[alfa] Backfire mystery

["John S." <roadtrip999@xxxxxxxxxxx>]

There are two fuel cutoff mechanisms on post-69 SPICA pumps. The first is a 
purely mechanical one where at idle throttle setting and rpms above 
1300-1600, the cam follower drops into a groove, and moves the rack arm to 
the closed position. The other is the fuel cutoff microswitch operating the 
fuel cutoff solenoid (FCS). The fuel cutoff microswitch is on the back 
bottom of the logic section and is operated by a pin that is moved as the 3D 
cam rotates as commanded by the throttle and slides back and forth as 
dictated by engine speed. The reason that the FCS was added was that it 
provides a much more positive fuel cutoff than just the mechanical one 
provided by the 3D cam groove. The FCS operates directly on the rack pivot 
and makes sure that the rack is in the FULL cutoff posiiton, when required. 
Of course, the movement of the rack controls the variable displacement 
plungers, which in the case of the rack going full-cutoff, literally 
prevents any fuel from being pumped by the plungers. Running the pump low on 
fuel with intermitten fuel starvation for periods of time is damaging, 
however. Hence the warning about not running out of fuel.

Here's a picture of a pump cutaway.

http://www.alfabb.com/bb/forums/showthread.php?t=3397

If you'd like a more specific picture of the area of the 3D cam, I can email 
one to you.

John

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