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  #1  
Old 08-04-2012, 10:36 PM
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testing distributor vacuum advance

I am continuing to sort out a friend's '74 450SL. The car has an issue with accelerating. When you lean on the gas, she does not want to go and you start hearing popping. This is to me is misfiring. The timing is set to spec. My thought was that there is a vacuum issue with the distributor. 2 things I found. I pulled the vacuum line off the vacuum advance, rev'd the engine to about 1100, but did not feel anything on my finger which I place over the open line. I turned the engine off and using a Mighty-vac tester, that particular line seemed fine. Though I sure thought I would have felt some vacuum when I rev'd the motor. Since I had the line off, I decided to use the tester directly onto the vacuum advance itself. I pulled the distributor cap off, applied vacuum and had expected to see the armature connected to the diaphram, to move. However, I got nothing except as the vacuum pressure dropped, towards the end of the vacuum loss, I can hear like a girggle/burp from the diaphram. If it helps, the most vacuum I could apply was 15 in. Hg., no matter how many times or fast I pumped. Again, it would quickly lose vacuum and "burp" at about 2-4 in Hg.

So, one, should I have seen some movement with vacuum applied. Two, is this "burp" I am hearing a sign of a torn diaphram.

The vacuum "trial" for the adavance is rather simple. It starts at the base of the "throttle body" for the lack of a better term. It's a very small hole, just below the throttle plate. This line runs over to the side of the engine compartment, where it connects to a valve of sorts. There is an electrical connection here as well as the next leg of the system, which is a vacuum line that goes directly to the vaccuum advance. This valve seems to be a swtich over valve.

I am at wits end trying to sort out this acceleration issue. Am I chasing my tail with this vacuum issue.

Thanks in advance for any input.

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Old 08-05-2012, 09:51 AM
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in the old days to check the advance was to connect a line to the vac adv nipple, suck on it and see if it would hold your tongue tightly, and if it didn't the diaphragm was bad

with a timing light on it, will it advance to spec when revved ??

sometimes the distributor plate will lock up and you can check that by using a screwdriver and prying on the plate where the arm of the vac adv hooks unto





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Old 08-05-2012, 12:51 PM
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Just a couple of thoughts. First, a vacuum advance will not cause a misfire unless the advance is stuck in the advanced position. You say the timing is correct. That is done at idle usually with no vacuum applied, so I don't think that is the problem. For example if the base timing is 0 deg and it is set at 0 deg then the car will run fine without the vacuum advance but use a lot more fuel at highway speeds.
Second, I believe that engine has electronic fuel injection. If it does, there are i believe 4 sets of points below the advance plate. These points are used for fuel injection timing. If they have a problem you will have poor performance.
Does this engine have ignition points or electronic ignition?
I may be all wrong about that car. It has been a long time since I have worked on one that old but the part about advance timing still stands and applies to all cars.

Paul
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Old 08-05-2012, 01:42 PM
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The car does have electronic fuel injection. It also has ignition points. Per the factory service manual, the timing is set with the vacuum line connected and set to 5 deg ATDC.

I do believe the timing advances when rev'd. I have been working this and other issues for a long while, so I have lost track. I will hook up a light and check to see if and how much it advances.

I have read about the injection points in the distributor, but haven't tried messing with them. Personally by the time I started messing with cars, it was the mid to late 80's and everything was electronic ignition. . So I am learning as I go. Hence my hesitation to start taking the distributor apart.

Teezer, as I mentioned I applied vacuum, but it quickly disapated. So I can only assume that using the "old days" method would yeild no tongue sticking.
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Old 08-05-2012, 02:02 PM
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Well I have only worked on 1 of these cars about 10 years ago. I am just trying to remember what I can and passing that on to you.
The vacuum advance should hold vacuum but still I don't think that will cause the lack of power. When you push of the go peddle vacuum drops so you will have no vacuum advance or retard. In other words timing will be at base or advanced some by the mechanical advance.
Another thing I remember. Under the dash on the right side there is a fuel computer that has a round black knob on it. The knob will adjust the fuel mixture. I don't know how much but I remember adjusting it for a smooth idle after I got all the other problems taken care of. It could be running a little lean or rich and causing the problem.

Paul
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Old 08-05-2012, 05:29 PM
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lack of advance will normally cause the engine to bog on acceleration

i believe these dist have some mechanical advance plus vacuum advance
a stuck dist plate will limit the mechanical advance too

check your specs ~~~ initial setting and full advance
then put a timing light on it, check the timing at idle and rev it to see what the full advance is, which normally full in at 3000 rpms

does sound like your vacuum advance is bad



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