Tuning Holley Double Pumpers (Part 2)

Second of a double-feature on tuning Holley's hot street four-barrels.

Hib Halverson - October 13, 2011 10:00 AM


O. J. Bretzing at the controls of the Carb Shop’s mill. He’s removing the fixed power valve channel restrictions (PVCR) in our primary metering block.


Bretzing threads the holes for the PVCR jets by hand. This cannot be done with a power tool.


This is one of our Carb Shop-modified metering blocks just before power valve installation. The PVCR jets are removed with a small hex key. The soft brass is easy to drill. We have three different sizes for each metering block.


Holley power valves for performance applications come in a variety of “sizes”. The power valve “size” is actually its opening point in inches of vacuum.


After installing .071-inch and .059-inch PVCR jets, this is what we saw on the Auto Meter O2S. Another 10 percent increase in PVCR area got us to 12.7 or so.


This is the air box I have on top of my 850. The Green Filter is a special-order,3½ by 14-inch unit.


The accelerator pump cam on the primary throttle shaft. The cams are held onto the bell crank with a sheetmetal screw which makes cam changes easy.


Road testing showed the secondary accelerator pump on our 850 was too large, so we replaced it with a “standard” 30cc unit.


To convert to the smaller pump, besides the pump cover, you need a 30cc pump lever which comes with the 30cc adjusting screw.


Holley accelerator pump shooters mount between left and right barrels. They are easily changed by removing the big screw. Once the screw and nozzle are removed, don’t turn the carb upside down or the check valve will fall out.


Two types of nozzles are used on 4150 series carbs. The “anti-pullover” nozzles (at left) are what come on a DP from Holley. We prefer the other type of nozzles because I think they “aim” the pump shot more accurately.


Holley offers a bunch of different accelerator pump cams for 30cc pumps. They come in a kit. Each is a different color and is stamped with a different number. Image: Holley Performance Products.


You’ve noticed how clean our Holley looks, but it’s never been disassembled and “soaked” in carb cleaner. We keep the exterior of our engine tidy. The inside of our 850 stays spotless because we regularly use Red Line Oil’s “Complete Fuel System Cleaner”, a detergent-based, carburetor and injector cleaner which, obviously, works quite well. We add about four ounces to a full tank of gas three to four times a year. As a result, we never have varnish, deposits or discoloring in our float bowls. Image: Red Line Synthetic Oil Corp.


Our 850 on the Carb Shop’s 540-inch carburetor test engine and ready for a test.


After testing our driveability-enhanced double-pumper, the Carb Shop’s, Bob Verbancic reviews its fuel curve data with us.


MSD makes Pro-Billet distributors for just about every passenger car and light truck engine that uses a distributor. What we like about this unit is its robust design which incorporates lessons MSD has learned building ignition parts for NASCAR racing.


Our Pro-Billet distributor triggers a new MSD Digital 6 AL multi-sparking, capacitive-discharge ignition amplifier. Image: Autotronic Controls Corp.

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Any more rich than that and the engine may respond sluggishly. Any more lean than that and the engine might have a flat spot.

Last month we covered sources of information, tools, wide-band oxygen sensors, initial modifications and setting up our 850’s idle and cruise air-fuel ratios. In the second article of our series on double-pumpers, we’ll finish off jetting and power valve restrictions then work on accelerator pumps.

Power Enrichment and the Secondaries

After jetting the primary main circuit more lean in Part 1 to have adequate fuel flow at wide-open throttle (WOT), the power valve channel restrictions (PVCR) need to be enlarged. While PVCRs can be drilled larger, I had the Carb Shop fit our 850’s metering blocks with removable PVCR “jets”. Replacing the fixed restrictions with removable jets makes tuning easier. The Carb Shop modified our metering blocks to accept 8-32 brass set screws, which are easily drilled. This process requires a mill and a special fixture to hold the metering blocks while the existing PVCRs are drilled out. A special tap with an unusual thread pitch is necessary to clean up the power valve threads after the installation of the jets. Also required is an experienced “touch” to thread the metering blocks by hand for the jets. Only a speciality carburetor service, such as the Carb Shop, is going to have the equipment and know-how to properly perform this mod.

With carb tuning, it’s not diameters of various jets, bleeds or restrictions through which fuel or air must flow, but their areas which are important. First, know that a Holley jet’s number is not its diameter. There is a chart of jet diameters in the Holley catalog. Figure the area of the old jet plus the PVCR area then subtract the area of the new jet from that. The result is the area of the new PVCR size. From that, derive the diameter then drill the PVCR jet to that dimension. If you’re math-challenged, read CarTech Books’ Super Tuning and Modifying Holley Carburetors which details figuring the area of jets and PVCRs.

