wicks

Hi - pro mechanics are stumped - 73 911s, everything rebuilt or new. Original spec engine, no mods.

The idle will either run at 500 RPM, or advance to 1200 RPM, and nowhere in between.

Thoughts were weakened distributor, so had PK rebuilt that. No change at all. In fact it is harder to get idling now.

Balancing air, checking exhaust richness of injection pump, all is in tune.

Air screws in to raise idle - it either stays at 500 RPM (almost stalling all the time), or jumps to 1200 RPM.

The motor also gets a bit misfirey at idle range, firing on 5 every couple rotations, and has to be "cleaned up" sometimes. Starts dirty when warm too, needs to run a bit before clears up.

Hand throttle is also exaggerated - very hard to move it into position for 1,500 warm up RPMs...it wants to "SKIP" everything between 1000 and 2000RPMs.

Motor runs strong and smooth under load.

Any experienced early 911 folks have an idea what could be wrong here?

Steve Weiner-Rennsport Systems

Hi,

Assuming (thats dangerous!):

1) There are no vacuum leaks since you checked for that.

2) All the bypass air passages in the throttle bodies are perfectly clean.

3) Pump is timed correctly

4) Correlator rod is adjusted properly and you have already completed the CMA procedures.


I would have the pump checked out by someone who is very good at that. Mark @ MFI Werks comes to mind.

dunlopnick

get the pump serviced the car will come alive - just been thru that myself ....

there is most certain a procedure to get everything working in concert ....

ask on early 911s registry - there's a wealth of assistance / guidance ...

wicks

Everything already rebuilt on this car (except the distributor wasn't done so had that done). Which was a waste as it turns out, it just got worse. Car only has 63K miles on clock.

Checking out who did the pump because maybe that wasn't set up right.

Two experts on the engine, can't get idle with all their knowledge.

Just hoping there might be something obvious to someone else when everything is set right but there is no way to get idling speed correct. Like - "oh that's an obscure problem when the edge of the bleep blank is just at the wrong angle from the blank blank and if you turn screw A by just 1 degree in either direction, your idle mix will come back in line."

But I guess that is hoping for sorcery.

Steve Weiner-Rennsport Systems

No sorcery needed here, just a VERY careful attention to all the details and never assuming anything!

When everything is perfect, these cars can run very well indeed.

wicks

Is there a procedure for cleaning the air bypass holes (just to be sure that is eliminated)? Carb cleaner spray? Or maybe that will melt the stacks.

I am thinking if not a bad or out of tune pump or some fuel delivery problem, I must have some throttle closed incorrectness, because the motor does not respond to adjustments of the air bypass screws.

Steve Weiner-Rennsport Systems

These can be cleaned with carb cleaner provided they are not too bad. If they are heavily carboned up, you may need to carefully use mechanical means and carb cleaner to do the job.

Remember,...every part of the system must be in PERFECT condition in order for these to idle properly. This means that the throttle bodies, linkage, pump, injectors, and distributor all have to have no wear in them. Further, there cannot be any vacuum leaks either as thats uncompensated air which leans the fuel mixture.

After that, one follows the CMA procedures as outlined in the factory MFI manual.

JCP911S

Steve is the guru, but I have a '70 2.2S and I found that it was very sensitive to spark plugs, of all things.

I used it as a track car, but when I put track plugs in, they would foul before the car warmed up, so I had to put street plugs in, and then swap plugs out at the track.

Go figure!

rusnak

How/ what did you use to check the mixture? Have you checked the fuel pressure?

Steve Weiner-Rennsport Systems

Excellent suggestion, as usual. All of the MFI-equipped S cars are the same way, and especially with today's fuels.

The factory manuals call for plugs (W265P21) that are far too cold for anything but extended WOT operation or heavy track use. When doing the CMA procedures, I install NGK BP6ES plugs to prevent fouling.

wicks

Set all the throttle stops to "3/4 turn just at where butterflies don't stick shut anymore" method. All the linkages cleaned, regreased, resting without moving the pump or throttles unless actuated. Pump arm back at 114mm. Someone had set it previously to 116mm and it would hold the pump open a little at rest. Pedal down the pump is at full open so that seems fine now. Microswitch reset. Air bypasses cleaned with chopsticks, air and carb spray.

