grainy;

...very good info from all, but I don't think I saw anyone mention that the crankcase gases are mixed into that (prewarming) chamber which the intake mixtures are routed through under part throttle (under full throttle, the seconcary throttles were opened and the mixtures ran directly into the head - to call it a rudimentary form of variable-tuned intake is not incorrect, Al).

But this mixing with oil vapor/droplet containing crankcase gases is what I believe causes the problem which this manifold is known for: Poor and/or unrepeatable idling.

From my observations, the crankcase gases cause deposits in that chamber when the oil components hit the chamber surface during hotsoak after shutting engine off (naturally, the higher the mileage on the engine, the more blowby, the more deposits), these build up and eventually catch fuel dropplets and can even cause them to puddle...after that, there's now way to get any kind of repeatable idle control...and we haven't even mentioned any additional false air getting past the secondary throttles...idle, what idle?

So the intake side is kinda hopeless, but the exhaust side is possibly the highest flowing 4 into 2 casting from the factory! That's why it's not unheard of to separate the two...replacing the intake side with a <'66 alu unit, and leaving the stock exhaust...it's not a header, but it won't rust through in 10 years either! I works great, I've done it too!

Cheers

My understanding is this:

At small throttle openings the intake charge is routed past
the exhaust manifold area facilitating the rapid warm-up and
easing cold start / cold running by insuring that the gasoline
is completely vaporized prior to going into the cylinder. At
larger throttle positions the secondary butterflies are held
open allowing the charge to directly enter the cylinder.

This system causes a couple of problems:
not all carbs have the arms to open the secondary butterfly

the secondary butterfly shafts wear causing an intake leak

at partial throttle the butterflies still obstruct the intake

the system is always on so the rapid warm up isn't useful if it
is really bloody hot out so heating the already hot air with the
exaust manifold only makes the car run hotter and reduces the
power of the motor because hot air is less dense than cold.

I'd take them out. They certainly didn't preserve the system
when they went to fuel injection.

chris

That manifold was an early, rudimentary version of today's variable-tuned intake manifold. It's designed to use the principles of ram tuning, where the specific length/volume of a column of air at a particular rpm has a (minor) supercharging effect. The linkage was supposed to tip-in the manifold butterflies to give a short, direct air path at higher rpms.
Maybe the previous owner didn't understand or care about the system, so he ripped it out. Or maybe the car was changed over to earlier carbs that wouldn't accept the system for some reason.

I think those were an emissions thing... I always pull 'em out or pin them permanently open. Don't know if they harm when they are in, but they sure don't help anything... I've been told they cost you power though.

I'd leave 'em out if I were you.

-matt