6 Common Causes for Holley Vacuum Secondary Not Opening (with Solutions)
Some Holley vacuum secondaries have been known to have problems. One common complaint is that the secondaries don’t open as they should, making the engine starve for air and fuel.
So why is a Holley vacuum secondary not opening? It is mostly a result of a punctured vacuum diaphragm. Also, other potential causes include stuck secondary throttle blades, a stiffer secondary spring, a damaged cork gasket, and obstruction in the primary vacuum passages to the diaphragm. Besides, the use of an oversized carburetor can be a problem.
While these issues may seem daunting, they’re actually quite easy to fix with the right tools and know-how. Read on for an in-depth explanation of potential causes and how to diagnose and solve this problem.
Holley Vacuum Secondary Not Opening: Potential Cause and Solution
Check out this potential cause and solution chart to figure out the problem and how to fix it once and for all!
Potential Cause | Solution |
Punctured vacuum diaphragm | Replace diaphragm if punctured or damaged |
Secondary throttle blades stuck | Clean the throttle blades of deposits Ensure the primary throttle is wide open |
Stiffer secondary spring cause delay opening | Replace with a lighter spring For Holley Quick Fuel carbs, use the adjustment screw to control the opening rate |
Damaged cork gasket | Replace the cork gasket |
Obstruction in primary vacuum passage | Blow through the vacuum ports with compressed air |
Oversized carburetor | Use correct carburetor size for your engine’s needs |
Understanding How Vacuum Secondaries Work
Before diving into possible causes and solutions, it would help if we first understood how Holley vacuum secondary systems function.
Opening the vacuum secondaries depends on the air speed via the primaries. On opening the throttle, there’s increased flow into the inlet manifold due to an increase in engine rpm.
The influx of airflow exerts a stronger vacuum effect on the diaphragm controlling the secondaries. This allows the secondaries to open gradually as your engine needs more fuel and air to sustain the demand of the throttle.
There’s another vacuum signal to the vacuum diaphragm once the throttle blades open, keeping the secondaries open for as long as necessary. The diaphragm spring regulates how fast the secondaries open initially. A stiffer spring means a slow opening rate and vice versa.
How To Check If Holley Vacuum Secondaries Are Opening
You can find out by putting a paper clip on the secondary shaft and then putting your car on full throttle. If, after that, the paper clip moves, you know the secondaries are working.
6 Common Reasons Your Secondaries may Not Be Opening
Let’s dive deeper into the six possible causes for vacuum secondaries not opening and how you can troubleshoot and fix these issues.
1. Faulty Vacuum Diaphragm
A punctured diaphragm will often cause vacuum leaks. You can do a vacuum test or a visual inspection to troubleshoot this issue. But first, you have to remove the diaphragm from the carb’s main body.
To do this, pull off the choke housing, undo the mounting screws on the diaphragm, and pull off the little E-clip. The diaphragm should come right off.
Vacuum Test
Pull up the secondary rod and cover the hole with a cork gasket on it. It should be able to create a vacuum block where the rod shouldn’t be able to move. However, if it moves, then your diaphragm is punctured.
Alternatively, you can put your mouth in the hole with a cork gasket and suck or blow on it. It should be able to move the rod around. If not, the diaphragm is punctured.
Visual Inspection
Sometimes, the puncture may be a pinhole size, making it difficult to pinpoint. For this reason, I don’t always recommend a visual inspection.
Note: When re-installing the vacuum diaphragm assembly, ensure it fits tight on the carb’s body to prevent vacuum leaks. Also, ensure to retain the check ball or brass restrictor, which keeps the throttle blades from opening too quickly.
2. Stiff Secondary Spring
The spring fits onto the underside of the vacuum housing’s cap and controls the initial opening rate. If it’s too stiff, the secondaries will delay to open.
Spring tuning & replacement
Basic tuning usually entails playing with different tension springs to control the opening rate. Holley makes this easy by offering a secondary spring tuning kit.
The kit comes with seven color-coded springs of varying stiffness to give you full control over the opening rate. It ranges from black (the stiffest) to white (the lightest). When opting for a lighter spring, ensure that the secondaries don’t open up too soon to cause hesitation.
Replacing a secondary spring on a vacuum will depend on the type of diaphragm housing.
For traditional Holley carb, remove the choke and the diaphragm’s housing to get to the spring. However, there’s a quick-change version where you don’t have to remove the choke and diaphragm housing.
Holley Quick Fuel carbs have an adjustment screw that regulates the speed at which the secondaries open. Turning the screw clockwise will slow the opening rate while turning counterclockwise will cause them to open up faster.
3. Secondary Throttle Blades Stuck
Airflow through the primary throttle bore causes the secondary throttle blades to open. Deposits often impede the free movement of throttle blades. If the primary throttle doesn’t open fully or the secondary throttle blades are stuck, you will not get enough vacuum to open the secondaries.
Over time, these deposits become thick enough to prevent the blades from opening all the way even if you put your foot on the gas.
4. Obstruction In Primary Vacuum Passage
Check the porting on the carburetor for obstruction. Open the primary throttle linkage and locate a little hole on one of the primary venturis. Blow compressed air across that hole with the engine off. It should clear any obstructions and open up your secondaries.
5. Damaged Cork Gasket
When looking at the diaphragm housing, you’ll see a small cork gasket for your secondary venturi air jet. A bad cork gasket will almost certainly lead to vacuum leaks. Get a new gasket if it’s damaged in any way.
6. Oversized Carburetor
Consider the sizing of your carburetor. If you have too big of a carburetor and the engine can never use the full CFM, it’s not going to open the secondaries because it can’t create enough vacuum across the porting to open the secondaries.
Conclusion
I hope this guide covers all of the most common causes of Holley vacuum secondary not opening. As long as you’re aware of these six possible causes and solutions, you should be able to get your Holley back up and running quickly.
Essentially, the problems arising with Holley vacuum secondaries aren’t so much of an issue with the carburetor itself. Rather, it’s the internal components attached to it that get obstructed or wear down over time.
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