In road testing, our Auto Meter SportComp wide-band oxygen sensor (O2S) showed the air/fuel ratio in the high part-throttle/high mid-range rpm area, just before power enrichment at 6.5 psi vacuum was a little too lean, causing some misfire and probably asking for detonation. We addressed the misfire with some MSD components discussed in the sidebar. As a detonation hedge, I went a little more rich with a 68 primary main jet and replaced the 6.5 primary power valve with a 7.5 (p/n 125-75) which enables power enrichment a little sooner. Holley power valves appropriate for use in performance applications vary in opening points from 2.5 to 10.5 inches of vacuum. According to the Auto Meter wide-band, all this gave us a cruise-speed air/fuel ratio of 13.8 to 15.5 from where the idle transfer was saturated to just before power enrichment.

I ran the car a little harder to test the air/fuel ratio at near wide-open throttle. In that part of the engine’s performance envelope, the carb was rich. On the secondary side, I took 10 jet numbers out of the main circuit and replaced the 6.5 power valve with a 3.5. The goal of all this was to lean the engine at high part throttle (7.5 to 3.5 psi vacuum – when the primaries are almost full open and the secondaries are partially open), but have the air/fuel ratio at wide-open throttle running 12.5:1 to 13:1.

After the Carb Shop’s O. J. Bretzing modified my metering blocks for removable power valve channel restrictions, I started with .071-inch primary and .059-inch secondary PVCR jets. At this point, I was road testing the engine at wide-open throttle to 6,500 rpm in second and third gear and noted that the air/fuel ratio was a little lean – in the mid-13:1 area – so I increased the PVCR area by about 10 percent. The final power valve channel restrictions were .073-inch primary and .063-inch secondary. That resulted in an AFR of 12.5 to 13.0:1 up to 6,000 rpm or so.

Starting at about 6,200 the air/fuel ratio went rich. Such behavior can indicate an airflow restriction. For many years I’ve run one of Fel-Pro’s handy, adhesive-backed air cleaner gaskets (p/n 2104) on the 850, an air box around the carb, a three-inch high Green Filter and a stock air cleaner top. A four-inch filter won’t fit, so the Green folks were kind enough to make a couple of 3½-inch high filters which I could use if I pitched the air cleaner top and used sections of square foam to seal the filter to the bottom of the hood. The taller Green Filter leaned out the motor just enough. I ended with the wide-open throttle air-fuel ratio from 2,500 rpm to 6,500 rpm at 12.3:1 to 13.0:1.


Double Pumping

The toughest part of our double pumper driveability project was getting the accelerator pumps dialed in. In fact, we worked on this right up to press time. With respect to fuel delivered by accelerator pumps, there are those who think: if “some” is good, then “more” is better. Not.

You want just a little more pump shot than is necessary to have no lean-sag or “flat-spot” when rapidly opening the throttle. The best pump tuning has the engine’s air/fuel ratio is as close to 12.5:1 as possible during pump discharge. It’s pretty easy to tune the pumps to keep the engine from spitting back or having a flat spot, but it’s hard to get the pump shots such that their duration and flow are just right. The response of the Auto Meter wide-band air-fuel ratio monitor and its “pseudo-analog” display were invaluable during this part of our tuning sessions.

During early road testing, when I whacked the throttle open at 2,000 rpm, the car would accelerate but the mixture would go way fat and stay there for a while, then the engine would “pull” out of it. Initially, I thought this was a problem with the main and power circuits but little I did there changed the situation. The standard 50cc pump had been on the secondary side of my 850 since the start and I never questioned it because, in first gear, the car had no problem spinning the 315/35ZR17 Goodyear F1 GS-D3s I had on the back.

After road testing with the Auto Meter wide-band, I was pretty sure the 50cc secondary pump was too big, so as a test, I put a 30cc pump cam in the secondaries. Whoa! Did that wake that car up!

From the Carb Shop, I ordered parts for a second 30cc pump (part numbers 26-139, 20-108-10, 20-114) and a second set of 30cc pump cams (p/n 20-12), installed the smaller pump and a white cam then went road testing. The lagging pig-rich condition was improved. Now we were cooking with gas — just less of it!

Holley 4150 accelerator pump function depends on the pump discharge nozzles, the pump cam and the pump capacity. Many call the nozzles “shooters” and, in a DP there are two, each with two discharge nozzles. Nozzle diameter determines the fuel flow and the length of the pump “shot”. Pump capacity and the pump cam affect the volume of fuel delivered.

