Rebuilding The 94In most cases, rebuilding a carburetor primarily consists of a thorough cleaning and replacement of the gaskets, needle, and seat, and in the case of a 94, the power valve. But when working with parts as old and with as many miles as most of these carburetors, some additional parts may be needed. on high-mileage carburetors it's not unusual for the throttle shafts or the holes in the carburetor base to be worn. new standard-size shafts, oversize shafts used to save a worn base casting, as well as extended shafts to allow the use of a linkage for multiple carburetors are all readily available.
Like the modern versions, Holley 94s use a power valve (also called an economizer valve) that opens to supply extra fuel to the engine when the vacuum drops to a certain point, usually 7 1/2 inches Hg or less. While they work fine in stock applications, if a more aggressive- than-stock cam is used or multiple carburetors are installed, the vacuum signal is altered and the power valve(s) open prematurely, making the mixture much richer than necessary. A common mistake in this situation is to rejet the carburetor(s), which results in even worse driveability problems. Another mistake often made is to plug the power valves, something that's not recommend. The best approach to this problem is to stick with stock jet sizes (or very close) and use power valves that open at a lower value. In the past, we've used 3 1/2-, 4 1/2-, and 5 1/2-inch Hg valves, depending on the cam used, with excellent results.
Selecting the proper power valve can be done a number of ways. The best method is to use a portable exhaust gas analyzer to check the air/fuel ratios under real-world conditions and try different power valves until you find the right combination. Another way is to use a vacuum gauge. drive the car and note the vacuum readings at idle, then install power valves with opening points in between the vacuum level at cruise and wide open. Then there's the seat-of-thepants method. If the car stumbles and puffs black smoke when you jump on it, the power valves are opening too soon, so go to a lower-number valve. If it pings, do the opposite and try a higher number. Finally, there's the best guess method. If you're using two carbs, go about half of the stock value of 7 1/2 inches of vacuum Hg, which is in the 3 1/2- to 4 1/2-inch Hg range.