Fuel cut off micro switch question.

[BOYER73S]

I have some backfiring occuring on my 73E so thought first place to start was the micro switch to check the fuel cut off was working.

With engine revving over 2500 rpm when I hit the microswitch it immediately cuts the revs but then while my finger is still on the switch
it revs up again then down again then up again and so on.

Is this normal???

I thought if you kept your finger on the switch the car should not be able to rev up again.

[Arne]

As I understand it, the fuel is cut off until the RPM drops to a certain point. (1800 RPM comes to mind, but don't quote me on that.) Then fuel resumes and it should rev up a bit, then fuel is cut off...

Shampoo, rinse, repeat...

[gled49]

In a perfect world the system would work. I’ve seen backfiring occur with system operational and defeated, no rime or reason to it. Try backing the micro switch screw out, as a test. G

[BOYER73S]

As I understand it, the fuel is cut off until the RPM drops to a certain point. (1800 RPM comes to mind, but don't quote me on that.) Then fuel resumes and it should rev up a bit, then fuel is cut off...

Shampoo, rinse, repeat...

That is exactly what happens

[frederik]

This confirms the system is working correctly. All that’s left is that you adjust the micro switch so it is triggered correctly when you release the gas pedal. Also when driving you need to purposefully release the pedal completely when slowing down, not keep it pressed just a little bit: in this case the fuel supply won’t be cut and you will get backfiring through the exhaust, especially after hard acceleration.

My fuel consumption and backfires decreased considerably after getting this system working, so it’s definitely worth it.

[NorthernThrux]

To adjust the microswitch, undo the locknut and back the screw off until it isn’t touching the microswitch. Then turn the screw in until you just hear the microswitch click. Turn another 1/2 to 3/4 turn in (I use 3/4) and then tighten the locknut while holding the screw in position with a screwdriver so it doesn’t turn.

The initial fuel cutoff should occur on a closed throttle when you are at 1500 rpm and dropping and the fuel should come back on at 1300 rpm so it doesn’t stall. So if you hold the throttle cross bar so that you are around 2k rpm and press the switch, the engine should hunt between about 1300 and 1500 rpm back and forth. That confirms your speed relay is working and the microswitch is doing what it is supposed to do.

[BOYER73S]

Thnaks to all seems like my system is working but will adjust the screw as Ravi suggested and be mindful of my right foot as it is often happening after hard accel and maybe my foot is still on the pedal a touch

[skinnerd]

I'm working my way through some problems on my MFI system as well.
This is my understanding.
The speed switch on the relay board supplies 12 volts to one side of the micro switch when the engine revs are above 1500rpm.
To be at (or above) 1500rpm, your foot is on the gas pedal and the throttle linkage has lifted off the micro switch button.
The micro switch is now in the "open" position and no current/voltage can flow through it to the fuel cutoff solenoid.
When traveling at speed (above 1500rpm) and you lift your foot off the pedal to slow down, the throttle linkage falls back to
contact the micro switch button (pushing it in) and allowing the 12V signal from the speed switch to pass through the micro
switch and energize the fuel cutoff solenoid.
This continues until engine revs have dropped to 1300rpm when the speed switch no longer supplies the 12V signal anymore
turning off the fuel cut off solenoid and restoring fuel to the engine.
The difference in the rising (1500rpm) and falling (1300rpm) is called hysteresis.
If both the rising and falling rpm set points were the same say at 1500rpm, then when you were cruising at 1500rpm, your
engine would be cutting out and starting back up very intermittently, it would be trying to stop and start at the very same
time. Not a very stable situation....hence the need for the hysteresis.

My engine began to start shutting down at speed....accelerating and cruising, but would idle OK.
Pretty sure this means I have a faulty micro switch.
It's letting the 12V signal through to the fuel cutoff solenoid when it's not supposed to.
Frustrating as the micro switch is brand new.
The car is coming back from the shop tomorrow and I'll check out my hunches.
For now the fuel shutoff solenoid has been electrically unplugged from the circuit and it's running fine.