9 Holley 670 Street Avenger Problems (Troubleshooting Guide)

For many, the Holley 670 Street Avenger is a go-to choice for bringing power and street reliability to their engine setup.

But when this carburetor doesn’t perform as expected—rough idle, fuel leaks, or throttle hesitation—it can be a real hassle and impact your driving experience.

Here, we’ll break down the typical issues, uncover their causes, and offer solid solutions so you can get back to enjoying the smooth, reliable performance. Dive in to find out how to tackle these problems and keep your engine running strong.

1. Idle Mixture Adjustment Difficulties

One common issue for Holley 670 Street Avenger users is achieving a stable idle. Some drivers notice their engine stalls or experiences idle speed fluctuations despite adjustments. Often, idle mixture screws seem to have minimal effect, leaving owners puzzled.

Several factors contribute to this issue, including incorrect factory settings, vacuum leaks, blocked idle passages, and improper float levels.

These adjustments may seem ineffective if the initial settings don’t match your specific engine configuration. For example, drivers with Chevy 350 engines report persistent stalling, with tweaks to idle screws providing slight improvement.

To address these idle issues, first inspect for vacuum leaks, a frequent source of unstable idle. s, and the carburetor base gasket while the engine is running.

Changes in RPM can signal a leak that needs fixing. If the issue persists, clean the idle passages by carefully disassembling the carburetor and clearing blockages with compressed air and carb cleaner.

Check the float level by removing the sight plug on the fuel bowl; the fuel should sit just at the bottom edge of the hole.

Finally, reset the idle mixture screws by turning them 1.5 turns out from fully seated, then adjust both screws equally to find a stable idle.

2. Secondary Throttle Plate Sticking

Some users experience issues with the secondary throttle plates sticking or not opening smoothly, leading to hesitation or a flat spot during acceleration.

This lag can diminish performance when a quick response is needed, creating a driving challenge. Secondary throttle issues may stem from improper plate adjustment, mechanical interference from debris, or an unsuitable diaphragm spring tension for the engine’s vacuum signal.

Owners of Ford 302 engines frequently note a delay in secondary engagement, describing a noticeable lag before the secondaries fully activate.

To address this, set the secondary throttle plates to open slightly—around 0.020 inches—at idle to prevent them from sticking closed. Inspect the plates and linkage for obstructions, and clean any dirt that might cause friction.

The diaphragm spring tension should match the engine’s vacuum characteristics; Holley’s secondary spring kit can help select the correct spring for a smooth transition. Use a vacuum gauge to ensure your engine generates adequate vacuum for the secondaries to operate as expected.

3. Fuel Leaks Around Fuel Bowls

Fuel leaks from the fuel bowl gaskets are both wasteful and hazardous. Common signs are visible wet spots around the carburetor exterior, a strong fuel odor, or dripping onto the intake manifold.

Causes include warped fuel bowls or metering blocks from over-tightening, defective gaskets, improper gasket alignment, and high fuel pressure overwhelming the needle and seat.

Many users on forums like ThirdGen.org mention fuel leakage issues after reassembling the carburetor, especially following jet changes. A Dodge 360 owner reported fuel seeping from the bottom corners of the fuel bowls.

To prevent leaks, replace damaged or misaligned gaskets with high-quality non-stick versions and verify that gaskets sit correctly during installation.

Check for warping by using a straightedge on fuel bowls and metering blocks. If surfaces are uneven, consider resurfacing or replacing them.

Tighten screws evenly to Holley’s specifications—typically around 25-30 inch pounds. Adding a fuel pressure regulator can also help manage leaks caused by excessive pressure.

4. Running Rich or Lean Under Load

A rich condition, indicated by black exhaust smoke or fouled spark plugs, suggests the engine is receiving too much fuel.

Conversely, lean conditions, which might cause backfiring or hesitation during acceleration, occur when fuel is insufficient.

Incorrect jetting is often the cause, as stock jet sizes might not suit your engine’s needs. Power valve issues, such as a blown or improperly rated valve, can also impact fuel delivery. Blocked air bleeds from dirt or debris further complicate proper metering.

For example, a Pontiac 400 owner reported a rich running condition despite factory jetting, while others have experienced lean backfires under acceleration.

Consider rejetting based on performance tests and spark plug readings to address these fuel mixture issues. Replacing the power valve with one rated at half of the engine’s idle vacuum reading can improve fuel delivery.

