MFI sans-microswitch?
 

This question is particularly aimed at you MFI guys- Grady,John,Jeff,and friends but anyone is welcome to contribute.

I have an acquaintance from the early 911S registry with a 2.7 liter MFI RS spec converted from a T spec. He was experiencing exhaust pops and backfires, though he claims no fire. It made "tick-tich" noises through the intake.

While trying various procedures to correct the problem, he disconnected the microswitch that is part of the fuel shut off circuit on over-run. Apparently, his microswitch and RPM transducer both tested well and were operational.

Now that he is running with the microswitch disconnected electrically, he reports smother running and no exhaust backfires; only a few intake ticks. He has heard several instances of this happening.

I have an RS spec 2.7 liter with correct RS MFI injection pump. I have a missing RPM transducer so my fuel shut-off circuit is disconnected like his. I experience lots of backfires accompanied by flames. These sometimes scare people like gunshots. I would like to get better range on my fuel so I will be getting an RPM transducer.

My friend and I are both curious as to how disconnecting the microswitch from the fuel shut-off circuit can make the MFI run better, resolving the backfiring problem. I would think that this action would cause richer running and more backfires/flames, as was the case with my car. Is my friend's car running to lean due to either climate,altitude,or the smaller volume of fuel from the 911T spec space cam and injection pump tuning? With the extra fuel without load on the over-run with the microswitch disconnected, this could cause the mixture to richen up to a better level to stop backfires?

Any thoughts? Thanks:)

Max -

I'm fairly new to the MFI cult, but there are a few things I'm certain of:

1) CMA;
2) CMA;
3) CMA.

Fuel serves only one productive purpose - to propel the car. Any fuel burned for any other purpose is both pollution and a waste of money. To get the car to run correctly, do as Grady recommends and follow CMA. Every step. No exceptions, no skipping, no cheating.

The purpose of the microswitch is to allow current (routed from the RPM Transducer) to close the fuel shutoff when the RPM is up and your foot is off the gas (propulsion is not desired - fuel serves no useful purpose).

If a car is backfiring on decel it is because fuel is being pumped into the exhaust and ignition is available. If the microswitch is worn or out of adjustment, it could be keeping the throttle plates from closing completely - fuel/air/ignition/backfire. Disconnecting the microswitch in this case could allow the throttle plates to close completely, but it is masking, not solving, the primary problem.

Which brings us back to ---- CMA.

I don't see how disconnecting the microswitch wires would allow the throttle to close further. Only increasing the spring's force or decreasing the friction in the throttle linkage could do this. The microswitch is still in place in his car.

I agree that CMA (and CMA2) is the proper course. I need that RPM transducer first, before CMA. I need a complete system to check, measure, and adjust.:rolleyes: I have not heard back from the seller I was getting the proper transducer from. He is looking in his garage, he said. Did you get your defroster?

Disconnecting the wires wouldn't make much diff unless the transducer is malfunctioning. Most people take the microswitch out of the equation by backing off the actuating screw because the microswitch has a bit of resistance that the throttle arm has to overcome. If the microswitch is worn or the return spring is weak then the microswitch can hold the throttle very slightly open. Doesn't take much. I replaced my microswitch because it was doing this.

You'll probably get the definitive answers from Grady, John, Jeff, or one of the other gurus tomorrow.

No defroster relay yet - not worried as it has only been one day enroute.

I thought that the microswitch was the last componenet in the circuit that tols the engine to cut off fuel flow when the throttle was closed. If the RPM transducer worked but the switch was disconnected, the signal would stop there and not tell the engine to stop fuel flow.

Am I mistaken?:confused:

First of all, your friend has a worn, or few, butterfly shafts. When closed the intake stand off bounces the butterfly, hence the ticking noise. Now, when this condition exists, additional air is getting into the intake making idle adjustments very difficult if not impossible. So, to counter this, the butterflys and air bypass are adjusted to as much closed as you can get. On decel with the adjustments made as such, the butterflys are sucked tight and no air gets through the bypass.

OK, no air, no combustion, but a build up of fuel still being pumped into the exhaust and unburned can and will ignite and cause one hellacious backfire. I can't explain why your friend has no backfires, but this should help you understand why you do. Also, you may have a leak in your exhaust system allowing air to mix with the unburned fuel. His system may be completely tight. There are a lot of variables.

Shutting off the fuel on decel is the way to go. I just put a helper spring on my linkage to make sure the switch closed. What Jim says is 100% spot on.

I already know my problem and have corresponded with Jim about finding an RPM transducer to replace my missing (!) unit.

I am just curious as to how someone else can disconnect their microswitch, basically disabling their RPM transducer like in my case, and actually cure their backfiring problem.

I guess I wasn't clear enough. Sorry.