Weber 32/36 DGAV Carburettor Woes

Encountering issues with your classic vehicle's carburettor can be a frustrating experience, especially when it's a popular and often reliable unit like the Weber 32/36 DGAV. This downdraught, progressive twin-choke carburettor, found on a vast array of classic European cars, is renowned for its performance enhancement capabilities. However, like any mechanical component, it can develop faults. This article delves into potential reasons why your Weber 32/36 DGAV might be acting up, drawing on common problems and diagnostic approaches, including the specific scenario described where an unknown carb's mixture and idle screws are proving elusive.

Understanding the Weber 32/36 DGAV

Before we dive into the problems, it's helpful to understand the basic layout of the 32/36 DGAV. It features a 32mm primary barrel and a larger 36mm secondary barrel, which opens progressively based on throttle position and vacuum. The 'D' signifies downdraught, 'G' indicates a progressive linkage, 'A' denotes a specific design variation, and 'V' signifies the vacuum-operated secondary diaphragm. Crucially, the location of its adjustment screws can sometimes be a point of confusion, particularly if the carburettor isn't original to the vehicle or has been tampered with.

Common Weber 32/36 DGAV Issues and Troubleshooting

Several factors can contribute to poor performance from a Weber 32/36 DGAV. Let's break down the likely culprits:

1. Fuel Delivery Problems

The most fundamental requirement for any internal combustion engine is a consistent and clean supply of fuel. If your carburettor isn't receiving the correct amount of fuel, or if the fuel is contaminated, performance will suffer.

• Stale Fuel: As noted in the provided scenario, the age of the fuel in your tank is a significant factor. Gasoline degrades over time, losing its volatility and potentially forming gummy deposits. If your car has been sitting for an extended period without a fuel stabiliser, this is a prime suspect. Testing fresh fuel from a separate, known-good container by temporarily connecting the fuel line is an excellent diagnostic step. This bypasses any issues with the vehicle's existing fuel tank, lines, and pump.
• Fuel Pump Issues: The fuel pump is responsible for delivering fuel from the tank to the carburettor. A weak or failing fuel pump may not provide adequate pressure or volume, leading to fuel starvation, especially under load. You can test the fuel pump's output by disconnecting the line to the carb and running it into a suitable container, observing the flow rate and consistency.
• Blocked Fuel Filter: A clogged fuel filter will restrict fuel flow. This is a relatively simple component to replace and should be considered as part of regular maintenance.
• Float Level: The float within the carburettor's float bowl maintains a constant fuel level. If the float is set too low, the engine can run lean. If it's set too high, the engine will run rich, and fuel may even spill out of the float bowl vent. Adjusting the float level requires careful measurement and is a critical step in carburettor tuning.

2. Air/Fuel Mixture Imbalance

The correct ratio of air to fuel is paramount for efficient combustion. The Weber 32/36 DGAV has specific adjustment screws for this:

• Idle Mixture Screw: This screw controls the amount of fuel entering the engine at idle. As described, this screw can sometimes be recessed. Getting this adjustment correct is vital for a smooth idle. The procedure typically involves gently seating the screw and then backing it out a specified number of turns (often 1.5 to 2.5 turns, but check your specific carb's manual). The engine should then be brought up to temperature, and the idle speed adjusted. Finally, the mixture screw is slowly turned in or out to achieve the highest and smoothest idle speed.
• Idle Speed Screw: This screw physically limits how far the throttle plate closes, thereby controlling the engine's idle speed. It's often located near the throttle linkage. Ensure it's not bottomed out or excessively wound in.
• Main Jets and Air Jets: These internal components dictate the fuel and air flow through the carburettor at different throttle openings. If the wrong size jets are installed, or if they become partially blocked with debris, it can cause rich or lean conditions.

3. Ignition System Issues

While this article focuses on the carburettor, it's crucial to remember that a faulty ignition system can mimic carburettor problems, often leading to rich running and poor performance.

• Timing: Incorrect ignition timing can severely impact how the engine runs. As suggested, using a timing light to set the ignition timing correctly is a fundamental step before attempting to tune the carburettor. If the timing is too advanced or retarded, the fuel mixture won't ignite at the optimal moment.
• Spark Plugs, Leads, and Coil: Worn spark plugs, faulty ignition leads, or a weak coil can result in a weak spark, leading to incomplete combustion and a rich fuel mixture.

4. Air Leaks (Vacuum Leaks)

Unmetered air entering the engine after the carburettor can cause a lean condition and an unstable idle. Common sources include:

• Carburettor Gaskets: Ensure the gasket between the carburettor and the manifold is in good condition and properly seated.
• Manifold Gaskets: The intake manifold gasket can also develop leaks.
• Vacuum Hoses: Check all vacuum hoses connected to the intake manifold and carburettor for cracks, splits, or loose connections. A common diagnostic technique is to spray a small amount of carburettor cleaner or unlit propane around suspect areas while the engine is idling; a change in idle speed indicates a leak.

5. Choke Issues

The Weber 32/36 DGAV often features an electric choke. If this is malfunctioning, it can lead to a persistently rich mixture, especially when the engine is cold, and even after it has warmed up.

