Help!!! 450 SL misfires on 3 cyls. also cuts out.
 

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 noticeable pause is strobe light pattern on cylinders 1, 2, and 8. The other cylinders didn't miss a beat. We checked under 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 then for a fleeting second the engine cutout but fired back up on her own.

Any and all tips would be greatly appreciated.

Is the rotor redish with a black sealer stripe from near center to near edge? Is the crack on the top of the rotor where the black is? Under the black sealer is a resistor used for ignition noise ( radio static )

The is an oiler, only the felt should touch. No big deal at this point if is isn't working, metal should not touch the shaft.
You are correct on the operation of points ( contact set ) . What you are describing as a sensor is likely the condenser. It would be a metal cylinder about 5/8" diameter 1" long with a single wire leading to the points.

With the key off, pull the cap. Rotate the engine until the points are open fully ( take a look at where the point arm rubs the shaft, turn until the arm is at the top of any of the 8 the high spots )

Measure the gap between the contacts, as a general rule it should be near 0.016" ( match book thickness is the time honored standard ) In reality anything from 0.012 to 0.020 will let the car run.

I'm guessing the point gap is sub 0.005.

The rusty shaft may or may not be an issue, if nothing rubs there don't worry about it.

Lastly, if you hold the rotor and rotate it CW and CCW, it should turn and spring back, this is the mechanical advance system.

The rotor is red and the sealer is actually red as well and it is in the sealer that this crack is evident. Logic tells me the resistor overheated and that created the crack in the sealer. Would this effect the operation of the sparkplug wires? Though that wouldn't explain why only 3 cylinders have firing issues.

I know where the condenser is and it is appears to be new. I looked at the picture of a new point set in the parts section and the part that I was talking about be closely located to the shaft appear to be plastic. If you look at the picture of the part it is a small tab about 1/4 tall, maybe less, and is vertical.

What is the purpose of an "oiler" in this instance?

When I go to measure the point gap and let's say it's our of spec. can it be adjusted or one just buys a new one?

I will be helping my friend again this Sat., so I am definitely taking notes.

Thank you.

Cracked sealer isn't much of an issue. Some rotors ( non MB ) have exposed metal from the center to tip. For the car in question, measure resistance from the center to tip. I _think_ it will be ~ 5,000 Ohms but don't quote me on that . ( 5 K ohms is the same as your meter might show that )





The oiler keeps the distributor cam lubricated, a thin wipe of grease is used at initial install. Not too much as it will fly off and cover the points.

They can be adjusted by backing the mounting screw (s) off a bit and pivoting the point base.

Check the plugs? if 1, 2, & 8 misfire is there black soot on the plug?

Also trigger points could cause misfires in certain cylinders.

Where are the trigger points? I assume that is different than the breaker points. Are they something that can be adjusted or cleaned?

I was able to look up the MB factory manual online and they talk about checking/adjusting the dwell. Without the use of a O-scope, how does one check the dwell? The spec says it should be 30 degs.

97SL320, will adjusting the points affect the dwell angle?

dwell meter.

For an 8 cylinder motor, dwell is 0 - 45 degrees (the portion of the circle representing one cam lobe). Having 0 dwell means the points never close. 45 degree dwell mean they never open.

-cth

Most automotive multimeters will measure dwell. Its another (and more accurate) way to set the points. The feeler gauge will work just fine however

Yes, dwell is directly related to point gap.( and ignition timing, smaller point gap = late ignition timing. )

Dwell was used as a quick check of point gap, you would hook one wire to the neg side of the coil and one to ground. If you set the point gap with a feeler gauge, it will be plenty close.

Thanks,

Since I only have a basic meter, I will just stick with the contact gap.

If I find any corrosion, discoloring, etc on the contact points, is it permissible to clean the points with carb. cleaner, or some fine grit sandpaper?

you could clean them with sandpaper to prove your theory, however they will need to be replaced for sure once this is done. your buddy wasnt sittin in the car with the key on engine off listening to the radio before this started was he?

Well, I wasn't thinking of using aggressive sandpaper, I was thinking 220 grit. What exactly are the contact points made of ? Short of a mild abrasive, how does one clean the contacts.

And no, she wasn't running the radio. The engine was running and it instantly stopped on it's own. I witnessed it once while the car was idling in her garage, and then again while driving. We had just turned into a gas station and it died. My friend immediately started the car and it fired up like a champ. Then about 15 mins later as we were driving along at about 35 mph, the engine for a fleeting second died (dash warning lights on), then caught itself and continued to run.

The contact surfaces are made from tungsten ( pretty hard ). In the old days you could buy a thin "points file" to dress them. Sanding is OK though in the old days some said not to. Have the key off, points closed then put sand paper between the points and scrub back and forth.

Turn engine so the points are at max opening, check gap. Note, points usually wear one contact in a cup shape so actual gap will be more than measured gap. In any event, + 0r - 0.002" is no big deal for the current situation.

Well, I just returned from my friend's house and the points are beyond shot. With the cam follower on the highpoint, the gap between the points was so small, we couldn't slip a piece of paper (Posti-note) through it. Never mind a .016 feeler gauge. Plus there was evidence of blue on the contacts and per the factory manual that is a sign of too hot of an arc due to too small of a gap, which was self evident. We went on ahead and ordered a new set of points.

The cam follower was worn down as well. The "oiler" felt was as I mentioned before is completely dried out and where it meets up with the cam, was all worn away, right down to the metal. So the felt is now in two pieces being held on by the rivet. This naturally lead to the cam being bone dry and wearing out the cam follower.

We checked all the spark plugs and all were nice and clean and consistent in color, though we did find the gap to be .035 when it is supposed to be .028- .032, so adjusted those.

So here is a parts question, where do I find a new oiler, or do I have to make my own using a new piece of felt? We went to a local NAPA auto parts store and they didn't show this oiler to be a replaceable item. I looked at our host's catalog and nothing there as well. Any ideas?