Air flow readings on the snail meter were, before air screw adjustment (this is with air screws all at 2.5 turns out, which I'm still not sure about as also I see people talking about 1.5 turns for 2.4 S motor, and in CMA that way):

1 - 6.1
2 - 5.2
3 - 4.8
4 - 6.0
5 - 7.0
6 - 6.1

Idling (barely) at 500 RPMs.

I then began to withdraw air screws as a test, and idle came up. Something must be better now because before all this work on the throttles nothing would bring up the idle, except for it to rage up over 1,200 all of a sudden.

Air screws opened somewhere between 1.5 - 3 turns depending on the cylinder, to get to all air flows at about 7 and now idling 1000~ RPMs.

Then I tweaked the timing again. The correct timing idle mark is the left notch of the pair of notches, yes? So, this brought the idle up to 1050RPM or so. I then began to close air screws. Closing screws anywhere from a quarter to 3/4 turn, idle came back down to 900~, but the motor gets choppy, when it was smooth when idling at 1050.

Retesting air flow, opening the slightly lower screws and all cylinders close to 6 on the meter. Idle up to 950-1000RPM.

Checking advance timing now - At 6K RPM, vacuum line removed, 35 degree advance mark still does not make it to the mark, it is shy of fan notch by about 3/8-1/2 inch to the right.

When I run the motor up and let it come back to idle, it falls quick to 500 RPM and quickly rebounds to 1,100 RPM, then settles to 950-1000. But doesn’t always settle. So idling smoothly at 1100 RPM after a couple of blips, I begin to close all air screws just 1/8 turn. The idle drops to 950~, but firing becomes poppy and burpy again, like little backfires up the stacks.

I then examine vacuum, clear the vacuum fittings with air (can’t get even a small needle into them!), clip the ends off, refit. Idle climbs about 250RPMs with vacuum removed. Seems to be functioning well, but I have this two stage idling thing now.

So, I am wondering, is the correct way to balance the air flow to adjust the throttle stops or to turn air screws? If throttle stop adjustment is better (and will clean up this choppy burpy stuff under 1000 RPMs), I will return all the air screws to 2.5 or 1.5 turns and instead turn in the throttle stops on the low-flowing cylinders to equalize that way instead?

I have read mixed comments about which method is correct to get a smooth idle. I just can’t work out in my head what it is that causes the engine to burp and sputter when idling below 1,000 and to run completely smooth at 1050 RPMs. In either range, disconnecting vacuum raises the RPM by 250, so I think the advance/retard is working correctly.

wicks

ps, there is oil oozing from the solenoid on back of the inj pump - do i have to replace to get it to stop leaking, and where can i get one of those?

thanks!!!

Steve Weiner-Rennsport Systems

Do you have a copy of the Check-Measure-Adjust publication?

Has someone actually put your distributor into a distributor machine to make sure the advance mechanism is working correctly?

Have you had a qualified professional (Pacific FI or MFI Werks) calibrate your MFI pump?

wicks

Yes it was done by Gus but some years ago.

Test driving today, it is alternating between 650RPM rough idle and 1050 smooth idle. Seems to be completely random. Else running very well with minimal backfiring.

wicks

And the switch test:

Engine at 2K RPMs with hand throttle (which is hard to do because the engine just hunts up and down from 1500 to 3000 randomly):

Push microswitch, after about 3 seconds, the motor slowly drops to 1300 RPM, but does not return to 1500 or do the 1500/1300 alternating thing at all. If I let go of the microswitch, nothing happens - motor stays at 1300 RPM. Blip the throttle a few times, retest. Same thing. Then re-blip throttle, motor races up to 3000RPM without changing the position of the hand throttle. Blip again, it comes back down to 1500. Blip it again and comes back down to idle.

Try the same test just by pressing and releasing the microswitch rather than holding it down - nothing happens, motor stays at 2000.