Clean air bleeds using carb cleaner and compressed air to remove obstructions. Adjusting the accelerator pump to fine-tune the pump shot volume can eliminate lean spots when the throttle opens.

5. Accelerator Pump Hesitation or Lag

A hesitation or “bog” when pressing the gas pedal, especially from a stop, is annoying and unsafe. This lag makes your car feel unresponsive, which is problematic in traffic or while overtaking.

The causes often include an incorrectly adjusted accelerator pump linkage, inadequate pump nozzle size, or a worn accelerator pump diaphragm.

Owners of classic cars like the 1978 Corvette frequently report acceleration hesitation, particularly when moving from a standstill.

To solve this, check the accelerator pump linkage to ensure no gap between the pump arm and the lever at idle. If necessary, upgrade to a larger pump nozzle for a more substantial initial fuel shot.

Inspect the diaphragm for wear or damage, replacing it if needed. Changing the pump cam can also adjust the timing and duration of the fuel shot to better match your engine.

6. Electric Choke Malfunction

An improperly functioning electric choke can make cold starts difficult or cause the idle speed to remain too high after warm-up.

A sticking choke plate that doesn’t open fully affects fuel efficiency and overall performance. This issue often results from improper adjustment, electrical problems preventing full voltage to the choke, or mechanical binding in the choke mechanism.

Users report cases where the choke remains closed even after the engine reaches normal temperature, leading to a rich fuel mixture. Others discover their choke isn’t connected to a consistent 12V ignition source.

Begin by verifying that the choke receives a full 12V from an active ignition source. Adjust the choke cap to ensure the plate closes at ambient temperature, fine-tuning as needed.

Ensure that all linkage and the fast idle cam move freely. Testing the choke coil with a multimeter can confirm whether it has proper resistance, helping you determine if replacement is necessary.

7. Fuel Boiling and Vapor Lock

Fuel boiling or vapor lock may be to blame if your engine stalls after a run or struggles to restart when hot. Observing fuel boiling in the carburetor sight glasses is a clear sign of this issue.

Causes include ethanol-blended fuels, which boil at lower temperatures, high engine heat, and insufficient insulation between the carburetor and intake manifold.

Owners in hot climates report these problems, especially with older vehicles like the 1970 Mustang. To mitigate boiling and vapor lock, use a heat-insulating spacer, preferably phenolic or wood, between the carburetor and intake manifold.

Reroute fuel lines away from heat sources and apply insulating sleeves where necessary. Due to its higher boiling point, non-ethanol fuel can help prevent boiling. Installing a fuel return line also helps keep fuel cooler by promoting constant circulation through the system.

8. Throttle Shaft Wear Leading to Vacuum Leaks

Throttle shaft wear can create vacuum leaks, resulting in an unstable idle or whistling noise from the carburetor.

Over time, wear and tear cause the throttle shafts to let air in, disrupting the air-fuel mixture and impacting drivability. High-mileage owners on forums like Camaro Forums report idle issues directly linked to throttle shaft leaks.

To address this, install new bushings on the throttle shaft, which restores proper clearance and seals the leaks. If wear is excessive, replacing the entire throttle base plate may be necessary. Lubricating the throttle shafts periodically can help extend their lifespan and prevent early wear.

9. Synchronizing Dual Carburetors Can Be Challenging

While running dual carburetors can boost performance, it also requires precise synchronization to ensure balanced fuel delivery. Imbalances may cause backfires, hesitation, or uneven performance.

This issue can arise from mismatched carburetors, improper throttle linkage adjustments, or inconsistent secondary opening rates.

Owners using dual 670 Street Avengers on V8 engines frequently encounter these synchronization challenges. Start by ensuring both carburetors are identical in model and settings.

Adjust throttle linkage carefully so both carburetors operate simultaneously. Balance idle mixtures by adjusting screws equally on both carburetors while observing engine performance.

Synchronizing the secondary operation by ensuring the vacuum secondaries open at the same rate will help maintain consistent performance across all cylinders.

Final Thoughts

When properly tuned and maintained, the Holley 670 Street Avenger carburetor offers robust performance. Tackling common issues—whether idle adjustment difficulties, secondary throttle sticking, or fuel leaks—requires patience, thorough troubleshooting, and periodic maintenance.

With careful attention to each problem and solution, you can enjoy a carburetor that delivers reliable power and a smooth driving experience, keeping your engine running at its best.