Who could resist the classic Holley 4150 double pumper carburetor with mechanical secondaries and a throaty voice—an American classic for generations of street performance weekend drag racing enthusiasts? Not many of us who crave the aroma of burning rubber and unburned hydrocarbons could resist the vintage Holley atomizer.

The 4150 has been serving hotrodders since 1957 when the Blue Oval put this guy on top of the 312ci Ford Y-block V8. Word spread quickly about the advantages of the new design Holley and it took the street performance world by storm.

Cruise SummitRacing.com and you will see Holley has been busy in its research and development labs birthing even better technology—the refined 4150 HP Series. Holley HP Series performance carburetors have been netting better elapsed times and speed for more than a decade. They excite the senses with power and they deliver reliability without being high maintenance. When they do need attention, they’re easy to service, with plenty of replacement parts on hand at SummitRacing.com.

The new lightweight 4150 Aluminum Street HP carburetor weighs 40 percent less than the traditional HP carb and is quite affordable. The Aluminum Street HP utilizes the best features from Holley’s time-proven 4150 HP carburetor, yet is quite tame for street and weekend racing. These features make it an excellent replacement for a vintage Holley 4150.

(Image/Jim Smart)

Bruce Couture of Modern Driveline in Boise, Idaho had grown frustrated with performance and drivability issues from his 1965 Mustang. He’d been through a succession of Holley carburetors, which didn’t yield the kind of performance he expected. There were idle quality issues, part throttle stumble, and serious lapses in wide-open throttle performance. He had custom built Holley carbs, which were never up to par. He knew his 289 was capable of more.

He looked to Holley for the 650cfm Aluminum Street HP Carburetor (PN HLY-0-82651SA) as a good first step toward better performance along with a professional dyno tune. The difference has been remarkable. Better idle. Crisp off-idle response along with a nice transition into the power circuit at high revs. Bruce also decided take his troubleshooting a step further with an in depth look at the ignition system. He was running a stock Autolite ignition, which was never very good to begin with.

Ford’s Autolite single- and dual-point ignitions have always been troublesome due to single and occasionally dual-bushed shafts, which suffer from instability causing point bounce and irregular dwell. Poor oil distribution to the shaft and bushings causes premature wear. Even with aftermarket drop-in electronic ignition drop-in retrofits, these distributors just don’t perform very well at high rpm.

Bruce opted for an MSD ignition system with vacuum advance (for street use) along with a Blaster coil which has made a significant difference in performance.

Performance issues aren’t always about one single issue, but collective problems that go undetected. Before you spend a bunch of money, check your engine’s current state of health—both static and working compression, manifold vacuum, valve adjustment, and air/fuel ratio, along with fuel system and ignition condition.

Static compression is checked with a compression gauge with the engine both cold and hot. Compression doesn’t always have to be high, but it should be consistent across eight bores. Working compression is checked with a cylinder leakdown test. Does each cylinder hold compression and for how long?

At idle, how is intake manifold vacuum? Does the needle bounce or is it steady? How is vacuum off idle? Have you checked valve lash? If valves are not fully closing (lash too tight), you can expect all kinds of issues. If valve lash is too loose, you will hear rocker arm noise and valves will not open fully.

Bruce has checked all of his engine’s vitals and has found he has two areas of weakness in carburetion and ignition. He has taken the time to improve both. Let’s get started.

We like the Holley 0-82651SA HP 650cfm aluminum carburetor for its lightweight design and ability to isolate engine heat. (Image/Jim Smart)
The Street HP sports all kinds of nice engineering refinements that have made the original even better with what has been learned in racing. The result is a fabulous street carburetor. (Image/Jim Smart)
Holley’s Street HP sports an aluminum contoured bore design taken from the Race HP and has a tumble polished finish along with screw-in swappable air bleeds for precision tuning. Cool thing about the Street HP is serviceability for street and weekend racing. There has never been a more user-friendly Holley. (Image/Jim Smart)
Bruce’s custom built 650cfm 4150 Holley, despite its custom-built status, made little difference in performance, which was not entirely the carburetor, but also the ignition system. View performance issues collectively because an engine is the sum total of all of its dynamics encompassing compression, manifold vacuum, valve adjustment, cam profile, and more. (Image/Jim Smart)
Here’s something so many of us overlook. Your fuel line is only as effective as its smallest segment. If you have a 5/16 inch body line and fuel tank pickup yet have a 7/16 inch fuel line between the pump and carburetor, you still have a 5/16 inch fuel line. Here, we have a 3/8 inch body line and a 7/16 inch pump to carb line. Both should be the same size from tank to carburetor.  (Image/Jim Smart)
Bruce uses a cotter pin to secure the throttle linkage. You may also use a clip. Ford’s factory throttle spring steel retaining clip will not work with the Holley. (Image/Jim Smart)
Four half inch nuts secure the Holley’s base. All that’s left are fuel and vacuum lines. Unless you’re swapping spacers, studs don’t need to be this long. (Image/Jim Smart)
removing fuel fittings from a holley carburetor
Bruce removes fittings and transitions them to the HP. Note the figures scribed into the plate—jet sizes and power valve specifics— which is a good idea when you don’t remember what you did a year ago. (Image/Jim Smart)
(Image/Jim Smart)
(Image/Jim Smart)
Best to always replace the carb gasket and go as thick as you can for good heat insulation and sealing. (Image/Jim Smart)
Make sure all contact surfaces are clean. (Image/Jim Smart)
Holley’s wonderful simplicity has never changed in 66 years of serving enthusiasts. Power valve failure issues (from engine backfire) have been resolved and jet swaps remain simple. Fuel bowls with drain plugs are also available to make light work of jet and power valve swaps. Note the generous array of vacuum ports—both throttled and constant. (Image/Jim Smart)
Because the 4150 has two metering blocks, idle-air mixture is adjustable in all four corners with a power valve for both primaries and secondaries. The 4150 offers you greater tuning options. (Image/Jim Smart)
During installation, be mindful of safety issues such as failsafe throttle linkage security. The linkage must be secure with a clip or cotter pin along with a dual throttle return spring even though Holley has thought of this for you within the carburetor. (Image/Jim Smart)
The dual feed fuel line with a fuel pressure gauge from Summit Racing is secured and checked for leaks. Always double check your work with a leak check before starting the engine. (Image/Jim Smart)
close up of throttle return spring on carburetor linkage
We cannot stress enough the importance of throttle linkage security and safety. We have a spring return here. It is suggested you opt for a dual-return spring (one inside of the other) as a back-up should either spring fail. (Image/Jim Smart)
Check idle-air mixture to seek a stable idle. Surging indicates a lean idle mixture. It’s a good idea to check ignition timing—both at idle and total timing at 3,000 rpm. This isn’t just about total timing, but the rate of advance as you increase rpm and goose throttle. How quickly does advance roll in? (Image/Jim Smart)

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Author: Jim Smart

Jim Smart is a veteran automotive journalist, technical editor, and historian with hundreds of how-to and feature articles to his credit. Jim's also an enthusiast, and has owned and restored many classic vehicles, including an impressive mix of vintage Ford Mustangs.