Diagnosing MFI idle issue

Phil Gilsdorf · 02:27 PM

Evening folks, I need some help in diagnosing a high idle issue on a 72 911 E 2.4L with MFI. The engine is a fresh rebuild and the pump was rebuilt not long ago by Pacific Fuel Injection. Distributor is a pointless style.

The actual idle is set to 1000 rpm, but when you rev it up the rpm hangs at around 1800rpm on the way down. Sometimes it comes down after 10-15 seconds, other times you have to force the rpm lower by slightly engaging the clutch with it in gear to slow the engine down and then it stays at 1000 rpm.

It isn't an issue with the hand throttle or the foot throttle cable as I can see that the butterflies are sitting on their stops at 1000 rpm and at the 1800 rpm.

Any ideas?

Thanks - Phil

BURN-BROS · 02-07-2013, 02:53 PM

Hi Phil,

There are 2 issues that come to mind.

one is the distributor advance is hanging up.

The other is the springs for both the cross bar and throttle bodies have weakened over time. The Microswitch for the RPM transducer can cause enough resistance to prevent the throttle from fully closing.

To verify if it is a spring issue, observe that the throttle bodies shafts are resting on their stops. If they are, then look to the distributor for the possible issue.

Add a helper spring to help the throttle to return.

There can be other problems causing this, like a faulty microswitch, shafts binding, or throttle cable wrapped around the clutch cable etc.

Phil Gilsdorf · 02-07-2013, 03:04 PM

Thanks for the reply Aaron. I checked the throttle bodies and observed them coming to rest on their stops. The resistance from the microswitch isn't the issue, it hangs up no matter how I set the adjustment screw. Its gotta be the dizzy then...

BURN-BROS · 02-07-2013, 03:07 PM

go ahead and verify with a timing light when the curve starts....They can start advancing before idle rpm so you will have to verify what timing is at 6000 rpm and subtract what you have at idle.

BSNMOE · 02-08-2013, 01:57 AM

Remove the dist. cap and suck on the vacuum line and see if the plate moves moves. It might be the vac retard is not puling the idle down and your're into the advance.

Phil Gilsdorf · 02-15-2013, 03:39 PM

Turned out to be something very simple. The wires inside the distributor for the pointless ignition were rubbing on the distributor cap and preventing them from moving freely. Thanks guys.