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Fuel Pump Problems: Ask Anything

Cam timing tricks and the advantages of an S-60 rear are the topics this month.

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Let's Get Regulated

Philip Peacock; Buffalo, NY I am a longtime reader, and I enjoy the average-car-guy-build way of doing things. I've got a fuel line or fuel pump problem for you. The car is a 2,400-pound Henry J with a 383ci small-block Chevy, Edelbrock RPM heads, a TR1X tunnel-ram with two 450-cfm Holley carbs, a Herbert mechanical flat-tappet cam with 238 degrees of duration and 0.538-inch lift, and 10.47:1 compression. The fuel system consists of a Mallory 110 pump, a Mallory 140 filter, and a 1/2-inch aluminum line up to the 12-803 Holley regulator. I've come out of the regulator with a single, -6 line to dual, inline banjo fittings connected to the carbs, with pressure set at 7.5 pounds with a gauge on regulator.

The engine sounds like it's starving for fuel at around 5,700 rpm, so it was suggested to run dual 12-803 regulators with a line out of each to each carb with a straight fitting instead of the banjo. What is the difference between just disconnecting the pressure gauge and running the second fuel line out of the one regulator? I have attached a photo of the current setup to get the visual.

Jeff Smith:This sounds like a fun ride - fuel pump problems always are. You get extra style points for that body. From your description, I'm not sure your plumbing job on the fuel line is really at fault. The Holley 12-803 pressure regulator has been around for decades. It uses a spring-loaded check ball to establish the required fuel pressure, and the ball is 0.220 inch in diameter. When compared with a -6 fuel line (a theoretical inside diameter of 0.375 inch), you might assume the regulator is the restriction. But let's start at the carburetor and work our way toward the pump. The typical small-cfm Holley carburetor is equipped with a pair of 0.097-inch-diameter needle-and-seat assemblies. We used the term "theoretical" when describing the -6 fuel line because common AN fittings generally restrict the inside diameter to something significantly less than 0.375 inch. But for the purposes of our discussion, we'll leave it at 3?8 inch. Moving further upstream, we come to the regulator. After adding the potential flow area of four 0.097 needle-and-seat assemblies and comparing that with a single 0.220-inch check ball in the Holley regulator, I initially thought I'd found the restriction. But when I did the math, it was revealed that the four needles are only 78 percent of the area of the regulator. A second regulator with more flow area would probably help by reducing the flow restriction, but perhaps this isn't really the problem. The assumption we're making is that we have good fuel pressure at all times.

I'd suggest plumbing at least a temporary fuel-pressure gauge to the cowl so you can watch the fuel pressure during a test pass. That Mallory pump is rated at 120 gallons per hour at 6 psi, which converts to well in excess of 1,000 hp worth of fuel flow. So in theory, your pump has the capacity, but we've seen many cases in which restrictions on the pressure side, restrictions on the inlet side of the pump, inadequate fuel tank vents, and even voltage drops on the electrical side all affect pump capacity and create fuel pump problems. Rather than testing all these separate systems, I'd suggest making a pass with the car and watching the fuel pressure. If the pressure begins to drop halfway down the track, you'll know you have a less than optimal system and can begin to check the above systems as possible corks. Another suggestion is that you may have to raise the static fuel pressure setting at idle to maintain that 6 psi under full load. If the fuel pressure drops as the car runs down the course, it's an indication of a restriction or a reduction in pump output. Your system is what is termed a dead-head system. At idle, there is very little fuel used when you set the pressure. But under WOT, fuel demand is high, and the pressure might drop. As an alternative to this setup, we installed a return system in my orange Chevelle that uses a return regulator to set system pressure at 10 psi, then attached the outlet to a dead-head regulator (like your 12-803) to set the pressure to the carb. This way, the fuel pump does not have to work as hard, as the fuel is returned back to the tank. And under load, we're using 10 psi (or more if desired) to push fuel forward to the dead-head regulator to feed the carbs.

Of course, it's also possible that the problem is not fuel-delivery related. We know of an enthusiast who upgraded his fuel-delivery system only to discover that his real problem was that the throttle was opening only 60 percent. The cure was a 30-second adjustment to the throttle linkage after he spent days building a killer fuel system. Other classic issues that could cause the engine to lie down at higher engine speeds include retarded ignition timing, or, my personal favorite, weak valvesprings. More than likely, your issue will be easily identified after first determining whether the fuel pressure drops during the run. Sometimes the simplest solutions are the best.

The Mighty 10-Bolt

Kory Errico; North Haledon, NJ: I have been reading your magazine since the '70s and enjoy it greatly. I've tried other subscriptions but always return to you as my main mag. This is my first time contacting Car Craft for help. I have a small-block Olds in my '72 4-4-2 making more than 450 hp and want to add a bottle. The car has the 8.5 GM 10-bolt. Is there anything I can do to get this rear to handle 600-plus horsepower? I was hoping to keep the sleeper look going by not swapping out the rear. I would love to hear your suggestions for either type of setup. I have PMT-Fab upper and lower trailing arms and factory frame braces with Lakewood traction bars, and I've been using the narrow 225/70R14 BFGoodrich radials to keep the rear in one piece. Thank you.

Jeff Smith: The common assumption about GM 10-bolt rearends is that they are all weak and all the same, but that's not true. The early, original 10-bolt was a lighter-duty version of the 12-bolt, using an 8.2-inch-diameter ring gear, a small-diameter shaft pinion gear, and spindly, 28-spline axles. Around 1971, GM essentially integrated much of the strength of the 12-bolt into a corporate 10-bolt. This new rear uses an 8.5-inch ring gear that is only 0.375 inch (diameter) smaller than the 12-bolt and uses 10 larger 7?16-inch retaining bolts. The pinion gear is also the same shaft diameter as on the 12-bolt at 1.625 inches, so it is just as strong. So you can certainly strengthen the 8.5-inch rear to be pretty durable, and there are plenty of parts to choose from.

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