• Electric Choke Operation: The electric choke uses a bimetallic spring that is heated by a small electrical current. This spring causes the choke flap to open as the engine warms up. If the heating element fails, or if the bimetallic spring is damaged, the choke may remain partially or fully closed, causing the engine to run rich. You can check for voltage at the choke housing when the ignition is on. Ensure the choke flap moves freely when the engine is cold and opens as it warms up.
• Manual Choke Conversion: Some owners opt for manual choke conversions, which can introduce their own set of potential issues if not installed or adjusted correctly.

6. Internal Carburettor Wear and Blockages

Over time, internal components can wear, or small passages can become blocked by sediment or debris.

• Blocked Jets and Passages: Even with a fuel filter, microscopic particles can find their way into the carburettor, blocking the small fuel and air passages. A thorough cleaning and rebuilding of the carburettor, using a quality rebuild kit, is often the most effective solution for persistent issues. This involves disassembling the carburettor, cleaning all components (often with an ultrasonic cleaner or carburettor cleaner), and replacing seals, gaskets, and often needle valves.
• Diaphragm: The vacuum diaphragm that operates the secondary barrel can tear or become brittle, leading to improper secondary opening.

The Mysterious Mixture Screw and Electric Choke Location

The scenario highlights a common point of confusion: the recessed idle mixture screw and the idle speed screw being behind the electric choke mechanism. This is indeed characteristic of many Weber 32/36 DGAV installations. The idle mixture screw is typically found on the base of the carburettor, often on the side facing the engine block, and it is indeed smaller and designed to sit flush or slightly recessed when fully seated. The idle speed screw is usually on the throttle linkage, but on many Weber 32/36 variants, it's positioned to contact the throttle linkage cam, which can be integrated or positioned near the choke housing. Identifying these correctly is the first step to successful tuning.

Tuning Your Weber 32/36 DGAV: A Step-by-Step Approach

Once potential issues are identified, a systematic tuning process is essential:

1. Ensure a Clean Fuel Supply: Start by confirming fresh fuel and a properly functioning fuel pump.
2. Set Ignition Timing: Use a timing light to set the ignition timing according to your vehicle's specifications. This is a non-negotiable first step.
3. Address Air Leaks: Check for and eliminate any vacuum leaks.
4. Initial Carburettor Settings: With the engine at operating temperature, set the idle speed screw to achieve a slightly higher-than-normal idle speed. Then, adjust the idle mixture screw to achieve the highest possible idle speed. Make small adjustments, then readjust the idle speed screw if necessary.
5. Fine-Tuning: Once the idle is stable, you can fine-tune the mixture screw. For a slightly richer mixture (often preferred for smooth running and preventing detonation), back the screw out an additional 1/8 to 1/4 turn from its highest idle point.
6. Check Choke Operation: Verify that the choke operates correctly and fully opens when the engine is warm.
7. Road Test: Take the vehicle for a test drive. Pay attention to acceleration, deceleration, and overall power delivery. If issues persist, further investigation into jetting or internal components may be necessary.

Fan Speed Diagnosis

Regarding the fan speed issue mentioned, the observation that the fan works on high but not low is a classic symptom of worn contacts within the fan speed switch. The switch likely has multiple positions, each connecting to different resistances or directly to the motor. The low setting might use a resistor to reduce the fan speed, and if the contacts for this position are corroded or burnt, the circuit will be broken, preventing the fan from operating on low. Checking the amperage draw on both settings with an amp meter, as suggested, is an excellent way to diagnose this. A significantly lower or zero reading on low compared to high, with no change in fan motor noise, points strongly to a faulty switch.

Table: Common Symptoms and Potential Causes

Frequently Asked Questions

Q1: What is the correct idle mixture screw setting for a Weber 32/36 DGAV?

A1: There isn't a single universal setting. The correct setting is found by adjusting the screw to achieve the highest and smoothest idle speed, typically between 1.5 to 2.5 turns out from a gently seated position. Always fine-tune based on engine response.

Q2: My Weber 32/36 DGAV is leaking fuel. What could be wrong?

A2: A fuel leak often points to a float level set too high, causing fuel to overflow, or a faulty needle valve that isn't sealing properly. It could also indicate a crack in the float bowl or a loose fuel line connection.

Q3: Can I use a generic carburettor cleaner for my Weber 32/36 DGAV?

A3: While generic cleaners can work, it's best to use cleaners specifically designed for carburettors, and ideally those that are safe for brass and rubber components if you are undertaking a rebuild. Some aggressive cleaners can damage delicate parts.

Q4: How do I identify the correct idle mixture screw on my Weber 32/36 DGAV?

A4: It's usually a smaller, finer-threaded screw located at the base of the carburettor, often on the side closest to the engine block. It's designed to disappear into the carburettor body when fully screwed in.

Q5: My car runs fine when cold but poorly when warm. What's the likely cause?

A5: This often suggests an issue with the choke mechanism not opening fully, or a vacuum leak that becomes more pronounced as the engine components expand with heat. It could also be related to fuel delivery issues that worsen as the engine compartment heats up.

Troubleshooting a Weber 32/36 DGAV can be a rewarding process, bringing your classic vehicle back to its former glory. By systematically addressing fuel, air, ignition, and carburettor-specific adjustments, you can overcome most performance issues. Remember, patience and a methodical approach are key.