Modifying a Holley 950 HP double-pumper for the street and strip
One ongoing problem with our Bold Beeper project was carburetor related. On the dyno, we had run a Holley 950 HP double-pumper, and it had performed flawlessly. Since we had misgivings about the true streetability of a double pumper, we had fitted a Holley Avenger 830 CFM vacuum-secondary for street use, where it worked OK. However, with the tall, sticky French Drag Radials installed, no matter what we did, the car would fall on its face about halfway through first gear. We diddled the secondary springs, pump shot, etc., until we were blue in the face—to no avail.
Finally, with our new AEM wide-range O2 display installed, we were able to do
some scientific testing. This affirmed what
we suspected—the engine was leaning out.
Normally, this would point to a fuel pressure
drop, but the car pulled cleanly in the higher
gears—even at top end—situations that
would seem to place even greater demands
on the fuel supply. The fuel pressure
gauge didn’t even wiggle!
That left the carb as the culprit. Not that it was defective— far from it. It was just mismatched, tuning-wise, to our hot-cammed, big-cube, tight-converter, tall-geared oddball setup. And, we were running out of patience.
As we were busy pulling out our few remaining hairs, we glanced over at the double pumper, resting comfortably under the bench. Could it be used on the street? Hmmm. We reviewed the myriad reasons why we had discounted it in the first place:
- No PCV hookup
- No choke
- No fast idle cam/linkage
- No ported spark advance fitting (for vacuum advance)
- Poor performance expected with automatic transmission kickdown
A quick call to the carb-trouble “go to” guys, the Vrbancic clan at the Carburetor Shop in Ontario, California, gave us the impetus we needed to think outside the box. The C.S. guys said that adding a nipple for PCV, and a ported spark fitting, was no big deal (easy for them to say.) They also indicated that they could lengthen the duration of the pump shot to improve kickdown performance, and that all that would be required to run on the street without the choke was a few pumps of the pedal and maybe 90 seconds of manual fast idling, as longs as the weather was over 40 degrees. That was all we needed to hear. A few weeks later, our trusty double-pumper was reworked, back in our hands and bolted on the Beeper’s Victor intake.
Sure enough, it works! While the pump shot duration isn’t long enough to cover a mash-it-at-25-MPH-in-drive-andwait- for-the-kickdown-to-first scenario, just a small dose of driver intellect, applied as manually downshifting (to at least second gear) before mashing the throttle, is a simple and effective workaround. All in all, the Carb Shop’s double pumper worked out extremely well.
Source: The Carburetor Shop, 1461 E. Philadelphia, Ontario, CA 91761; (909) 947-3575