I was getting ready to increase the size of the fuel restrictor in my metering Promax billet metering block based on a lean wideband reading I was getting at idle. This is on the old Direct Connection 2300 series "racing" carbs with the mechanical outboards that have no idle circuits and blocked power valve holes in the metering blocks. I love these carbs and have been running them for years.

I was thinking that I had enough of an understanding of emulsion circuits and metering blocks to do what I was about to do. Heck, I was able to adjust the main jets using a wide band, so I thought I would just increase the main restiction jet 3 sizes from .031" to .034" and recheck the A/F ratio. I figured it would be richer at idle afterwards. After all, I had been getting readings of 22 & 24 to 1 before the gauge zero out due to leaness at idle. More fuel should work like increasing a main jet at cruise and wide open throttle I thought. It might have.

I knew I had too much of the idle transfer slot uncovered from watching Youtube videos on Holleys. I tried opening the outboard carbs throttle blades very slightly to simulate drilling holes in the center carb throttle plates. As I closed the center carb's throttle blades using the idle screw, a surprising thing began to happen. After a few minutes, I had the center carb closed up significantly, and the outboard carbs open about a half a turn of their idle screw from fully shut.

To my surprise, the A/F ratio was now in the mid to low 15's at 900 rpm. The idle is much more stable and the response off idle is as good or better than it ever has been.

I expected that bypassing more air through the outboards would have leaned out the A/F ratio even more than it had been. It actually richened it up and the car is running better than ever now. Can anyone clarify the physics behind that?

On a side note, I have C&S building me a set of aerosol 500 cfm mechanical carbs using the body of a spare outboard I found on Ebay. I'm looking forward to a back to back chassis dyno test to see if they build more power.


1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)