The Holley 94 and 2100 two barrel carbs came as the standard equipment 2 barrel carbs on the 1938 thru 1956 Fords. When converting the distributors on the Y-Block Ford engines from the original Load-O-Matic (LOM) design to the later model Ford (1957 and up) distributors, the Holley 94’s original distributor vacuum port for the distributor will supply an excess of negative pressure or vacumm signal to the late model distributors. The original Holley 94/2100 carb port design uses an anti-spark valve as well as a speed sensing venturi port and a ported vacuum signal just above the throttle blades to provide the proper vacuum signal to the original LOM distributors. All three of these work in unison to supply a vacuum signal to the distributor even at full throttle with the LOM system. This is a good thing for a LOM distributor equipped vehicle but a bad thing for the later model distributors.
The LOM distributors did not incorporate a centrifugal advance and relied solely on the vacuum signal supplied by the carburetor in which to maintain the proper advance curve under a variety of driving conditions. When using these carbs with the later model distributor which incorporates both a centrifugal and vacuum advance system, hooking up the vacuum advance without any carb modifications will provide an excess of vacuum signal to the distributor’s vacuum advance chamber at both idle and full throttle. This in turn creates tuning and drivabilty issues as well as increasing the propensity for detonation. Here is a pictorial in which to convert the Holley 94’s and 2100’s so that only a ported vacuum signal is supplied to the distributor and in turn makes this older model carburetor function similar to the ’57 and up carbs in both performance and fuel economy.