Hib Halverson's Big Block From Hell Series

by Hib Halverson
© December 2012

With four-corner idle carbs, the bypass hole process is more involved. With an idle circuit for each barrel, for best idle stability and drivability, idle speed and mixture adjustments are made equally to primaries and secondaries. Adjust the primary throttle plates so you see .020-.030 of the idle transfer ports. Hold the T.B. up to light and adjust the secondary stop screw so the secondary plates are open the same amount. The location of the transfer ports in the secondaries is different than in the primaries, so the bottom of the secondary transfer ports may not be visible. This will be easier if you install a Holley Secondary Adjusting Kit (PN26-117) which adds a conventional idle speed screw to the secondaries.

After running the engine, if you need bypass holes, start with a #70 bit (.028-in.) and drill all four plates on the side of the throttle shafts closest to the transfer ports. The holes in our 850 ended-up .043-in., on the low side of what can be needed. Generally, the more camshaft you have in the motor, the bigger the bypass holes need to be.

Now, do an idle mixture adjustment using the vacuum gauge and your ear. Take each screw a quarter turn lean then, listen to the engine and watch the vacuum gauge. If the engine runs more rough or the vacuum drops, go back the other way. If the engine smooths and vacuum increases, go another quarter turn lean. You want the highest vacuum level you can get. On the wideband, the AFR will vary depending on your camshaft profile, spark timing and engine configuration. I was able to lean my idle to the high-14s/low-15s and still have it relatively smooth.

Smooth Transfer

Coming into this tuning session, the BBfH 850's main jetting was 72 primary and 78 secondary. I dropped the primary mains five numbers to 67, then road tested with the vacuum gage and the Autometer wideband. My goal was, at 2500-4500 rpm with the engine loaded between 10 and 7 psi vacuum which is just above power enrichment at 6.5-psi, to have the AFR between 13.5:1 and 15.5:1 and with the 67s, I was close enough for now.

Now, get the primary idle transfer mixture in the ball park. With Double-Pumpers on engines having modest duration/high lift roller cams typical of hot street duty cycles, it will be too rich. Fuel is metered to the idle screws and the idle transfer ports by "idle feed restrictions" (IFR), little tiny jets inside the metering block which are not removable. You can't make IFRs smaller, so the DIY fix is to restrict them with small wire "V"s. You'll need either a good set of dial calipers or a 0-1-inch vernier micrometer to measure their diameters.

A great source of wire are the strands of automotive electrical wire, the diameters of which vary greatly. I've found as small as .008-in. and as large as .025. Another source for larger sizes are straight pins—find a friendly tailor or seamstress who'll give you some, then snip-off the heads and points with wire cutters and bend them into Vs. Medical forceps or small needle-nose pliers may be necessary to manipulate these tiny wires.

I started with .0175-in. wires which, in a .0355 primary IFR, is a significant reduction in fuel flow. That was a pretty good guess as the light-throttle AFR I saw on the AutoMeter went from low-12s to high-13s. With the AFR headed in the right direction, I tried .0205-in wire. That got me to the low-15s, a little more lean than stoichiometric, where best fuel economy occurs. Now, the BBfH's 460 Big-Block still "felt" good at cruise and was capable of better gas mileage. My idle feed wires are on the high side of what most Double Pumpers will need. Once the wires are in the IFRs, redo the idle mixture adjustment.

One caveat about leaning the part throttle that much: you may find that, if you did your idle and part throttle calibrations in warm weather, in cool and cold weather, the AFR will be too lean due to the dense air. That may force you into changing the IFR wires periodically which is no more difficult than changing jets. Nevertheless, if you don't want that hassle, do your part throttle calibration in cold weather and live with the more rich mixture when it warms up.