34mm Venturi, for 40 Dcoe Weber Carburetors

OVERVIEW

When it comes to fine-tuning the performance of your air-cooled Volkswagen, the venturi is arguably the most critical component inside your carburetor. This EMPI 34mm main venturi, designed specifically for the 40 DCOE Weber series, acts as the heart of the carburetor''s airflow dynamics. By narrowing the barrel of the carb, it creates the necessary vacuum (the Venturi effect) to pull fuel from the main circuit and atomize it into the intake stream. A 34mm sizing is widely considered the "sweet spot" for many street-performance VW engines, providing an exceptional balance between high-velocity airflow for crisp throttle response and sufficient volume for top-end horsepower. Whether you are running a spirited 1776cc or a torquey 1914cc engine, swapping to the correct venturi size allows you to "right-size" your 40 DCOE carburetors to your specific displacement and camshaft profile, ensuring you don''t suffer from the "bogging" associated with oversized induction systems.

Precision-engineered by EMPI, a leader in VW performance since the golden age of air-cooled racing, these venturis are manufactured to exacting tolerances to ensure a perfect fit and consistent airflow across all barrels. Replacing a worn or improperly sized venturi is one of the most cost-effective ways to restore drivability and optimize fuel efficiency. If you find that your engine feels sluggish at low RPMs or struggles to transition from the idle circuit to the main jets, it is often a sign that your venturis are too large for your engine''s vacuum signal. By installing these 34mm units, you increase the air velocity through the carburetor, which creates a stronger signal at the auxiliary venturi and allows for much smoother acceleration. At Appletree Automotive, we understand that every engine build is unique, and having the right internal carb components is the difference between a car that just runs and a car that screams on the open road.

FITMENT

This 34mm venturi is designed for use in the following carburetor models and applications:

• Weber Carburetors: 40 DCOE Series (All Variations)
• EMPI Carburetors: 40 DCOE Clones / HPMX Side-draft Series
• Common Engine Applications: Volkswagen Type 1 Bug (1600cc-2110cc with side-draft manifolds)
• Common Engine Applications: Volkswagen Type 2 Bus / Transporter (Type 1 upright engine conversions)
• Common Engine Applications: Volkswagen Type 3 Fastback, Squareback, and Notchback
• Common Engine Applications: Karmann Ghia (Low-profile side-draft setups)
• Custom Builds: Sand Rails, Dune Buggies, and Manx-style bodies using side-draft induction

SPECS

Below are the technical specifications for the EMPI 43-5604-0 main venturi:

• Part Number: 43-5604-0
• Venturi Size: 34mm (Internal Diameter)
• Carburetor Compatibility: 40mm DCOE / Side-draft
• Material: High-grade Die-cast Zinc Alloy
• Finish: Precision Machined Natural Finish
• Quantity: Sold Individually (Note: Most dual-barrel carbs require two)
• Function: Controls air velocity and vacuum signal for the main fuel circuit
• Manufacturer: EMPI

PRO TIPS

1. The 4mm Rule for Street Performance

When selecting the right venturi for your 40 DCOE Weber, a general rule of thumb for street-driven air-cooled Volkswagens is to choose a venturi that is approximately 4mm to 6mm smaller than the throttle plate size. Since you are using a 40mm carburetor, this 34mm venturi is the ideal "upper limit" for a high-performance street engine. It provides enough airflow to support higher RPMs without sacrificing the air speed needed for "off-the-line" torque. If you go too large, such as a 36mm, you might gain 2-3 horsepower at 6,000 RPM, but you will likely lose the crisp, snappy throttle response that makes a VW fun to drive in traffic or on winding backroads.

2. Proper Alignment and Set Screw Tension

Installation of the venturi is straightforward, but precision is key. When sliding the 34mm venturi into the carburetor body, ensure that the machined indentation or "flat" aligns perfectly with the venturi set screw located on the side of the carburetor body. If it is misaligned, the set screw will not seat properly, which can lead to the venturi vibrating or shifting during high-vacuum events. Always use a small amount of thread-locking compound on the set screw and tighten it firmly, but avoid over-torquing, as the carburetor body is made of relatively soft aluminum. A loose venturi will cause erratic lean conditions and impossible-to-solve tuning issues.

3. Re-Jetting is Mandatory

Never change your venturi size without adjusting your main jets and air correctors. The venturi size dictates how much air passes through the carb and how hard it "pulls" on the fuel. When you move to a 34mm venturi, you are changing the air-to-fuel ratio requirements of the main circuit. Generally, as you increase the size of the venturi, you will need to increase the size of your main jet to maintain the correct mixture. After installing these 34mm units, we highly recommend performing a "plug chop" or using a wideband O2 sensor to verify that your engine isn''t running too lean at wide-open throttle, which could lead to overheating or piston damage in an air-cooled engine.