vacuum & timing

On my 72 911T w/ MFI I noticed that when I pulled the vacuum hoses from the MFI that there was a distinct difference in the amount of vacuum from the right side vacuum fitting vs. the left side vacuum fitting. I noticed this because when I removed just the right side vacuum hose from its fitting there was no difference in the idle. However, when I removed the left side vacuum hose the idle increased as per the instruction of my new timing light. I out my finger on the little tube on the right side manifold and could barely feel any vacuum. Then I put my finger on the left side vacuum tube and it seems as though all the vacuum is being create from this side of the MFI> QUESTION: Is this normal or do I have a vacuum leak on the right side of my MFI and could this explain the loud clicking I hear in the area of number 4's injector air stack. Note: Timing is dead on and valves adjusted correctly but the damn thing barely idles at 400 rpm without the help of the manual throttle. Any help would be appreciated.
Phil Deering
72 911T Targa

It sounds like you have a lot more going on that concern over uneven vacuum! A few basic background questions ...

What was the idle speed before you pulled off the vacuum hoses? Where was the timing with the vacuum hose connected to the vacuum retard diaphram? Do you have the owner's manual or factory 'Spec. book' for your car?

The timing is checked on a '72-'73 MFI engine at two points:

1. 900 rpm with the vacuum hose connected to the distributor, which should be 5 degrees After TDC

2. 6000 rpm (VERY BRIEFLY) with the vacuum hose pulled off the distributor, not to exceed 38 degrees Before TDC

Centrifugal Advance Checks with vacuum hose disconnected:

1000 rpm -- should be 5 degrees Before TDC, note that the vacuum retard is 10 degrees, if your vacuum is acceptable, and the retard diaphram is working correctly! The acceptable range is 3.5-6.5 degrees.

At 1500 rpm - acceptable range 4.5-14 degrees

At 2000 rpm - acceptable range 16-24 degrees

At 4000 rpm - acceptable range 24-31 degrees

At 6000 rpm - acceptable range 31-38 degrees

Have you checked the flow (balance) through the individual injection stacks with a Uni-Syn or Synchrometer?

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Warren Hall
1973 911S Targa

The idle speed with all the vacuum hose connected to each side of the MFI and to the dashpot on the distributor is less than 400-barely readable on the cars tach. In order to time the car @ 900 rpm I need to pull the hand throttle. Once I remove the left side vacuum hose and lower the hand throttle to the off position the idle increases to about 1100 rpm and the timing advances to the opposite side of the Z1 mark from the 5^ ATDC at the approximately correct spot(+/- 3^). I have in the past checked and reset the airflow in the air intake of my MFI as per the "blue" MFI book. It was at that time that I notice the distinctive clicking sound in the #4 airstack. I have only the Vol. 1 & 2 factory manual and the blue book I bought from Pacific Fuel Injection.

I'm thinking there may be a vacuum leak in the manifold or gasket which is on the right side and that is causing the weaker vacuum reading. The click in the airstack may be something esle unrelated. I was hoping that some simple solution to increase the idle speed without messing up the whole balance of the MFI. All in all the car runs decently with only a spot where the engines bobbles (+/- 3800)rpm.

Thank You
Phil Deering

Well I went and got me some more accurate test gear and performed some checks and measurements. I checked the timing advances with my Sear timing light w/ advance dial here are the results:
A) With all vacuum lines attached engine idle at 550 rpm; dwell 36 degree; -5 to -7 degrees from the 5 degree ATDC mark.
B) With all the vacuum lines connected and idle at 950 rpm using the hand throttle assistance--5 degrees ATDC.
C) With all vacuum lines removed; engine idle at 950 rpm without hand throttle assistance and the timing mark advanced 10 degrees to 5 degrees BTDC.
D) RPMs stated with all vacuum lines removed: 1500--17 degrees; 2000--26 degrees; 4000--30 degrees; 6000--35 degree.

