Join Date: Mar 2008
Location: Kapuskasing Ontario Canada
Mentioned: 3 Post(s)
Idle speed is the only speed where the throttle body vacuum synchronization is required for efficient engine starting, good idle quality and driveability.
Once the butterfly valves begin to open, the area of the opening that permits additional airflow rises quickly. It’s only at the very slight throttle opening at idle speed that minute changes in the bypass adjustment screw opening are apparent. Remember, the throttle is almost closed. Very small changes in throttle opening affect idle speed and smoothness. Once the throttle begins to open farther, these small changes are insignificant and irrelevant.
Throttle valves are joined together by a fixed shaft, and their synchronization is actually not adjustable. The phrase “synching the carbs” is used as if it were a performance issue. It comes from an era when each carb or TB in a multiple rack were operated by separate cables or linkages, rather than a common shaft (or, in the case of CV carbs, by vacuum alone)... In those cases, you really did have to synch the throttles to ensure that they were even at part-opening and that all reached wide open position at the same point.
When you adjust your TB vacuum sync at 1,300 rpm, you'll notice that it goes off synch when you open the throttles as rpm rises, and then goes back to being balanced at higher rpm. The difference you're seeing on your manometer is only at very slight throttle opening and under no engine load... Opening throttles too fast may cause the liquid in your manometer to be sucked in the throttle with a higher vacuum level.