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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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member