MFI RPM Transducer Switch Help

[Gearbox]

Trying to eliminate possible causes for a slight bruply back fire on deacceleration. I have a 71 911S and when cold, I get a pop pop pop from the exhaust when I lift off the throttle. It gets a little bit better when warm, but it's still there.

I understand the MFI RPM transducer switch may be at fault. From what I have read, the switch cuts off (reduces?) the fuel to the pump on deacceleration. I like to know the location of the switch (I think it's the one activated by the throttle linkage on the rear driver side bank of trumpets, but not sure) what does it do, and how do I test it to see if its working? I assume the switch is just a on or off switch when the throttle linkage returns to rest, but how do I test to see if the fuel is being shut off?

I also released propane around the trumpets to see if there was a vacuum leak, but didn't find any. Any other help would be appreciated. Thanks Allan

[HawgRyder]

The speed sensor unit is a silver rectangular can on the electrical panel in the engine bay (on the left side).
If I remember correctly...its about 3" high x 3/4" thick.
I took mine out of the circuit..as well as the micro-switch on the side of the left stack that triggers it.
The idea is..(I think)...the fuel is cut off when the throttle is closed and the engine RPM is above 1300-1500.
When the engine slows down past 1300, the fuel is allowed to come back on.
This is supposed to prevent the back-fire or "burbble-ing".
There was a circuit diagram posted a couple of years ago.
There is not a lot inside the can, and any competent tech should be able to test or repair it.
Bob

[rs dreamer]

From the engine compartment, rev the engine to 2000 rpm using the crossbar and while holding it there, push on the microswitch. The engine speed will drop to 1300 rpm and then increase to 1500 rpm and will continue to cycle between those two speeds. This procedure is described in the Check, Measure, Adjust manual for MFI engines. Gary

[210bhp]

Originally posted by rs dreamer
From the engine compartment, rev the engine to 2000 rpm using the crossbar and while holding it there, push on the microswitch. The engine speed will drop to 1300 rpm and then increase to 1500 rpm and will continue to cycle between those two speeds. This procedure is described in the Check, Measure, Adjust manual for MFI engines. Gary

Lets say the revs do not drop in this little test. What's the next step by step proceedure to find the location of the problem?

Regards
Mike

[304065]

Mike, I would test the microswitch for continuity next, then check the connections to the speed switch, then finally, check the speed switch itself.

But MUCH has been written here about this. . . I suggest the following thread to begin: Ultimate MFI resources thread

[Gearbox]

Thanks guys, I'm on it. Now why can't I find this stuff when I search the forums lol. I'll let you know what happens, Thanks again Allan

[210bhp]

Thank for your pointer John

Regards
Mike