With my handy vacuum gauge and the engine idling @ 950 rpm I checked the right air stack vacuum and the gauge read about 10-12 in HG. The guage was easy to read and moved only when the clicking noise in the air stack occured. However, on the left side vacuum the gauge just bounced around so violently I was unable to get a reading. I was only able get a reading at 1500 rpm of about 8 in HG. THe left side certainly has a distinctive sunction and it is much stronger than that on the right side. I also tried to check the vacuum with both side connected together with the "tee" and I was unable get a decent reading due to the gauge jumping all over the place.

In the Clymer's manual the were some graphics the depicted some possible causes of various readings one might get from the vacuum gauge. Are you familiar with this and is it accurate/applicable.

I took the car for a ride and all seems to be normal except the idle is just too low and a slight miss @ +/- 3800 rpm. At idle I did notice a slight pop in the exhaust at 950 rpm. I hesitate to change anything for fear of screwing up the whole system.

THANKS AGAIN
Phil D

Well, it looks like your ignition is within spec and working fine!

Have you done a compression test lately? If so, what were the results, and what did cylinder #4 look like?

Have you looked carefully at the valve springs, both inner and outer, intake and exhaust, for cylinder #4 ... to see if any valve springs could be broken?

My suspicion is that you have either a burnt valve or broken valve spring on cylinder #4.

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Warren Hall
1973 911S Targa

Well it looks like I might have to invest in some more tools. But, that the fun of it. Next on the agenda is a compression test. Is there some simple method of checking the valve springs?

If it is a broken outer valve spring, like 99% of the ones I have ever heard about, it will be very easy to depress the valve 1/4" with one or both thumbs at the ignition TDC point where you check clearances. Just jack up the right side (to keep the oil in the sump from running out from under the the right-side exhaust valve cover) of the car 10-12" higher than the left, put a jack stand under that side, rotate the engine to the firing point for cyl. #4, pop the valve covers. You may be able to see the broken outer spring, but the majority of the normal 'seat' pressure of approx. 100-150 lb will be gone, so pushing the valve an initial 1/8" to 1/4" will be VERY easy with a thumb on one or both sides of the valve spring retainer.

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Warren Hall
1973 911S Targa

Well it took some time so here it is.
Readjust exhaust valves after inspection for broken valve springs. I tried to press on the springs, but they seem very firm so I even pushed with a flat screw drive and got no movement. I did not attempt this with too much muscle as I did not want to scar the caps.
After warmup I removed the spark plugs and all but 1 seemed to be fuzzy with carbon deposits-too rich fuel mixture? The one that was not fuzzy (No. 5) was light grey with no sign of carbon deposit. Keep in mind these plugs have only a few hundred miles on them.
Took compression reading as follws:
1-165
2-160
3-150
4-155
5-160
6-155

As I was warming up the engine for the compression test I remove the air box that covers the air stacks. I notice in the #4 air stack a noise that sounds as if the fuel in detonating in the stack. This sound like a loud click. The sound repeats itself every cycle. Is that even possible that the fuel is igniting inside the air stack?
My next adjustment might be to buy hotter plugs and/or adjustment the fuel injector as I still do not have an adequate idle RPM.
Any suggestions.

PHIL
72 911T MFI

Phil,

When facing the engine on your car, is the #4 cylinder on the right side, closest to the fron of the car? If so, I had the same problem on my '73. I had the "pop", and I had jerking. The pop was fixed when I had my gas tank boiled/coated, my hard fuel lines flushed, new fuel filter installed, and a new fuel pump installed. I had a lot of rust/garbage in my lines. Anyway, the pop is now gone, but I still have the jerking after the car warms up(after 5 minutes or so). My car also has a low idle. In fact, it quits sometimes when I depress the clutch to stop at a stoplight. I think this can be fixed when I adjust the idle on the MFI rack. Of course, I need to by the MFI tools just as you do, along with a stack air flow meter. Make a post if you get everything adjusted properly. I'll do the same if I get everything squared away before you do.

Bill


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William Armentrout
1973 911T
2.7 carerra rs specs
www.geocities.com/william_armentrout