HOLLEY Classic Carburetor Questions


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Jul 14, 2023
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My Car
Mustang Mach 1 Fastback 1973
Hey guys,

The previous owner of the Mustang installed a different carburettor. I want to make sure I don't make any mistakes during the restoration.

I can see the fuel inlet here.

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Am I right in assuming that the inlet for the vacuum for the secondary flap is on the opposite side?

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But what are the two inlets at the bottom of the petrol inlet on my carburettor?

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Thanks for you support
The rear port is vacuum for either the brake booster or can be used for PCV - it has nothing to do with the secondaries. Vacuum secondary carbs take the signal from passages in the main body. If you have power brakes, this is usually the default port to connect to the brake booster.

The two front ports are just extra vacuum sources. usually, one is ported (signal is only active when throttle blades are opening/above idle), and one is "manifold" meaning vacuum active at any speed or throttle position. All ports, when active, have the same amount of vacuum present, based on engine load.

Depending on your preference, and especially what your engine likes, you can choose either for the distributor advance. There is also another ported source, capped in your pictures, just to the front of the electric choke assembly.
I am not an expert on Holley carbs but I will offer some shade tree advice. Realize that Holley Carbs are very tunable and adjustable. This is both their benefit (to racers) and folly (to most of us). Most of the 'tunability' can be found with proper float adjustment, proper accelerator adjustment and proper jet and power valves. Set the idle and choke and you'll be 'on your way'. And make sure you have the proper fuel pressure. Bogging./stalling can usually be corrected with some accelerator pump adjustments and making sure the float levels are not too high or the power valve is incorrect On vacuum secondary carbs, once you have it running ''pretty good' changing out the spring in the vacuum pod can improve the driving experience.

But generally, beyond that, most of the rest of adjustments and changes are to glean a bit more power out of the engine, often for specific purposes like drag or road racing or if you have a heavily modified engine that pulls poor vacuum at idle due to a big cam or makes major horse power. Or perhaps you have a setup with multicarbs, etc. But for most of the rest of us we can accomplish a good driving experience by keeping it simple.

If you are having trouble with adjusting it don't be tempted to drill things, replace squirters or buy custom parts like metering blocks or externally adjustable jets. Don't do things like 'plug the power valve. You don't need a bigger accelerator pump either. You might find some improvements with different accelerator cams but otherwise there isn't much more you want to 'replace'.

I probably forgot something, I am doing this out of memory, so our friends will correct me if I did.
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Thanks @will e . I will not change or "optimise" anything on the carburettor. The previous owner installed it and I will keep it as long as it is in good condition. If the engine no longer runs smoothly, I'll see what I do next. The rest of the car will be returned to factory condition, all original.
The two front ports are just extra vacuum sources. usually, one is ported (signal is only active when throttle blades are opening/above idle), and one is "manifold" meaning vacuum active at any speed or throttle position. All ports, when active, have the same amount of vacuum present, based on engine load.
One little thing. The two vacuum ports shown in the bottom view picture are both manifold vacuum. It seems that Holley has chosen to include two different sized manifold vacuum ports, on newer carb production, for various accessory connections. The ported, or veturi vacuum port that most people use for vacuum advance is visible in the front view view photo on the choke side of the primary metering plate with a black cap on it. It is true you can use either ported or manifold vacuum for your vacuum advance connection, there's a good reason to use ported vacuum. Manifold vacuum is full at idle on the average and moderately modded engine. When you give hard throttle opening off idle your manifold vacuum goes way down, basically removing all vacuum advance. Ported vacuum will start at zero and will pull in as engine speed (airflow) rises. There's no reason to work the advance unit from in to out and then pull in. Tuning to a no advance vacuum at idle is typically more stable and easier to make initial tuning on. Of course, things change on ultra big cam motors, but that's where jetting, squirters, pump sizes, and power valve selection really come heavily into play.

Oh, and if your tailpipes are black, try going two sizes smaller on the primary jets. Conversely, if they're white and your engine runs a little hot, try going two sizes larger. One set in jet sizes is often inside the Holley factory production tolerance and may flow nearly identically to what you just pulled out!

Have fun and remember to only change one thing at a time, otherwise you'll likely not have any idea what helped or hurt!
I agree that based on the photo posted above, both ports are likely manifold. Whether you use a ported or manifold source is usually just whatever you prefer, but ultimately should depend on what your engine responds well to. I've had cars that "liked" both - depending on the engine build and tune.

Generally, more timing at idle is OK and keeps engine temps down, but isn't necessary for many builds. Once you are above idle, both vacuum sources do the same thing. It takes a little experimentation to see what you and the car like best.