72 MFI, engine idles but won't rev for power
 

Any ideas as to what would cause this. I was driving, 3rd gear @ 5K and the engine stuttered. I drove several more miles and then I lost power. I pushed in the clutch & expected to see that the engine had stalled, but it was still running at idle. Raising the revs past 1400 & the engine cycles between idle & 3k, but no power. I can put it in gear & drive slowly at idle, but anything more & the engine balks

Thanks
Tom

Hmm. That almost sounds like the speed switch for the fuel cutoff on the MFI pump is working all the time. But that would mean that the microswitch on the throttle crossbar was stuck closed. I've never heard of that happening as it is a nominally open switch. And the RPM transducers are often removed or not working after all the years of heat and vibration.

That said, have you checked under the distributor cap? Ignition is usually the best place to start. Wires, plugs, distributor, points.

Classic symptoms of fuel pressure and or volume.

sounds like you are having fuel issues. I dont know much about MFI but I will offer some general gass advice.

1) Im not sure about the older 911's but I know my SC has a small screen at the outlet of the gas tank that can get clogged, see if you have one/its clogged

2) check fuel pressure, could be the pump, could be a clog

3) check fuel lines for leaks or cracks. You should smell this if its sever.

4) Could be a problem in the MFI, I dont know enough to speculate on what it may be.

5) check your air filter and air box

Regards
Dave

Fuel pump rebuilt last year. tank removed & cleaned & new fuel lines & filter.

Microswitch clicks when pushed, but I will remove a wire. Linkage not touching switch

Checked under the dist cap. dry & clean

Where the RPM transducers & how do I check them?

???
Thanks for the help
Tom

The first step in a diagnosis is to quantify what volume and pressure you have.

A recently rebuilt pump does not automatically make it O.K.

The RPM transducer is a little cigarette shaped aluminum box on your electrical console, held in place by tab with a screw. The transducer only activates when the microswitch is activated. It sends a signal to the fuel shut off solenoid until the RMPs fall to roughly 1700 then de-energizes the solenoid. It basically stops the rumble and backfiring as you decelerate. As pointed out, it's probably not working. You could pull the unit from the electrical console by removing the screw on the tab and gently rocking it out of it's connector. Usually, they have one or more pieces loose in the box meaning it's broken for sure.

If you find it's broken, we may be able to help. In any event, it's probably not part of this problem.

Run CMA!

Have you had any backfiring, loud as a shotgun shot from the exhaust pipe? I did at one point, in addition to your symptoms, and it turned out to be fuel starvation. I actually had the flexible fuel lines at the console mixed up. My here was an incorrect diagram being shopped around as the right way to do it (it was wrong).

Minor correction, but the transducer activates below 1500 rpm or whatever you set it at whether the switch is open or closed. The solenoid only works when the transducer is sending a signal and the microswitch is closed so that the signal can get to the solenoid.

Check the easy things first. Check that your distributor is tightened down and hasn't rotated.

I went to start checking on your suggestions. I started the engine & it ran fine???. So I am still at a lose as to what happened. Thanks to everyone who helped! Maybe you chased it away with your mind powers

Tom