Dave,
Well, you answered your own question as to why the microswitch was disconnected! It is shorted out, and MUST be replaced! With a shorted switch there would be a rather limited operating zone for the engine between idle and 1800-2000 rpm ... as in [/b]NO FUEL TO THE INJECTORS above 1500 rpm![/b] I guess the previous owner decided he could live with backfiring, as opposed to an 2000 rpm rev limit! Apparently he didn't want to spend the money to fix it.
Explanation of circuit logic for Speed Switch + Microswitch + Cut-Off Solenoid/Magnet:
Refer to items #11, #13, & #15 in the following circuit (also in Haynes on p. 219) schematic: http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_1971_Part-2.jpg
Item #11 is the Speed Swictch, and is mounted on the engine 'console' in close proximity to the Voltage regulator. It gets the same ground-pulse signal from the distributor that drives the CDI-unit and tach. It is 'activated' and closes a set of relay contacts at 1500 rpm, and the relay contacts do not open up until rpms fall to 1300 rpm.
Item #15 is the Microswitch mounted on the throttle body, and operated when throttle rod is at rest, i.e, throttles closed!
Item #13 is the Cut-Off Solenoid (or magnet) mounted on the injection pump.
The need for the fuel cut-off during overrun is farly easy to understand, since the MFI injection pump operates primarily based on engine rpm and throttle position ... there is no air flow or vacuum input! So, an electronic relay operates when the engine rpm is above 1500 rpm and provides switched 12 Volt power, fed to the Microswitch. So, an overrun condition exists ... meaning the engine is above 1500 rpm and the throttles are closed, the Microswitch provides 12 Volt power to the Cut-off solenoid, causing the fuel to the injectors to be stopped while the engine speed drops to 1300 rpm. The difference in the switching action on rising rpms vs. falling rpms is called hysteresis, and the electronic action of the circuit with different thresholds on rising and falling triggers is called a Schmidt Trigger.
This interupted fuel supply is why properly-operating MFI cars always get better gas mileage than Weber-equipped cars -- even found to be the case empirically in 906E race cars at Le Mans!
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Warren Hall
1973 911S Targa
[This message has been edited by Early_S_Man (edited 09-08-2001).]