re: Clean Air Tube - Small Block

Jim,

Thanks - I think I understand. Are you saying that air is drawn through the entire length of the tube by the vacuum in the choke housing - which means that the air is flowing counter clockwise in the tube - starting at the air filter assembly, through the rubber then metal tube to the manifold, it is then heated as it passes through the manifold, and then the heated air travels up the tube to the choke assembly and heats the bi-metallic spring. Is that correct?

[Jim D.]

Re: Clean Air Tube - Small Block

Jim,

Thanks - I think I understand. Are you saying that air is drawn through the entire length of the tube by the vacuum in the choke housing - which means that the air is flowing counter clockwise in the tube - starting at the air filter assembly, through the rubber then metal tube to the manifold, it is then heated as it passes through the manifold, and then the heated air travels up the tube to the choke assembly and heats the bi-metallic spring. Is that correct?

That is correct.

[Ed S.]

Re: Clean Air Tube - Small Block

Jim, Mark - thank you. Makes sense now. I always thought the warm air flow was purely a convection process. I didn't realize the air flow started at the air cleaner assembly. One more question please, the "hole" in the exhaust manifold. I haven't pulled either "tube" out to inspect the hole but, from what you explain, I assume that the hole goes straight through the cast iron manifold - but definitely does not infiltrate orifices in the manifold that are carrying hot exhaust gasses from the cylinders. Thanks again - I am glad I asked.

[Jim D.]

Re: Clean Air Tube - Small Block

Jim, Mark - thank you. Makes sense now. I always thought the warm air flow was purely a convection process. I didn't realize the air flow started at the air cleaner assembly. One more question please, the "hole" in the exhaust manifold. I haven't pulled either "tube" out to inspect the hole but, from what you explain, I assume that the hole goes straight through the cast iron manifold - but definitely does not infiltrate orifices in the manifold that are carrying hot exhaust gasses from the cylinders. Thanks again - I am glad I asked.

Correct. There is a separate tube installed in the exhaust manifold that the other two metal tubes connect to. Over the years, that tube can sometimes erode and get holes in it. That will allow exhaust gases with the associated carbon to enter the choke mechanism causing it to "gum up" and not work correctly.

[Ed S.]

Re: Clean Air Tube - Small Block

Thanks to all - this has been good - you (meaning I) learn something new every day.

[Ed S.]

Re: Clean Air Tube - Small Block

This is a follow-up to my "clean air tube" question. Providing a few pics of the tube I was asking about and its proper connection to the block just to close the loop on the issue - for those than may not be aware of its existence. The car is not mine - it is a friend's '64 that is being restored - a frame off. The engine is sitting in the frame, body not yet lowered. Before anyone comments, the owner knows that everything you can see in the pic is not to original GM (or NCRS) standards. The objective here was to show the "clean air tube" fastened to the block.