If the car bogs when you nail the gas, you may need larger pump nozzles. If it responds, but in a doggy manner, and/or with a puff of black smoke from the exhaust, you may have too large a nozzle size. If the car first goes, then sags like it’s loading up, you may have too long of a pump shot and might need a pump cam change or, like me, even a smaller pump. A graph of all the pump cams is in CarTech’s Holley book and there is a full-color version in the Holley catalog. Of course, where there are rules, there can be exceptions to those rules and we ran into one. We ended up with much less accelerator pump cam on the secondaries due, in part, to the intermediate circuit in our 850’s secondary metering block.

Now, back to the accelerator pumps. Start with the primaries. With the engine warmed and idling in the shop and keeping your upper body away from the top of the carb, hold the secondaries closed with your right hand and use your left to rapidly open the throttle. If the engine hesitates, jump the primary nozzle size up three sizes. On the other hand, if you snap the throttle open and the engine responds lazily, go down a couple of nozzle sizes. Now, do the same with the secondaries. If the engine hesitates as the secondaries open, jump the secondary accelerator pump nozzle up three numbers. If it responds but in a doggy manner or you see a puff of black smoke from the exhaust, go down three nozzles.

Once you have the pump configurations in the ballpark, go road testing with your wide-band O2S. You want the air/fuel ratio during accelerator pump action to be between 12:1 and 13:1. Any more rich than that and the engine may respond sluggishly. Any more lean than that and the engine might have a flat spot.

If you drive the car at a slow speed and light throttle in a lower gear then quickly go to one-third throttle and see the air/fuel ratio spike lean, you may still need a bit larger primary pump shooter. Make changes to the primary pump and the early part of throttle opening, first. Then do transition to wide-open throttle. If the engine sags, go to a smaller nozzle on the secondaries. If you punch it and the engine spikes rich and stays there, you may need to both make the nozzles smaller and use a less aggressive pump cam.

Many factors affect accelerator pump tuning — atmospheric conditions, engine size, camshaft, intake manifold, carb size, vehicle weight, gearing, tire size, duty cycle and driving style all can play into pump tuning. To end this discussion, I’ll repeat my earlier observation: it’s fairly easy to get the accelerator pump tuning such that there are no flat spots, but it’s more difficult and time consuming to get the pump shots just right. Fortunately, with a Holley’s tuneability – two pump sizes, multiple pump cam profiles and many different pump nozzles – it’s possible to tune the accelerator pumps to perfection. I have my 850’s pumps set such that at 1,500 rpm in second gear, I can floor it and the engine pulls hard – no bogs, no sags, no flat spots.


The Bottom Line

So, what were the results of our Holley DP tuning sessions? The benefits were snappier throttle response, improved driveability around town and better performance at wide-open-throttle. What about fuel economy? Our highway mileage went from 15.9 mpg to 18.4 mpg. Once we were through changing things, to validate our work, I took our 850 to the Carb Shop and had owner Bob Verbancic run it on the Carb Shop’s 540ci dynamometer test engine. After making a couple of dyno pulls with my 850, Verbancic commented, “Not bad,” then added, “For an 850 without a lot of the mods a high-end carb would have, your fuel curve at wide open throttle is pretty good. On our carb test engine (540ci), part throttle was a little lean, but I’d expect that. Also on a 540, at wide open throttle your carb was generating two inches of vacuum, which means it’s a little small for a 540, but ... it’s probably OK for a 460 with your camshaft and the rpm you run. If you were to install modern aluminum heads and a bit more camshaft and extend the rpm range to 6,800, you’d be better with a 950.”

While the driveability enhancements covered in this series apply to Holley double-pumpers on a wide variety of engines, remember that your final carburetor calibration and results will likely differ from what you’ve read in this series. The calibration chart is only a starting point. Our carb tuning specifications will be close only with engines similar to ours: 450 to 500ci, 2,500 to 6,500 rpm nominal operating range, roller lifter cam with short duration (234°/244° at .050-inch) and high-lift (.567-inch), dual-plane high-rise intake, heads with modest port volumes and big (but not huge) valves, 9.5 to 10.0:1 compression, headers, high-performance ignition, a manual transmission, 8.5:1 final drive ratio in first gear and a curb weight of about 3,400 pounds. Engines which are either more aggressive or less aggressive than ours, or cars with different drivelines or curb weights, may need a different carburetor calibration and in some cases, vastly different.

For Your Information:


Auto Meter Products

(866) 248-6356


Carb Shop

(909) 947-3575


CarTech Books

(800) 551-4754


Green Filter USA

(724) 430-2050


Holley Performance Products

(270) 782-2900