MFI - Surging at Part Throttle - 911E '71

[webco]

I know there are a million threads on MFI but I couldn't find one that describes the issues I'm having. I have a '71 911E that I restored a few years ago. MFI was redone by Gus Pfister. Bosch CDI box redone by Ashcroft. The car ran perfectly for over 2000 miles. Yesterday I went on a 1 hour drive and it ran fine but started to stumble a bit after I got off the highway. When I headed home after several hours, it was stumbling and popping a bit on startup. But, ran fine for 30 minutes or so, did a few stumbles, then completely died. I pulled over, turned off the key, started it again and it ran perfectly for a while - then same thing happened. I discovered that if I turned off the key when it was stumbling and then turned it back on, it would run fine for a while.
So today I fired it up. Didn't drive it but warmed it up to near 180 degrees and discovered that as I revved it up to 1400, it would cut out, almost stall, then go back to idle. Likewise, when I held the revs at say 2500 and slowly let off, it would do the same thing once it dropped below 2000. Micro switch is set right. Where else should I be looking? Two separate problems or related? Seems like a weird coincidence if it's two separate problems. Any help appreciated!
Thanks,
Bill

[roblav]

Bad or dirty fuel?

[RSTarga]

clogged fuel sock at bottom of tank and/or points closing up.

[Jeff Higgins]

Sounds like a plugged vent on the gas tank. There is a little rubber hose that goes from the charcoal canister up at the left side of the cowl down to a steel tube that runs through the fender, kind of up behind the fuse panel and battery box. That steel tube goes through the fender to the outside, inside the wheel well. I have seen them all but rusted shut.

When this vent plugs, the car will start to miss and sputter as the fuel pump is no longer able to overcome the vacuum building in the gas tank as it empties. Turn the car off for awhile, and air slowly sucks back into the tank, and it runs well again until it develops that vacuum.

You can either try to clear that little steel tube out or remove it completely and just run the rubber hose through the fender. The steel tube is hard to clean out - it has a bend in it.

If that steel tube is in fact clear, look for other obstructions in the tank vent system. The steel tube is the most likely suspect, though.

An easy check for this is to simply remove the gas cap with the car running. You will hear a kind of a "whoosh", or sucking sound as air rushes past it into the tank.

[john walker's workshop]

Ohmmeter on the micro-switch, make sure it opens and closes properly. Pull the wire off the pump solenoid temporarily and drive for a while. The speed relay may be freaking out.

[webco]

Originally Posted by john walker's workshop

Ohmmeter on the micro-switch, make sure it opens and closes properly. Pull the wire off the pump solenoid temporarily and drive for a while. The speed relay may be freaking out.

Thanks John. I'll check the micro switch. Do you mean disconnect the solenoid wire to the MFI pump and drive the car to see if it stumbles and surges? And which one is the speed relay?
Bill

[john walker's workshop]

Yeah, just wondering if the solenoid is being activated when it's not supposed to, which would pull the main rack back to the idle position. Long shot though.