Hot Rods Carters on the 409, which is the primary

On a 409 motor, which carburetor is the primary, the front 4bbl or the rear?

 

The rear.

 

should be pretty obvious from the picture?

 

Anybody know they 'Why' of it? The idle richness and idle speed adjuster are so difficult to get to and all.

 

I run mine both as primary's

 

I don't understand the question...

you have to adjust the idle on both carbs. They are so close together, because that's how they figured out to fit the carbs on the intake. The primary is on the rear carb because it makes the linkage simple.

 

The primary being at the rear carburetor places the air/fuel more centered in the intake.

 

That too...but that might not have as much effect as you might think.

 

The original front carburetor should be closed off completely at idle and have thicker throttle blades being they have no idle circuits.

 

That's a very complicated matter (flow pulses). So many variables.
Only way to nail it down completely is IR intake manifold?

Joe

 

The primary carb usually has all of the utility provisions, choke, throttle pressure lever, idle controls, etc. The secondary often has blocked vacuum bosses, limited idle adjustments and sometimes a simple lever for a single linkage attachment. They gave very little thought to the convenience of the adjustments on the rear carb. They used to make kits with flexible, extended “knobs” for multiple carb setups. Someone needs to repop those. A bracket or two, an old speedo cable, a couple small Bakelite knobs, some silver solder and you could make your own.
If you’ve got a real 409 with two fours, I’m jealous!

 

We have a few in are stable. Going to pick up another one tomorrow.

 

Had a friend in HS w/ a cherry bubbletop. That was a crazy car

Joe

 

In squirrells pic it's set up as progressive linkage. No need for primary idle circuits on the secondary carb as it doesn't come in till approx 3/4 throttle. For ones who were wondering.

 

Odd combination really, its a 348 with 409 heads, carbs and intake are original, its in an old hotrod '40 coupe that was abandoned and in the family for years and the next generation is restoring it.

 

You have to watch valve to block clearance when putting 409 big port heads on a 348.

 

As I recall, the "front" carburetor has...no...idle mixture adjustment screws as released from Chevrolet on the 425hp, 409.





I'm not sure about the dual quad Z-11 combination. Being a race combination only, I'd guess that it's the same, no first carburetor mixture adjustment.

Mike

 

The carburetor manufacturers built what the customer was willing to pay for.

A number of dual quad carbs on other engines, and tripower center carbs came with a hex-headed idle mixture screw. Tune with an offset open-end wrench.

The 283 dual WCFB carbs had idle screws front and rear, and they were of the hex head design.

The 409 dual quad idle screws have a knurled (well, vertical slots) head. Since these slant upward, very easy to get a couple of pieces of neoprene tubing and push on the heads of the screws. The wise tuner will paint a dot on the tubing denoting where the screw is bottomed out. Since the tubing is flexible, easy to rotate using one's fingers.

Jon

 

It's not suppose to be easy. All old engines draw a little blood.