1971 heat riser for big block- components and operation?

[71CHEVYSHORTBED402]

The stove pipe wasn't on the truck when I bought it long ago, and I believe I threw the shield away.
Because there wasn't one, we added a manual choke. At any rate, looks like I've scored a heat riser and shield.
Kind of stoked. Unfortunately, I don't have a thorough understanding how this operates and have some questions.

1. What are the components of this operation and how does each one function?

2. Do you know how it mounts? I believe the shield mounts to the manifold on the driver's side,
the pipe runs from the shield to the air cleaner and that's it. Pictures would be great.

3. What's the component on the air cleaner "breather tube" with a hose on it?
I suspect that's for the heat riser operation. A picture of said component on another air cleaner is below.

4. Any issues using this just "for show" and keeping the manual choke operation?
The automatic choke was disconnected when the manual choke was installed.

5. What do you recommend for restoring these pieces? A picture of the parts are below.
They look to be in great shape.

6. Any other considerations?


Thanks for any input. It's appreciated.

[SS Tim]

You would do well to sit down with your factory service manual or download the link I posted.

Starting with 6M-18 you will get a good overview on how the THERMOSTATICALLY CONTROLLED AIR CLEANER works.

Also look at 6T for some information on the PCV, CCS (which the air cleaner is part of), Evap and CEC operation/components.

The heat riser tube has nothing to do with the choke.

RTFM ... Read The Factory Manuals... download 'em here

[71CHEVYSHORTBED402]

Originally Posted by SS Tim

You would do well to sit down with your factory service manual or download the link I posted.

Starting with 6M-18 you will get a good overview on how the THERMOSTATICALLY CONTROLLED AIR CLEANER works.

Also look at 6T for some information on the PCV, CCS (which the air cleaner is part of), Evap and CEC operation/components.

The heat riser tube has nothing to do with the choke.

RTFM ... Read The Factory Manuals... download 'em here

Yeah, I do need to RTFM. I have one sitting in the garage. Didn't think to look. I'm presently downloading the link as well. Pipe has nothing to do with the riser huh?.......All these years I thought it was a heat source to operate the choke on the quad.

Your opinion Tim, am I wasting my time with this setup? I'd like to mount it, but don't want to uck anything up. Good price if you ask me. More than 219 but less than 221, with shipping. That pipe was the only piece missing when I purchased the truck.

[71CHEVYSHORTBED402]

Probably ought to be careful downloading the files from that link. It now comes with a Trojan virus...........[/U][/B]

[leddzepp]

You won't mess anything up installing it. I have the set up on my big block.

[71CHEVYSHORTBED402]

So now I know how it operates, as motivated by SSTim. Anyway, for anyone curious, this is how it functions.

1. Snorkel passage should be open when the engine is off and it's under 85 degrees.

2. Snorkel passage closes somehow (must be the vacuum motor) when the engine is started and it's under 85 degrees.

3. Snorkel passage reopens at/over 85 degrees.

Parts: The "Vacuum Motor" (sensor/vacuum) moves the Damper Door in the air cleaner, and the Damper Door moves the Snorkel Passage. Very cool

........I'm going to buy it. Still not sure how the shield mounts, though I believe it's with two bolts used to mount the manifold. Not certain how to restore these pieces yet.

A new question comes to me brain. If they don't operate and all else works, then you need a new sensor unit. I don't imagine those are easy to locate, but I'm just guessing at this point.

[MagmaJct]

Heat riser and preheated intake are two different things, although, they do sound similar.

The preheated intake is one device that assists with leaner running carburetor calibrations needed to improve exhaust emissions. Now, before you get all "damn environment Nazis" on me, hear me out. This is a simple device that helps keep the fuel in a vapor state once it leaves the carburetor. When everything is cold, the engine, intake manifold, carburetor, and intake air. Gasoline does a poor job of vaporizing under these conditions. Any cold surface it hits, causes it to condense and stick. Thus, it doesn't make it into the combustion chamber very well.

Before carburetors were calibrated to run leaner, this wasn't a big deal. Back then, carburetors provided a rich enough mixture to compensate for this condensate.

The preheated intake solves this by raising the temperature of the air entering the carburetor. That air cleaner has two parts that make this system work. The blend door that mixes the heated air and the cooler air, and a thermostatic vacuum modulator. The blend door is connected to the thermostatic vacuum modulator through one of the ports. (I'm not sure which, it does matter!) The other port on the thermostatic vacuum modulator connects to a manifold (read: constant!) vacuum source.

This system was used through many models of vehicles all the way up to the latest throttle body fuel injection. Port fuel injection doesn't need a preheated intake, as no fuel travels through the cold intake system until its injected at the intake valves. That area of the engine heats up quickly anyway.

[71CHEVYSHORTBED402]

Mag: That thermostatic vacuum modulator must be the part marked A/C. Interestingly enough, I have a piece with three outlets (not sure what it's called) that fits onto the intake manifold. One is a vacuum to the brake booster, two is a vacuum to what looks like the transmission, and three is just plugged up.

Seems that third port is suppose to go to the second port on the thermostatic vacuum modulator. The other port is as you say, connected to the "blend door".

Till now I thought we plugged up that vacuum port during the engine rebuild, because it went to the smog canister (somehow), which we removed. Looks like I was wrong. We plugged it up because there wasn't a stove pipe.

[SS Tim]

Busy with the critters, but for now... a clip from a 71 mailer.

[71CHEVYSHORTBED402]

Doesn't get any better than that picture Tim. THANKS dude.