Howdy,

Working on my 280 SEL 4.5 and came across a problem. I think I have found a solution so I'd like to pass it along. There may also be the "correct" way to do this that some of you will I'm sure educate me about.

The problem is distributor phasing. Correct phasing is where the rotor pole points directly towards the appropriate cylinder pole in the distributor cap WHEN THE IGNITION FIRES. Incorrect phasing is where the rotor pole is not pointing towards the distributor pole when the ignition fires.

I noticed I had a problem when I replaced the distributor cap and rotor. After a few weeks of duty, I removed the distributor and found that there were arc marks on only the leading edge of the distributor poles. This meant that when the ignition system fires, the rotor hadn't rotated far enough towards the appropriate cylinder distributor pole.

I found absolutely no way to adjust this, so I removed the distributor and dissassembled and cleaned the vacuum advance/points plate. I noticed that there is a ball bearing and retainer that helps hold the vacuum advance (or retard) plate down. This was also rotated all the way over to one side, which would be consistent with a phasing problem.

Here's how I solved the problem: I used a moto-tool to grind out the holes in the vacuum advance (retard) unit. This allowed it to be slid over affectively adjusting the phasing. Some of the end of the foot of the vacuum canister had to be removed in order to get it to slide over far enough.

To set it up, I put the distributor foot in a vise. I then set the point gap to book tolerance when fully open. I then put the rotor and distributor cap back on and marked the point on the outside of the distributor body where cyl no 1 distributor pole is located. After removing the distributor cap again, I then adjusted the vacuum canister mount so that the points just open when the rotor is pointing at the mark.

A few more days of driving have confirmed this to work. Now there are firing marks all over the copper distributor pole instead of just one side.

I suspect that there may be a difference in length of the actuator arm for vacuum canisters. This probably accounts for the phasing problem.

I was wondering if there was an adjustment built into the distributor? I have a book by Christopher Jacobs that uses an old distributor cap to adjust phasing. To do this, you cut a hole in the cap so that the rotor and number 1 cylinder pole is visible. With the distributor on a test bench, you use a timing light to view the position of the rotor relative to the distributor pole.

Sholin

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What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.

An ineresting point, But (there's always a "but") the distributor spends very little time in a "static" position. It's always influenced by either the vacuum retard or advance unit on the distributor (early cars with one fitting only have vacuum retard) or the mechanical advance. With static phasing, how can you compensate for those variables? What difference does it make where the marks are on the cap contact? I've never seen this "problem" before.
I'm trying, but I just don't get it. Though I understand that the existence of "IT" is not JUST to be gotten by me. What was the problem you were fixing?
Regards,
Randy D.

Randy,

Yes, both vacuum retard/advance and centrifugal advance/retard affect the phasing adjustment. That's why there is, what, about 30 degrees between the point where the rotor and cap pole first meet versus the point where the rotor and cap pole last meet. I think that this is enough to account for the variation of timing and, yes, ideally one would adjust the phasing on a distributor bench under "normal" advance conditions. But I don't have one of those.

I DO know it was off based on the strike pattern on the cap pole. I made an adjustment based on this and I'm confident it is closer to where it should be.

Of course, if phasing is way off, the gap between the cap pole and rotor will be so large that the arc will find some other more desireable path to ground. I don't think this was happening as I wasn't experiencing any miss. If the phasing is a little off, the larger gap under some advance conditions would cause a weaker spark leading to either poor idle or a weaker top end depending on where it was off.

But then again, it wasn't running poorly before and so the adjustment didn't really fix a problem other than the strike pattern.

Sholin

__________________
What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.

Because the spark jump pattern is determined by the angle
of the point mounting plate. the required phasing is determined by the point dwell/gap. Small gap= early spark, large= late.
This is why the gap spec. should be as close as possible.
This will give you the phasing the factory has confiqured to allow spark rotor jump throughout the advance/retard range.

Crane conversion instructions actually recommend a spare cap w/hole to line up the rotor w/timing light for fine tuning set-up.