Hi,

I have a 1972 350SL (4.5) - with M117 engine.

In setting the timing to 5 degree ATDC, the distributor is rotated to the end of the range allowed by the slot and bolt holding down the distributor. Looking from above the distributor body is rotated CCW as far as it will go.


Is this end of range condition an indication of a problem in chain and tensioner?

I know I'll need to replace these one of these years.

Paul

__________________
1983 500 SL (Gone )
1972 350 SL 4.5

Hello Paul,

It is unlikely that the tensioner or the timing chain is worn enough to cause the situation. It most likely was set up wrong by a previous tech. Make sure your vacuum advance cell is good. The distributor can be lifted up after removing the hold down screw and the shaft rotated one tooth clockwise so you can have proper range to adjust your timing. Consider cleaning your trigger points when it is out.

If your engine has over 100,000 miles and has never had a fresh chain installed, it is overdue. Replace the chain tensioner also. Replacing worn chain rails is also good insurance. The lower ones are a major ordeal requiring front timing cover removal. However, replacement of the chain, upper rails, and tensioner will usually keep you out of trouble and will not require taking apart the front of the engine.

Stretched worn timing chains, slapping on old brittle plastic chain rails have destroyed many of these engines. The rails get brittle after years of heat and chemicals from the engine. Eventually the top ones will snap off falling in the chain sprocket, jumping or breaking the chain, damaging valves, cams etc.

If I recall some of the early V-8s like yours may have had the rubber/aluminum chain rails which did not break. However you should make plans to do the work if you are overdue.

You will also need two early (small version) cam oiler kits for the camshaft oiler tubes (very inexpensive) and a couple of valve cover gaskets. Plan on adjusting your valves. I like to check all the spark plug wires and ends with an ohm meter while everthing is off.

Take care,

__________________
Joe Alexander
Blacklick, Ohio
1964 220SE Rally (La Carrera Panamericana someday)
1966 Unimog 404s (Swedish Army)
1969 300SEL 6.3 (sold)
1969 280SL Pagoda
1973 280SEL 4.5
1974 450SLC FIA Rally car (standard trans)
1982 300D turbo (winter driver)
1986 560SEC
1989 Unimog FLU419 (US Army)
1991 300TE (wife)
2002 SLK 32 AMG (350 hp)

Joe, thanks for the advice on the distributor rotation. I'll take it out, clean the trigger points, and rotate one tooth clockwise.

I believe that I still have the original timing chain as the mantainance records do not show it ever being replaced. The car has 125,000 miles on it now and is overdue - although I've never heard any chain slap noises at all on starting.

I bought the chain, tensioner, rails and pins over a year ago, but I've been nervous about installing them. Partly because I do have an early cast iron engine and that the original rails may be sturdier than the replacements.

I plan to adjust my valves and check the alignment between TDC on the cams and on the harmonic balancer, which will tell me how bad it is. Also I can see the condition of the old top rails then. I'll probably install the new chain tensioner in any case since it's better off in the car than in the basement. And also refresh the oiler tubes.

Making a list and checking it twice over the holidays

Paul

__________________
1983 500 SL (Gone )
1972 350 SL 4.5

On those engines ( I have same one in my SL), they should be set at about 30 BTDC with vacuum plugged at 3000 rpm. At that point, the distributor is turned CCW and IS at or near the end of the slot.

I wouldn't move it a tooth!

By all means though check the chain for stretch - The manual gives instructions and I think there is a link on the 116 forum to the CD version. I recently replaced both the chain and tensioner on my engine because stretch was at b8 deg, I think. My rails were just fine and all advice I received said not to replace them with the newer plastic units.

Good Luck

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Paul ;

Those engines have a wierd vacuum setup in the distributor , the vacuum signal is on FULL @ idle and it retards the timing , as you give it throttle the diminishing vacuum signal lets the spring advance the timing...

This is why it is set to 5° ATDC , vacuum connected & engine idling .

Look in the book , it shows this setup and the nipple on the vacuum advance biscuit is on the _retard_ side...

I've seen more than one incorrectly connected so there was NO vacuum signal @ idle .

As Graham mentioned , after setting the timing correctly you should have 30° ~ 32° BTDC @ 3,000 RPM's or more , easy to check with a dynamic timing light with advance dial .

DO NOT run with the vacuum hose disconnected ! this is a bodge fix by folks who don't understand how this wierd system works as it works backwards from most other engines .

Be _gentle_ with the D-Jet trigger points ! use a bit of lint free stiff paper (think business card) between the trigger points until it comes out without any black mark or soot on the card .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Hi All,

The vacuum advance does work- changes the timing about 12 degrees at idle. I have not checked the timing at 3000 RPM.

I did move the distributor a tooth. but found that I could not bring the timing right in that position. With the distributor body rotated clockwise as far as it would go, the timing now read 12 degrees BTDC with vacuum on. So each tooth moved it like 20 degrees - so an 18 tooth gear?? Sounds about right.

Anyway, I moved it back a tooth, and was back where I started. I think that Joe is right that the position is not likely to be due to chain stretch or tensioner. Probably designed like that.

