Timing retard not working.

[Raymond]

My timing has stopped adjusting according to RPM.  Right after the rebuild, the timing retarded like normal.  Yesterday, performance fell off.  I have the timing set according to the BBB spec for my car at 3 degrees ATDC. I have an early 280.  It sits pretty much on 3 degrees throughout RPM range.

I checked for binding inside the distributor and the push rod moves easily. I removed the vacuum line and sipped on it and I can see it move.  I did that with the engine running and the timing changes.  I ran a toothpick inside the vacuum port of the intake.  I checked the connections on the brake vacuum line and found one loose, but tightening it didn't help.  

What am I missing?  And, I want to drive it about 50 miles each way on a trip tomorrow.  What could be harmed with a trip that long with the timing not retarding?

Ray
'68 280SL 4-spd Coupe

[Cees Klumper]

I don't think driving it without the vacuum changing (retarding OR advancing) is going to harm the engine, only performance will suffer. Are you sure it is the retard function that ceased working, or could it be the advance function? Either way, it seems you checked out the usual suspects ...

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[A Dalton]

<<It sits pretty much on 3 degrees throughout RPM range.>>

 If this is true , than I suspect you have a broken spring jammed up in the weights below the vac plate. The vac is only retard. The mechanical advance should work regardless of the vac being on or off.
 A constant 3 degrees throughout the rpm ranges is indication of mechanical advance failure...see if you can turn the rotor and does it spring back...

[Cees Klumper]

Thanks Arthur - so vacuum is retard, while the centrifugal weights in the distributor take care of the advance. So the advance malfunction is likely caused by the spring problem. I still wonder why the retard does not work on Ray's car.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[A Dalton]

We will know more when he replies.
 I think he is unsure  or mistaken about the rpm range/timing staying at 3 degrees .. but we shall see....[ that is why I recommended the rotor spring test]
 As he has posted that the timing changed when he manually supplied vac , then one would have to assume he has no engine vac coming to the port.
 But that does not explain timing staying at 3 dergees at higher rpms , which should increase [ advance] as rpms increase due to the spring/weight mechanism of the mechanical advance..regardless

[TheEngineer]

Put a tee into the vacuum line and connect a vacuum gauge. Check the advance: It should be 30º at 3000 RPM. That's the important part. The retard just makes it idle much better (at lower RPM) But driving it without advance is no good.

[Raymond]

Thanks gentlemen.  The rotor does spring back when twisted.  I did check that.  I have a pretty decent timing light with an induction sensor for the #1 wire.  Just to be sure I'm not getting stupid in my old age, the BBB says Timing should be 3 deg. ATDC with vacuum, 12-19 deg @ 1500 RPM.  That means After TDC as well correct?  In any case, I am not getting any mechanical change of timing, and it does move farther after TDC when I manualy add vacuum, and returns to 3 deg when I release the suction.

Vacuum gauge connected directly to intake shows no suction at all.  Going now to try to find a blockage.

Ray
'68 280SL 4-spd Coupe

[A Dalton]

If you show no vac at idle, then the blockage is where the vac line hooks to the nipple at the intake, just rear of the throttle valve. There should be full vac at idle to retard the timing.. as soon as you open the throttle, the vac collapses and the dist no longer retards ..that is the BB spec of 12-19 @1500rpm.. that is NOT ATDC, it is now BTDC , meaning advanced.
Do you have any advance at higher RPMs ??  At 3K rpm you should have  about 30 degrees advance from the  mechanical advance..

[Raymond]

Dalton, thanks. That last bit triggered my brain to check the fallback logic.  "When all else fails, go to the begining and start over."

I took the air cleaner off and put my hand over the intake, and got vacuum.  I had the adjusting screw on the throttle turned in too far and the butterfly was not closing completely.  With that, I got the vacuum back.  I now have the timing back to 3 deg ATDC, but even at high RPM, I don't get more than 10 deg. BTDC.  So at least now, I am getting some retard function, but not enough.  

I was thinking about sending the distributor out for overhaul. I guess that is my best bet, but I can limp by for tomorrow.  Other suggestions would be welcome too.

Thanks to all three of you.  You guys rock!



Ray
'68 280SL 4-spd Coupe

[TheEngineer]

Raymond: I took my distributor apart: There are two flyweights under the plate which holds the points. Small springs are attached to move them back in when the RPM drop low. I can imagine, that, when you don't get the 30º advance, these weights are stuck. Dirt, rust, grease? Or maybe you haven't been oiling the wick. It's not that big a deal to take the distributor out and clean the works. Just mark the position of the shaft. Best to turn the engine to No. 1. There is a square protrusion on the shaft and it is offset a very small amount. So it fits into the slot of the drive only one way. Don't force it! There is also a spring in there. Don't loose it. Figure two hours.

[A Dalton]

<<, I got the vacuum back. I now have the timing back to 3 deg ATDC, but even at high RPM, I don't get more than 10 deg. BTDC. So at least now, I am getting some retard function, but not enough.
>>>


...which indicates the vac retard is now working close to spec,[ if you have 3atdc w/vac and 10 w/o vac at idle , the retard is now working ], but the mechanical advance is not working at all, as we suspected from the start.
..so, just take the dist out and take a peek under that plate..the problem should be  evident once you can see in there ...I doubt that the shaft itself is frozen as you said you were able to turn the rotor assembly. Most likely frozen weights or broken spring....

[Raymond]

I rebuilt the engine, so a distributor shouldn't scare me.  I've just never taken one apart before.  I'll give it a shot.

I did drive the car to the Oktoberfest car show in St. Augustine today completing the first 100 miles since I reinstalled the engine.  There were 115 cars in the Peoples Choice show and my car won a "Top Twenty"  Fortunately, no one heard the rough idle and miss. :oops:

Ray
'68 280SL 4-spd Coupe

[TheEngineer]

When it is properly adjusted, it should idle very smooth at 600RPM, even at 500RPM out-of-gear. But 500 is too low. It's just a test. Also, with automatic, the car should drive smoothly on level road at idle: Like maybe 2MPH. With gear shift, it should do it in 3rd gear, smooth(ly). If it doesn't, something is out of adjustment.

[Raymond]

I haven't sorted out the distributor yet.  I tried to disassemble it yesterday and couldn't get the plate out.  There are three screws on the bottom that appear to need a three pointed screw driver.  I cleaned it as best I could and put all the screws back in being mindful of how deep they penetrated.  I installed another new set of points and re-installed and timed it.  

When I first re-started the car, there was some loud clicking coming from inside the distributor, (pretty scary).  But the noise stopped after a couple of seconds and hasn't recurred.  The engine runs smoother than before, but it still only advances 15 degrees from retard.  Can anyone suggest a rebuild shop in the U.S.?

Ray
'68 280SL 4-spd Coupe

[jmela]

I rebuilt my distributor with a kit provided by Dr. Benz. Took a few hours but I took my time and enjoyed it. Messed with the plate advance slightly and it affected acceleration quite a bit.
 
Here's a good step-by-step on the distributor rebuild.
http://glenn-ring.com/010/

[Benz Dr.]

Rebuilding distributors is a lot more difficult than you might think. I bought a very exspensive distributor testing machine to do this with as it takes out all the guess work.
Having said that, 10 years of doing this kind of work still hasn't taught me ALL of the secrets to sucess. Yes, it really can be that tricky. Some of them come back to life easily enough while others can be very frustrating.
Knowing what to do when one won't work right and when one is too worn out to fix is only part of the stuff you need to know about. It takes me all day to do 2 of them depending on the model.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061