2.2 MFI -- Engine surges at idle. Any help?

[Gary R]

Hi,

I am currently working on my 1970 2.2 liter 911E. I recently had the engine rebuilt by a mechanic that was recommended to me. However, when I got the car back it would not idle under 1100 RPMs. I began doing some research on the MFI system and decided to try to figure out the problem. What I found was that the Microswitch was not set at all. So, I tested the microswitch with a test light and the light goes on and off as the book says it should. So I then set the set screw in 3/4 turn past the point where the switch clicks. I also found that all the air correction screws had been set open only 1/2 to 3/4 turn rather than closer to the 5/2 turns the manuals call for as a starting point. (I believe the mechanic did that to get the engine to idle down to approx 1100 rpm with the micro switch effectlively by-passed. After making the changes to the microswitch and air correction screws, the RPMs now pulse (between 900 RPM and about 1500 RPM) with the hand throttle closed and at idle.

Because of the pulsing I am not able to test the RPM Transducer (Speed Switch) with a test light to see if it goes on at 1500 RPMs and off at 1300 RPMs.

My thoughts are that the RPM transducer may need to be replaced. Is anyone familiar with this condition and may be able to help shed some light on this for me?

Gary

[glaverbel911]

sounds more like an ignition distributor problem than MFI,, perhaps a spring fell off one one of the weights etc.

Robert

[edmayo]

Since the engine was just apart more than likely either the linkage rods to the throttle bodies were reinstalled incorrectly and/or the ignition timing is off. There are a whole series of tests to do that would take longer to explain than I have space here, but start by disconnecting the main rods that go from the cross bar to each throttle body, then disconnect the long rod that comes from the bell crank by # 3 cylinder to the cross bar. Now you have removed all outside influence except the pump control rod. Now see what your idle is doing, if it has calmed down check your timing with a timing light, should be about 5 degrees advanced at 900 -1,000 RPM. Make sure you are using the Z-1 mark on the pulley, real easy to use the wrong one.

[Gary R]

Good Morning Robert & Ed

So far I cleaned the connections for the Microswitch, RPM transducer and the Stop Switch. I then removed the distributor cap & rotor and check the distributor. The springs were ok. While I was in there I reset the gap for the points. In most of my books it says to set the gap to 0.4mm, and that was the gap it was set at however in my book titled 101 Projects for Your Porsche 911, it says the point gap for 1970 models (thru 1976) should be 0.012. I decided to change the gap to 0.012 (since before the surge at idle the dwell was not at 38 degrees) and put it back together. I started the car and no surge. The car was idling cold at 850 RPMs. So I checked the dwell and with the new gap setting it was @ 37 degrees. I looked at the timing at idle and it appears to be off. It was late and time to come in. So I stopped there.

Today I will start the car and bring it up to normal driving temperature. If there is no surging, I will reset the timing. In the book it says to static set the idle timing, by turing the engine to TDC, putting a test liight across the points and turn the engine back slightly towards cylinder #5 and then back towards #1 until the light comes on, etc. I have never done it that way. Is it better to do it that way or just use that timing light for the idle, as well as the 6000 RPM setting?


After that I will continue to follow the Check Measure and Adjust manual.

Thank you very much for both of your inputs.

Sincerely,


Gary Rebar

1970 911E Targa (vehicle 733 of the 933 made of this model in 1970). I have owned this car since Spring of 1980.

[edmayo]

The static method you're describing was the method for timing the 356 engine, and would also be used for basic setting when building an engine to get it close enough to start. The 911 timing specs are assuming using a timing light. Check both at idle and at 6,000 RPM. I don't have my spec book here, but I think yours calls for about 32 at 6,000 and 5 to 7 at idle at 950 RPM. As I mentioned before, be sure you are using the Z-1 mark on the pulley for idle, the advance marks will be about an inch and a half to the right, 30 and 35 degree marks.

[Gary R]

Thanks Ed,

I started the car yesterday and let it warm up to normal temperature. Three things:

1. It would not idle lower than around 1100 RPMs. Any idea on why I can't get it to idle lower? Remember the day before it idled around 850 cold.

2. Is there a special 13mm wrench to loosen the distributor clamp bolt? I cant seem to find an angle that works using my standard Craftsman 13 mm combination wrench. I tried a 13mm crow foot, but I am concerned that the open end might round the corners of the bolt. So I didn't use that.

