Hmmmmmm . . . . That's really unusual that a pump from Wes would have a bad microswitch, but it's' probably the "lightbulb" theory . . . . one day you turn it one and it doesn't work.
First, the microswitch is not essential to the operation of the pump. There is also an internal mechanical cutoff. Actually, it's a specific position on the 3D cam when the engine is running above 1300 rpm. In fact, my '74 SPIDER's SPICA pump has a shorted microswitch right now. I've simply remove the electrical connector from the Fuel Cutoff Solenoid and relied on the internal cutoff. Works fine with no backfiring. When I get some time, I'll remove the pump and replace the microswitch. The microswitch is available through International Auto Parts (
www.international-auto.com) and several other vendors for about $60. That kind of money for a simple and (very) little switch is pretty steep but it's the only one that I know of that fits.
If you're engine is still backfiring after deactivating the Fuel Cutoff Solenoid, it's likely that your throttle manifold butterfiles are out of adjustment and slightly open at the idle stop. At idle, the throttle butterflies should be fully closed and the only air delivered to the manifold should come throught the idle system. In addition, it's likely that the rods are holding the FI pump throttle lever open slightly causing the stylus to never get into the fuel cutoff portion of 3D cam and continue to deliver fuel during what you "think" is throttle closed decel. (sorry about the run-on sentence).
In fact, if the microswitch fails open, it's likely you'll never know it. If it shorts (like mine) and delivers a continuous fuel cutoff signal, then the engine dies and will not restart. Simply pulling the wire off the FCS fixes the no start/no run condition immediately.
It is also possible that the adjustment on the microswitch is not quite right, but I'm sure Wes calibrated the pump correctly before he sent it to you. I would definately drop him an email or phone call. I'm sure he can source a microswitch.
Regardless if you need to R & R the microswitch and/or adjust it, it's going to require a R & R on the pump. It's a bit of a PITA, but not really complicated.
Never heard of anyone fabricating a distributor driven cutoff switch. That'd cost a lot more than a $60 switch.
Drop me a PM if you'd like more info.
Here's a couple of links that may be of interest to you:
http://www.alfaclub.org/techstff/spica.htm
http://www.alfaclub.org/techstff/tunespca.htm
Linear Mode
