Delayed return to idle when shifting ('72-MFI)

[sballard]

This problem cropped up as I sort through various shifting & clutch issues on my '72 911T ("Shop recommendations for shifter linkage/transmission repair?"). When I lift off the accelerator as I shift through the gears, it takes between 2-5 seconds for the high RPMs to return to idle values when I push in the clutch. It does not appear that the linkage is binding but I noticed that the linkage pin that contacts the shutoff solenoid is not pressing hard enough to activate it. Is this normal for this switch, or do I need to adjust the linkage so that the switch is activated when I let off the gas?

[john walker's workshop]

The micro-switch needs to be depresses at closed throttle to activate the speed relay, which pulls the pump rack back to the idle position.

[gptom]

I have this problem too. I will check the switch

[gptom]

I checked. the adjusting screw isn't long enough to push the switch button. There is no adjustment for the switch or the bracket. what's my next step? should I get a longer screw and round the end? elongate the holes in the bracket?

[sballard]

The shaft of the adjusting screw on my car is 20 mm and it easily reached the button. I don't think there is anything special about screw although it does have a conical end that presses on the button. You can probably replace it with a longer screw of the same pitch and diameter.

It turns out that, in my case, adjusting the screw had no effect on the prolonged revs. (I suspect that the clutch may be sticking)

[Jeff Higgins]

Even if the linkage does not appear to be binding, I've seen them close darn near all the way and hold up in the last 1/8" or so, then slowly creep shut. Make sure the throttle arm on the side of the pump actually returns to the stop when you let off the gas - don't do this by grabbing the linkage, actually have someone step on the gas and let up as you watch.

It seems like when the linkage gets a little dirty, that last 1/8" gets to be the sticking point. The individual springs on the throttle bodies get pretty gooey with crud over the years. Blast those with some carb cleaner. For good measure, pop each ball socket loose, blast them as well, and grease.

[cabmandone]

Never asked you this on the other thread but do revs drop slowly when the car is in neutral? And have you checked for binding in the throttle linkage? You said most of this came on after the engine had been dropped. If the problems didn't exist prior to the engine drop, something didn't go back together properly.

[john walker's workshop]

Often the linkage has some wear, which keeps all the throttles from closing at exactly the same time. I've made a couple of L shaped spring brackets mounted to the rearmost nuts on left and right stacks with a short spring attached to the #1 and #4 linkage balls to help them close better.

[gptom]

Jeff: I have already popped all of the ball sockets, cleaned & greased. helped a little to return to idle. I sprayed the return springs on the throttle bodies. helped a little

I just put in a longer screw. will see if that helps

[john walker's workshop]

Another issue is tired distributor advance weight return springs. If they don't pull the weights back at idle, the timing stays a bit advanced and that raises the idle.

Did you try revving the engine and pushing the switch closed with your finger with throttles closed and still idling high to see if the system is working as it should?

[tobluforu]

Originally Posted by john walker's workshop

Often the linkage has some wear, which keeps all the throttles from closing at exactly the same time. I've made a couple of L shaped spring brackets mounted to the rearmost nuts on left and right stacks with a short spring attached to the #1 and #4 linkage balls to help them close better.

That sounds like a great idea, I might try that with my org stacks. I use a spring attached to the pivot point under the tranny which pulls them all closed, but it makes for a stiff pedal.

[sballard]

Thanks for the comments. Really good ideas, especially possible binding of the linkage. It's been too hot to work in the garage the last day or so (Alabama in July!). I will dive in again tomorrow and report back.

[sballard]

Well the rack was definitely sticking. It doesn't seem to matter how much I adjust the screw that contacts the micro-switch, the rack stops just short of the switch.

I pulled off most of the throttle links that I could easily access and cleaned and re-greased them. My test drive was the same - throttle takes several seconds to return to idle.

John, your suggestion about the spring sounds great although I can't tell which bolts you are referring to for mounting the springs. Would these be the bolts that hold down the black plastic stacks or the ones that bolt the intakes to the heads? It looks like I could just screw the L-brackets to the sheet metal at the back of the engine compartment.

[tobluforu]

Originally Posted by sballard

Well the rack was definitely sticking. It doesn't seem to matter how much I adjust the screw that contacts the micro-switch, the rack stops just short of the switch.

I pulled off most of the throttle links that I could easily access and cleaned and re-greased them. My test drive was the same - throttle takes several seconds to return to idle.

John, your suggestion about the spring sounds great although I can't tell which bolts you are referring to for mounting the springs. Would these be the bolts that hold down the black plastic stacks or the ones that bolt the intakes to the heads? It looks like I could just screw the L-brackets to the sheet metal at the back of the engine compartment.

Before you go that route, do what I did as a test, Its simple and cheap and works.

[Flieger]

You can run the car without the shut-off solenoid working and it will return to idle normally. The shut-off solenoid comes into play more for engine braking. When the revs are above the cutoff and the throttle is closed enough to close the microswitch it will push the rack past the idle position so that there is no fuel going into the engine. This stops the backfiring (which some people like) and makes the engine braking stronger. Just shifting between gears though the engine should spin down just fine with the idle setting rather than the 'off' setting.

Do the test though, since it is easy. Use one hand to raise the revs and then push the microswitch with the other and the revs should fall, then come back up, and oscillate if things are working. The transistor box is probably non-functional since the leads get fatigued and break. Not that hard to repair.

So, I'd say your throttle is sticking. By the way, how is the idle- is it high?

[sballard]

Thanks very much for the replies and suggestions.

Today, dialed in the screw that actuates the shut-off solenoid all the way in and it seems to be working. The driveability is definitely improved. The idle now falls to 800-1000 RPM when I let off the gas pedal each time. I am still crunching gears when shifting, but not nearly as bad as before. I can tell that I need to get back to dialing in the shifter linkage. And I definitely need to do something about my shifter. I've read good things about Rennshift.

[gptom]

I installed a longer screw and now closing switch. Engine now returns to idle correctly.

I now have a new problem, especially when cold. The idle hunts between 2500 and almost stalling very rapidly. what has the microswitch circuit revealed as a hidden problem

[matthewb0051]

Check your injectors and replace as necessary. I spent over 2 years chasing problems on MFI that were being impacted by old injectors.

[Flieger]

Originally Posted by gptom

I installed a longer screw and now closing switch. Engine now returns to idle correctly.

I now have a new problem, especially when cold. The idle hunts between 2500 and almost stalling very rapidly. what has the microswitch circuit revealed as a hidden problem

To me it sounds like you are activating the shut-off solenoid but something is holding the throttle open. What you describe sounds exactly like the result of doing the test on the microswitch/rpm transducer/shut-off solenoid. Did you do that test by the way?

My guess is that your screw is too long and it is causing the throttle shaft to not return all the way to idle. It mashes the switch closed and shuts off the fuel until it almost stalls, then the speed switch reaches the end of its window and fuel comes back on but revs climb into the range where the transducer works again. This is exactly what you do to test the switch- hold the throttle open by hand and press the button with the other hand.

It is a fine line between too short and too long on the screw. I don't know if you can ever get it perfect. On mine if I want it to activate I need to make sure I let off the throttle abruptly enough at the end to close the switch.

[tobluforu]

Remove the micro switch from the equation in regards to the car idling. Unscrew the screw so it does not depress the switch, then unplug it. Now, get your car running/idling correctly, once that is complete then incorporate the switch back into the equation. Lots of people disconnect this switch and never use it, I'm one of them. Its set to work between 1300 and 1800, give or take, so if your car is idling high and its depressed then its going to hunt. The transducer can the biggest issue with the system as they go bad.
Rpm Transducer