As I read it - the chain goes in a V pattern, with the crankshaft & harmonic balancer at the bottom of the V, the distributor gear midway up the V on the right side, and the cams at the top of the V. Since, the slack is taken out from the left side of the V (passenger side) - then chain stretch would have the greatest offset on the passenger side cam, about half of that offset on the drivers side cam, and 1/4 of the offset on the distributor. I've read that 12 degree stretch (measured on the passenger side cam) is not acceptable, and at the distributor - this would only be a few degrees.

I just wonder why those brilliant German engineers would design the distributor rotation to be at the end of it's adjustment.

__________________
1983 500 SL (Gone )
1972 350 SL 4.5

On my 450SL(1974 model)there is a bolt on the bottom of the distributor which holds the grooved part the hold down screw clamps to.I assume,but could be wrong,this would allow for some "adjustment" due to chain stretch.I would not think the adjustment should be at the extreme of it's range at any time.Loosening this and re-centering would give you more "range" to set the timing.

Yes - I do feel a bolt on the opposite side of the distributor holddown - the same as in your '74 SL.

Perhaps this bolt would allow me to adjust the distributor body independent of the holddown screw. Or perhaps it breaks a vial of acid into the motor.

I'll have to wait to find out - it's getting dark.

Two ways to adjust the distributor & it's timing. Those brilliant German engineers - using both belt and suspenders.

Thanks for the clue!

Paul

__________________
1983 500 SL (Gone )
1972 350 SL 4.5

I have a spare distributor - Just went out and checked it. The bracket on the bottom of the ditributor has a large hole that fits over the shaft. On each side of that , there are slots. Mounting bolts through those slots clamp the bracket to the distributor body. This allows for somthing like 5 or 10 deg of movement ( I didn't measure it - it is FREEZING out there!)

So yes, the bracket could be positioned differently - But the normal position would be in the centre of the slot and then the distributor would be rotated to near the end of the adjusting slot for the clampdown bolt. I know of several owners who all have the distributor clamped there for 30 deg BTDC at 3000rpm. It actually makes set up easier - just turn the distributor all the way then maybe back of an 1/8" before checking timing.

I at first just changed just my chain. But marks on passenger side cam still showed apparent stretch. I put in a new adjuster and stretch was now almost nil. Doesn't make sense to me, but that was my experience.

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

IMO it sounds like your mechanical advance is sticking on the advance position, hence why you can't retard it enough. Try rotating the rotor clockwise (advanced) or counter-clockwise (retarded) and see if it moves. These are often neglected (not given a drop of oil during oil changes) and freeze up.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

I haven't checked this in a while!

The plate that the points sit on in the distributor is not frozen, I check that when I replace points - that's part of the vacuum advance (retard) - Not sure about the mechanical part, tomguy??

anyway

I replaced my timing chain & I checked if I could center the distributor adjustment slot using the mystery bolt that is underneath it on the opposite side of the holddown.

Anyway - with it loose I was unable to budge the distributor, so I guess if I want to do this I would have to remove the distributor and get the the slotted bolts on its underbelly, sooner or later.

Maybe later than sooner

__________________
1983 500 SL (Gone )
1972 350 SL 4.5

That's the vac retard. Seperate from the mechanical advance. I have a thread with escapades on adjusting it...

4.5 Distributor modifications!

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Hi all,

I just installed a new timing chain on my 1972 350 SL 4.5 - M 117.

Before the installation the timing was set to 5 degrees ATDC (vacuum on), with the distributor body rotated CCW as far as it would go.

After the installation, the timing came out as 18 degrees ATDC, and in setting the timing to 5 degrees, I rotated the distributor body CW about 1/8" - so it is no longer at the end of the adjustment as it was before.

The dwell remained the same at 31 degrees.

The 13 degree difference is about the amount of chain stretch I measured before the installation.

So, I'm thinking that the impact of "chain stretch" is experienced mostly at the crankshaft gear, allowing the crank to rotate some, before it pulls the distributor and cams along.

I still have about 5 degrees of apparent chain stretch which I bet will be reduced with the installation of new tensioner - and that this will affect the distributor timing as well.

__________________
1983 500 SL (Gone )
1972 350 SL 4.5

Maybe - but probably not. The tensioner picks up slack at the end so the chain doesn't flap around or jump a tooth on startups and shutoffs. The leading end of the chain pulls and it pulls the same amount whether the tensioner rail is worn and the tensioner is weak or not. Did you replace the rails? If not, they need to be done on the top to fix the additional stretch. But I would not replace them if they're the aluminum rails. Someone somewhere a while back said you can still order the replacement rubber for these as well.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Here's my new theory on chain stretch.

(btw - my 37 year old chain measured 73 & 3/16 " for 196 links)

A single crank sprocket is like a U with the chain link sitting at the bottom of it.

When it turns it tries to lift the chain - and can do so somewhat depending on chain tension.

When the tension is tight it will look like this.

|.....|
.\.O./

When loose (longer or bad tension) it will look like this.

.|....|
..\O../


If really loose (like mine was??) it will look like this

..|O...|
...\... /

So maybe a tight chain will lead to less offset on cams and distributor.

Tomguy - If this new chain stretch theory proves correct and wins the Nobel Peace Prize, I will share it with you. Stockholm in 2010?

__________________
1983 500 SL (Gone )
1972 350 SL 4.5