I've now replaced a good chunk of the ignition system on the Mercedes—result: much easier starting and far less sputtering off idle. Part of this involved replacing the ignition points which, for those of you without a serious automotive background, are the parts which cause all of the glorious toasty electricity into each spark plug so it can ignite the fuel mixture. Since it isn't very effective to pump power constantly through the active plug, the points allow a field to build up in the coil, and then (when the points contact) force the field to collapse and dump all of the energy through the plug at once.
The thing is that these points are adjustable—they can be pulled closer and further apart and, because they're really fairly springy, essentially results in the points being in contact for a greater or less amount of time, respectively. This is the dwell angle, and is so called because it's the rotation angle through which the points are in contact for a given cylinder.
Now, if a car is equipped with points, its service manual will generally specify the dwell angle. So configuring the points requires setting the point gap to some starter value (just to get the car running), then firing up the car and measuring the angle with a dwell meter. And then when the value isn't right (which, invariably, it isn't) then you get to pull everything apart and open or close the points to try to get closer to the specified value.
The problem is that they Haynes manual that I've got (I know—I'm surprised the dwell angle is even printed in there in the first place) specifies that the dwell angle is 29 degrees. And this is all well and good...except that setting the points to the start distance (12 thousandths) gives me a dwell angle of nearly 39 degrees. Cranking the points open to 19 thousandths gets me a hair under 33 degrees...and anything past that doesn't let the car start. It just sortof sputters and chugs, but doesn't run.
So a question for all you car guys out there: how much tolerance in the dwell angle would you expect? The engine runs great at 33 degrees, so I don't see much of a reason to go further—but I have to wonder if my base timing is off. I don't have a timing light, and so I haven't checked it...but really, could I be five degrees off on the timing, and five degrees off in the dwell, and still have the engine run?
Important update (read this before commenting!): I've disabled comments for this post because oddly, it's a huge spam magnet. If you want to comment here, just go to the main page and comment there, with a note that you're really commenting to the "dwell post." I'll move the comment over manually for you.
I dont know the answer, I would however like to know what the resolve is. My guess is unless someone knows (Justin might know - if like taking advise from someone under 5foot tall(can he even see into the engine bay to know what you're talking about here)).
If you cant figure it out, I'd say run it at 33* or get a timming light.
I thought that's why Justin bought the 2002. So he could stand on it to see into the Mustang's engine bay.
*rimshot*
Thanks, jackasses.
I actually bought the BMW because when I stand in the engine bay, it doesnt appear that the water pump is hitting me in the groin (i think we all remember that picture).
As far as your points. . . UGH. The first thing I thought when I started reading is "why doesnt he just install a distributorless ignition, I am going to do that on the BMW, and he makes more money than God." But, for the sake of aguement, I will assume that God is broke, and it has something to do with the Pope's Muslim comment.
The few times I have had to set dwell, it hanst been at a given angle, but rather with the points wide open, and a gap. I think that with a bit of trig you can do the same. As far as tolerance goes, its more critical as you add more cylinders, as the coil has less time to charge.
If you can, post a pic of the distributor. On the Ford's I have done, there is a hexagonal or octagonal (depending on V6 or V8) trigger which is on the center of the distributor, and then the points are activated by a rocker arm, which at the mounting base is slotted for adjustment. If yours is the same, I dont see why you wouldn't be able to do it the same way.
Justin-
Indeed, the Merc has the standard points configuration—a hexagonal cam (this is an inline 6) driving a lever which contacts a stationary point, the position of which is determined by how far along its slotted groove the set screw is cranked.
The thing is that I haven't been able to find a correlation between the point gap and the dwell angle. It's monotonic, but it sure as hell isn't linear. And as you mention, it's measured as the points are driven completely open with the cam.
And the thing was that when I replaced/adjusted the points on my Dart years and years ago, I thought I was able to get it dead on in the first pass. So clearly, it's achievable with this method—just not guaranteed.
And how's that groin working out for you then?
--Dan
Shorty, post up the pic... the only groin pic I distinctly remember ( there must be a number of them ) is the one of you dressed as Mr. Hulk Hogan - groin poissed at the ready.
...groin poised at ready?
Eww.
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--D