The dual Solex PDSIT carburetors found on 1972-1974 Volkswagen Type 2 buses often present a challenge even for experienced mechanics. Misunderstood and frequently replaced with less sophisticated alternatives, these carburetors offer reliable performance and excellent drivability when properly tuned. This guide delves into the intricacies of the Vw Solex Carburetor, providing a step-by-step adjustment procedure to unlock its full potential.
Understanding the Central Idling Circuit
The key to the Solex PDSIT’s efficiency lies in its Central Idling Circuit. This system allows for a lean idle mixture while maintaining drivability by utilizing a high-velocity air passage to optimize fuel delivery. It compensates for minor throttle bushing wear and complements the accelerator pumps, ensuring a responsive engine across the RPM range. Proper adjustment prevents hesitation and ensures smooth acceleration.
Preliminary Steps: Setting the Stage for Success
Before diving into carburetor adjustments, ensure the following prerequisites are met:
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Valve Adjustment: Adjust valves cold, targeting .006″ for both intake and exhaust (except for 1974 models with sodium-filled exhaust valves, which require .008″).
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Points Adjustment: Set points to .016″-.018″ for a dwell angle of 44°-50°.
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Leak Detection: Thoroughly inspect all hoses and connections for leaks, paying close attention to brake booster elbows and EGR valve screws. A comprehensive vacuum leak check is crucial.
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Vacuum Retard Test: With a warm engine idling at 850-950 RPM, disconnect the vacuum retard hose from the distributor. The engine speed should increase. If not, troubleshoot for leaks in the hose or diaphragm, or a lack of vacuum signal from the left carburetor.
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Timing Adjustment: Set timing to 25°-28° BTDC at 3,200-3,400 RPM with both vacuum hoses disconnected from the distributor. At idle, with vacuum hoses reconnected and the central idling solenoid wire energized, timing should be 5° ATDC (1972 and automatics) or 10° ATDC (1973-74).
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Spark Plug Maintenance: Clean, gap (.028″), and reinstall spark plugs with a light coating of oil on the threads. A compression test at this stage can provide valuable information for subsequent adjustments.
Initial Adjustments: Fine-tuning the VW Solex Carburetor
With preliminary steps completed, begin fine-tuning the carburetors:
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Throttle Stop Screws: Adjust throttle stop screws to ensure a proper base throttle setting. This impacts vacuum signal for the vacuum retard.
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High-Speed Air Flow Synchronization: Use a Uni-Syn or similar tool to synchronize airflow between the carburetors at high RPM (2,500-3,000).
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Idle Air Flow Synchronization: Synchronize idle airflow using the Uni-Syn and throttle stop screws, aiming for balanced airflow at idle.
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Mixture Adjustment: Adjust the mixture screws on both carburetors using a “mixture reference hose” to achieve a slightly lean mixture. Balance mixture between the two carburetors by observing RPM drop when individual cut-off solenoids are disconnected.
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Central Idling Speed/Mixture: Adjust the central idling speed and mixture screws to achieve a stable and slightly rich idle, compensating for the lean dual carburetor setting.
Choke Adjustment: Ensuring Smooth Cold Starts
Adjusting the chokes ensures smooth cold starts and warm-up:
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Choke Index Adjustment: Synchronize the choke opening times on both carburetors.
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Fast Idle Link Adjustment: Adjust the fast idle links to achieve the desired engine speed during warm-up, gradually decreasing as the engine reaches operating temperature.
Fine-tuning and Environmental Considerations
The VW Solex carburetor system is sensitive to temperature changes. Minor adjustments to the mixture screws may be necessary to compensate for extreme hot or cold weather conditions.
While most emissions equipment is no longer required, maintaining functional evaporative emissions and crankcase ventilation systems is recommended. Consider disabling or removing potentially harmful components like the EGR and air injection systems. Consult local regulations.
Conclusion: Achieving Optimal Performance
By following this comprehensive guide, you can unlock the full potential of the VW Solex carburetor. With patience and careful adjustment, these often-maligned carburetors can deliver impressive performance, smooth drivability, and excellent fuel economy. Remember to re-evaluate adjustments periodically and adapt to changing weather conditions for optimal performance.