3. I noticed slight intermittant "burps" in a couple of the velocity stacks. I can hear them at idle as well as during loads (when drvivng the car). They are soft, not hard or loud.


Thanks again

Gary

[edmayo]

As I said earlier, this could get involved, I don't know, nor do you, what linkage rod settings may or may not have been changed during the rebuild. When everything is set correctly the idle speed is adjusted with the air by-pass screws at the base of each air stack, BUT that is assuming the linkage rods aren't holding the throttle plates off their stops. It is correct that the engine idle speed will pick up as the engine reaches its operating temperature, but you should be able to idle it to about 950- 1,000 rpm via the air by-pass screws. Also the distributor clamp is loosened for timing by loosening the 10 mm pinch bolt, not the 13mm nut. The 13 mm nut is for removing the distributor and the clamp.You will need to remove all linkage influence from the throttle bodies to determine if the plates are being held open, or if it really is just a by-pass adjustment. Pop off the ball socket ends from; each main link rod from cross bar to throttle body, the pull rod from bellcrank (behind #3 cylinder) to cross bar, and back off the screw that bears on the micro switch. Do all this after the engine is warmed up so that it will restart easily since you have now removed all throttle control. Does it still idle at 1100 rpm, if so check and adjust timing if incorrect. Now, while engine is running pop off each individual rod to each throttle ONE at a time to see if any of these are holding the throttle plate open, (an open 10mm wrench works great) if they are the idle speed will drop since the throttle plate can now close like it should. If so you will need to adjust that rod length so that it will pop back on the ball end without any pretension. Now with only the correctly adjusted throttle body links on you should be able to adjust the air by-pass screws and the timing to get your proper idle speed (this is where you'll need a synchrometer to balance the air stacks for equal air flow). As you start reinstalling the rest of the linkage every rod should be adjusted so as to not influence the idle speed anymore. The one rod I haven't mentioned is the pump control rod (from the injection pump to the cross bar) It MUST be 114 mm in length from center to center of the ball sockets. What all this is saying is that the two starting points MUST be that the pump rod length is correct, and the throttle plates MUST be against their stops. EVERYTHING else is adjusted accordingly based on those two criteria. Now all this is also assuming that you don't just have worn throttle plate shafts letting air past them. If so the only fix for that is to send throttle bodies off for rebuild. Sorry for the length of this, but there are so many possibilities here.

[Gary R]

Good Morning Ed,

Sorry it has been a while since I posted a reponse, but I followed your advise with the rods and it made no difference with the RPMs at idle. I then reset all the idle set screws with all the linkage disconnected. I did this by backing out the idle screws until the throttle valves slighty stuck in the closed position and then turned the idle screws back down just until the throttle valve no longer would stick. I then turned them in 3/4 turn more. The idle was still too high. I redid the exact same procedure only using a 1/2 turn this time. Still at 1100 RPM. I readjusted the throttle valve linkage to make sure there is no pretension, did the same with the push rods. The link to the pump is 114.0mm exactly. I noticed that I am getting "ticking" sound caused by the idle stops "flapping" against the idle set screws. My thought here is that my bushings are worn and I probably need to have my throttle body rebuilt. If you concur, any suggestions as to who to use?

Also I checked my timing at idle (before it is totally warmed up it will idle at around 925 RPMs the timing is greater than 5 degrees, it is closer to 9.5 degrees. After it warmed up I checked the 6000 RPM timing and it was close, but not 30 degrees. I set it to 30 degrees, but the idle (at 1100 RPMs now that it is warmed up) is still showing around 10 degrees. If I change the idle timing to 5 degrees at 1100 RPM the 6000 RPM timing is lower than 30 degrees. I am thinking that even though I am not able to check the timing at 950 RPMs with the engine warm, this may indicate that I need to have the distributor recurved. Any thoughts here?

Thanks again for all your time. I hope sometime I may be of help to you as well.

Sincerely,

Gary

[Cornpanzer]

This is the same problem that I have with my 2.2S. At idle, especially in cool crisp days where the is running a bit leaner due to air density, I get a surging idle.

As you have found, my butterflys flutter due to wear and this is causing the idle fluctuation. I keep saying that I will get them rebuilt every winter, but the car runs so perfect otherwise, that I am afraid to mess with it

On another note, if your transducer is in fact not working correctly, I can repair this for you. Let me know...

[Gary R]

What does it run to have my transducer rebuilt?

Gary