by Hib Halverson
© December 2012
After jetting the primary main circuit more lean, 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 are easily drilled with various-sized restrictions. 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 knowhow 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 the jet's number is not its diameter. There is a chart of jet diameters in the Holley catalog. You must figure the area of the old jet plus the PVCR area then subtract the area of the new jet from that. That 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, Big-Block from Hell's Auto Meter SportComp Wideband 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 was asking for detonation. We addressed the misfire with some MSD components discussed in a sidebar. As a detonation hedge, I went to a 68 primary main jet and replaced the 6.5 primary power valve with a 7.5 (PN 125-75) which enables power enrichment a little sooner. Holley power valves appropriate for use in performance street or racing carbs very in opening points from 2.5 to 10.5 inches of vacuum. According to the AutoMeter wideband, all this gave us a cruise-speed AFR of 13.8-15.5 from where the idle transfer was saturated to just before power enrichment.
Now 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 (7.5-3.5 psi vacuum—when the primaries are almost full open and the secondaries are partially open), but have the AFR at WOT running 12.5:1-13:1.
After the Carb Shop's O. J. Bretzing modified my metering blocks for removable power valve channel restrictions, I started with .071-in. primary and .059-in. secondary PVCR jets. At this point, I was road testing the engine at wide-open throttle to 6500 rpm in second and third gear and noted that the AFR a little lean—in the mid-13:1 area. I increased the PVCR area by about 10%. The final power valve channel restrictions were 073-in, primary, and .063-in. secondary. That resulted in an AFR of 12.5-13.0:1 up to 6000 rpm or so.
Starting at about 6200 the air-fuel went rich. Such behavior can indicate an air flow restriction. For many years I've run 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 Green folks were kind enough to make a couple of 3.5-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 WOT AFR from 2500 rpm to 6500 rpm at 12.3:1-13.0:1.
The toughest part of our Double Pumper drivability project was getting the accelerator pumps dialed-in. With respect to fuel delivered by accelerator pumps, there are those who think: if "some" is good, then "more" is better.
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 is when 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 rapid response of the Auto Meter Wideband 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 about 2000 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 tuning I did changed the situation. The standard 50-cc pump had been on the secondary side of my 850 since the start and I never questioned it because, in first gear, the Big-Block from Hell never had problem spinning the 315/35ZR17 Goodyear F1 GS-D3s I had on the back.
After road testing with the AutoMeter Wideband, I was pretty sure the 50-cc secondary pump was too much fuel, so as a test, I put a 30-cc pump cam in the secondaries. Whoa! Did that wake that car up!
From the Carb Shop, I ordered parts for a second 30-cc pump (PNs 26-139, PN 20-108-10, 20-114) and a second set of 30-cc pump cams (PN 20-12), installed the smaller pump and a white cam then went road testing. The lagging pig-rich condition was much improved. Now we were cooking with gas—just less of it!