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  #1  
Old 11-24-2013, 12:16 PM
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Help!!!! '74 450SL misfires on 3 cyls and cuts out.

I am posting this on 3 different forums to garner the greatest exposure.
The is for a friend's '74 450SL that I have worked on and know fairly well, but not everything.
First some background on the car. The valves and timing have been set to spec. The wires (Beru), cap and rotor (Bosch) are relatively new. The coil (Bosch) appears to have been there a while. The distributor has the old points system.

The car has a loping idle as if she is misfiring. Using my timing light, I found that there is a pause is strobe light pattern on cylinders 1, 2, and 8. The other cylinders didn't miss a beat. We checked un der the cap and there is no corrosion on any of the tips, they all look the same, clean. We did however notice that the rotor itself has a lengthy crack in it. It runs almost the whole length of the rotor, meaning from the center point out to the edge. It's a hairline crack, but noticeable.

Now, this car as I said has the old points ignition system and this is where my knowledge is a little thin. At the base of the rotor shaft, on one side is a metal tab which is secured to body of the distributor that has what appears to have felt on it. This however appears to be very old and the felt part that touches the rotor shaft is all worn down. It looks as if the metal may be touching the shaft. On the opposite side there appears to be what I guess would be the "points". There is a spring load device and clearly there is a "make or break" contact point on it. I also notice near this and a hair away from the shaft, what appears to be a magnetic sensor which I assume senses the position of the rotor as it spins. We noticed that this part of the shaft has evidence of surface rust/grime/dirt.

The other and probably related issue, is that the engine at times just ups and dies. The engine would be idling and suddenly just die. No stumbling, just cuts out, yet it would start right back up and be fine. This also happened yesterday as we pulled into a gas station. It has also happened while underway. We were driving along at about 35 mph and the a fleeting second the engine cutout but fired back up on her own.

Any and all tips would be greatly appreciated.

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  #2  
Old 11-24-2013, 02:14 PM
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start with a rotor. A crack isn't OK.

Regarding the points, first check the dwell at different RPM ranges. If it's not consistent, +/- 3 degrees from like 800rpm to 3,000 rpm, you need the distributor rebuilt. Then set the dwell to the right place and then you set the timing as per the book regarding vacuum & rpm.

If you can't set the dwell with a little trial and error, then you may well need a fresh set of points. Just remember that that any time you mess with the dwell, you have to re-time the ignition system.

If you've not done so, a well place bit of white-out directly on the TDC line makes using a timing light a lot simpler.

Coils can die in many ways. Heat induced wire damage can cause the ignition to cut out. Though usually it stays out for a bit.

-CTH
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Old 11-24-2013, 04:20 PM
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Thanks for the quick reply.

I also got your PM and thank you for the generous offer. In hindsight I wish I had taken pictures to help show what I saw. Help me understand what the parts are in the distributor. Again, all my cars have either been diesel or had electronic ignitions.

That metal tab I mentioned with felt on it, what is that for? Also, would that "dirty" rotor shaft play any part in this?

As for the rotor, in the past when I have replaced rotors, I just gave it a good yank and the rotor came off. When I tried to lift this particular rotor, the whole shaft came up about a 1/4 of an inch and stopped. Since I have never had that happen to me before, I didn't even try to give it a good yank to get the rotor off. Any thoughts on this?

This crack I mentioned on the rotor is in the resin that I guess they pour over the brass to hold it in place. Is the crack in this resin indicative of anything? How does it effect voltage traveling through the metal?

As for using white out, someone had previously marked it as you said.

As for dwell, I assume this requires a dwell meter or can it be done with a basic volt meter?
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Last edited by rickjordan; 11-24-2013 at 04:57 PM.
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Old 11-24-2013, 10:36 PM
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while looking for distributor pictures on google, I found this old thread here...

http://www.peachparts.com/shopforum/vintage-mercedes/121451-4-5-distributor-modifications.html

It has some pictures of various parts.

-CTH
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  #5  
Old 11-25-2013, 12:21 AM
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Thanks. Through some additional research I learned that the metal tab I described that has felt on it, is apparently some type of oiler. I don't understand what it is supposed to lubricate. It touches the base of the rotor shaft (octagon section). I also figured out what I thought was some type of magnetic sensor was really a plastic tab that gets hit by the ridges/high points of the octagon section. This in turn pushes/separates the contact point. I was given instructions on how to measure the spacing of the contact. When I work on the car again, which will be next Sat. I will check it out.

My next question is, is the contact spacing adjustable or does one simply just replace the contact mechanism?

