Racing Fuel Systems

Seems like nobody posts on this forum, but here it goes anyway...

Let's say you have a fixed hanger and secondary metering rod combination. How does the air door spring setting affect WOT A/F ratio?

It seems like going tighter on the spring would tend to make things richer, but I am also thinking about how the cam lifts the metering rods higher with more opening.

So, on one hand, you get more jet area with the door opening sooner, but on the other hand you probably have less "venturi" signal when the air door flops open early. Anyone have any real world experience they can share? Plan to do my own tests, but curious what others have found.

Once the secondary air flap/door is wide open there is no difference to WOT AF ratio, if it has recovered from opening too soon or too late.

If the air door opens too soon you create a bog. Initial lean spot.
If the air door opens too late you could have a loss of power production/torque.

It gets more complicated.
The choke pull off release time also has an effect here because it is connected to the secondary air door.
Not all secondary air doors open to the same distance. This varies the cfm.
Some truck quadrajets open the secondary butterflies at a later position. They use a different linkage.

Here is an example. 750 quadrajet 350 HP sbc such as 1969 Corvette 350/350HP
Air door full open distance 1.36 inches
Secondary flap spring : 3/4 to 7/8 initial wind up
Choke pull off release time : 2 to 3 seconds
Secondary rod BG .400” medium length tip

If secondary door opens too soon/fast the bog is created because fuel is heavier than air. So you have a sudden lean condition until it recovers. Sudden flopping open without control and it may not recover.

Quadrajet carbs do have a secondary pull over enrichment system when the secondary air door opens.
This system can be tailored somewhat .

I have worked on a lot of quadrajets, and I keep detailed records of each carb.
I don’t no if this helps your question , hopefully it does.

Thanks. I think I understand all of that, but my question was more around what happens in the period between the air-door being closed and the air door being fully open.

As you said, that is controlled by the spring and vacuum break. Intuitively, one would think that tighter spring and/or slower break would be richer at all "in between" positions of the air-door. However, I think that rod taper from ~0.134 to the power tip also greatly influences what happens in the "middle" positions.

I think I'll play with various spring settings while keeping rods and hangers constant and see what happens.

"I think that rod taper from ~0.134 to the power tip also greatly influences what happens in the "middle" positions. "

I see what you mean. For example the .0400" tip rod was available in short , medium, and long tip lengths. At say 60 degree opening of the air door this varied the rod diameter in the secondary jet on these different tip lengths from about .0800 to .1090 according to a chart I have.

Interestingly the short and medium tip lengths were considered better for most street use , giving a clean crisp transition.

Looking forward to your test results.
May I ask what quadrajet number you are working with?

It's a 17080204 (small "750" primary venturi), but nowhere close to stock. 0.046 main bleeds (not the 0.120 this unit came with), the "tuner" secondary mods from the old "racing fuel systems" board, Upper IAB relocated to airhorn, etc, etc.

Mine goes rich 11 on secondary opening if I have the spring much tighter than 1/2 turn.

Have you tried loosening it about an 1/8 turn?

One of the runs that recorded from a few years ago at the track. Not a very flat AFR curve.

Am I reading that right? You are hitting high / mid 13 A/F right before (or after) the gear change?

I haven't played with air door tension yet, but I will probably try your suggestion of backing it off 1/8 turn and see what happens. I really need to datalog... too hard to read the gauge and drive at WOT at the same time.