Also, how do you do pull off the rotor. I know it's a simple question, but as I mentioned before, there is play in the shaft. By this I mean, I can lift the rotor shaft up about 1/4 of an inch and I didn't want to yank on it harder to remove just the rotor. Is this play normal?

Thank you again.
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  #6  
Old 11-25-2013, 11:39 AM
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It is not normal for the shaft to pull out more than 1/8" but when I checked a distributor in my basement I can pull it out a bit, 1/8" is my guesstimate... The rotor can spin counterclockwise by hand a bit thanks to the mechanical advance, but the shaft should not wobble nor be "Liftable" any more than this tiny amount. The fact that yours is hints at a possible bad rebuild in the past, or a missing/broken circlip (snap ring) holding the top part of the shaft (that has the mechanical advance) to the bottom portion. Or maybe it just seems like it's lifting more than it is.

You do remove the rotor by just lifting. The point gap is adjustable. That's what is referred to as "Dwell" - it is set by angle on these cars, not by point gap, mainly because it is quite difficult to manually set the gap exactly right. That having been said, it is less of a pain in the arse to adjust by gap than it is to measure by dwell. If you search on the forums, you may be able to come up with results if you do something like "m117 point gap" or "4.5 point gap" for a measurement with a feeler gauge vs dwell angle.

The felt-tipped tab just helps grease stay on the "Octagon" so that the points themselves have a supply of grease. Otherwise, they will wear a little faster and that will throw off the gap / dwell, affect the timing (etc.) - so you would want to keep that lubed. Also, you put a drop or two in the tip of the shaft during each oil change. That keeps the mechanical advance lubed up. However, I bet your mechanical advance is stuck and that is a very bad thing for engine performance and economy. Another thing to search if you would like would be mechanical advance. I have played with the 4.5 distributor and measured timing / curves at different RPMs and have a guide with limited info but with teardown pics in a thread here somewhere!
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  #7  
Old 11-25-2013, 12:27 PM
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As for the advance, this distributor has a vaccuum diaphram on the side of it, which is mechanically connected to the workings of the distributor. Is that what you mean by it having a mechanical advance? I have a vacuum tester, so I assume if I applied vacuum I should see the advance mechanism move, correct? The performance of the car is not the best, nor is the mpg.

When you say grease, do mean literally grease. Looking at the "octagon" or whatever that part's offical name is, is only touched by 2 things. The "lube strip" and a plastic tab which is part of the "points" part. What can I safely spray or lube the mechanical pivot points with?

Also, and again I just want to get the proper names to help understand you and everyone else that has been helpful. When I look up the part "ignition points", it shows a spring loaded device about 2 inches long. At one end is where there is a point where pull it can be pulled apart, but due to the spring, it closes on it's own. Is this what evryone calls the "points"?

How does one adjust the gap? I suppose if i were able to be looking at it I could propbably figure it out, but I don't have access to the car right now.
I will be working on the car again this weekend and it is at my friend's house. So I am going off memory of what it looks like under the dist. cap.

Thanks
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Old 11-25-2013, 12:58 PM
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I just downloaded a generic diagram of a distributor and now I understand what all the parts are. The "octagon" as I called it is a cam follower. So when you say to lube it regularly so that the points don't wear out quick, you mean so that this cam follower doesn't wear down. It appears to be just made of plastic so I can see how striking the high points on the cam would wear it out.

So, what shall I lube this with?

I also saw a diagram on what dwell is and I also see what you mean by it being a PITA to measure. So, if one is adjusting by the points gap, at what point do I measure? Is it with the cam follower on the highpoint (open) or inbetween the ridges (closed)?
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  #9  
Old 11-25-2013, 01:44 PM
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Quote:
Originally Posted by rickjordan View Post
As for the advance, this distributor has a vaccuum diaphram on the side of it, which is mechanically connected to the workings of the distributor. Is that what you mean by it having a mechanical advance?
These cars have vacuum RETARD, not advance. At idle, a vacuum is drawn on the diaphragm and this retards the ignition.

The distributor has mechanical advance. This is achieved by weights that are activated by centrifugal force. They are located below the plates that the points are mounted on.

More here: D-Jet Distributor Removal & InjectionTrigger Points Repair - Mercedes-Benz Forum
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  #10  
Old 11-25-2013, 03:47 PM
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Looks like your distributor needs a full overhaul. Also, the trigger points in the base of the distributor that control the electronic fuel injection are probably worn and/or contaminated with engine oil...a common problem with an aged D jet system. Much has been written on this forum about these problematic trigger points, so do the search on them and you will learn a lot. Or make your life easy and send the bloody thing to Pierre Hedary in Florida. Contact: mbownersforlife@bellsouth.net

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