1972 450SL distributor's centrifical advance - PeachParts Mercedes-Benz Forum

erubin · #1 08-15-2003, 05:13 AM

I replaced my archaic point, condensor, ignition module and ballast resistors system with a drop in Pertronix (hall effect) ignition. Big improvement (car no longer shutters under hard high RPM acceleration) and only costs $69 and 1/2 hour. Looking forward to improved MPG as well (I hope).

My original stock distributor end play is fine but I have a concern.

The centrigical advace spring does not feel like it pulls the rotor completely to the end. When i rotate the rotor by hand and then release it the centrifical advance mechanism does not snap the rotor all the way back. I estimate that there is ~3 degrees of rotor rotation left that the advance mechanism does not pull back to. Does this mean my centrifical advance is worn (springs?)? Should it pull completely back or is a little wear here not worth fixing? Can new springs be bought and is it worth pulling out the distributor to improve this?

Elliot

ctaylor738 · #2 08-15-2003, 10:51 AM

The real test isn't when you are diddling it by hand, but what it does when the engine is running. The manual has the specs for advance at different RPM and you should make sure that it is hitting those advance values, that it is steady, and that it is returning to the proper setting at idle.

The other thing to check is slow cranking caused by too much advance on a hot start. That means that the springs are not pulling the counterweights back to their proper position when the engine is shut off.

erubin · #3 08-15-2003, 02:28 PM

Thanks ctaylor738,

I will dig into the service manual and perform the test you mentioned relating RPM to timing and smooth operation of the advance.

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My engine when hot takes a little more time with the starter than when the engine is cold. How do I measure if i have too much advance while hot engine is cranking? There would only be a few seconds with the timing light before the engine started. It would not be enough time to see with the timing light.

Are the springs available/serviceable if i determine that the centrifical advance is not smooth or accurate across the RPM range?

Elliot

afmcorp · #4 08-15-2003, 02:59 PM

hello elliot

chuck is right it's near impossible to gage a distributor in your hand.

you need to stick a timing light on it rev the engine to what ever the rpm is that you get X amount of advancement then release the rpm back to idle and then watch were the timing mark is sitting. i'm quite sure there is some backlash that's built in the vacuum advance mechanism. so if it returns to X degrees then it should be just fine.

now i have a question for you.

i just spent time replacing quite a bit of primary ignition wiring and the resistors. now the 72 runs just fine. I want to keep the original system for general principles but I'd like to put a system like you did for general driving. (my 350sl rarely comes out of the garage) so can you tell me where you got your kit?

tks much
craig

ctaylor738 · #5 08-15-2003, 03:02 PM

This is literally slow cranking, not taking longer to crank. It sounds like you have an almost dead battery.

It occurs because the centrifigal advance is sticking (advanced), and you get pre-detonation against the starter. If it ain't doing this, don't worry about it.

If it is, you should be able to have someone crank the car and get a look at the timing if it turns over a couple of times.

psfred · #6 08-15-2003, 08:03 PM

Chuck:

Any pointers on disassembly? I've not got that part in the manuals I have and don't want to end up with a non-driveable car on short notice!

I assume (probably foolishly) that there is a roll pin in the gear on the bottom I need to drive out to take the distributor apart. I know the advance weights aren't working right and need to fix it -- 15 mpg is killing me driving to work and back (the higher milage cars are all sick at the moment).

Peter

erubin · #7 08-17-2003, 11:19 PM

Craig (afmcorp),

The ignitor--model 1885--can be found for $70 at this link.

http://www.vintageperformance.com/retrorockets/mercedes.htm.

This add on retains a stock look and is obvious only by the two wires that go from the distributor to the coil instead of one as is the case when stock. Only draw back i experienced is that the distributor seal-off grommet that came with the kit is too small and must be sealed off with RTV or other means. No longer have to worry about worn cam lob, point float and dwell or whatever was giving me less than optimal performance. Huge difference in performance as something was causing the car to shutter whenever i hit the acceleration hard with the breaker point system in place. Alot cheaper than replacing the primitive transistorized ignition module as well when it fails.

Chuck,
My starter cranks a little longer (a few more seconds) with a hot engine but it is not spinning any slower. It's not a big deal but would be nice if the car started right up when hot as quickly as when at room temp. One day i'll look into this.

Still need to perform timing check at different RPMs to verify good operation of the centrifical advance.

Elliot

afmcorp · #8 08-18-2003, 01:06 PM

Elliot

thanks much on the info. i'm going to buy one of these puppies today.

took the 72 out around the block (in 2 years since i've owned it) for the first time sunday night. spent several hours just buffing and waxing on sunday. sure was fun!!

thanks
craig

PS i had to replace a bunch of primary wires due to age. the insulation was cracked or missing. this included the + wire to the distributor. what i did was make a hole thru the existing rubber piece. i used the old piece to seal the distributor just like stock. i also used a sealed male/female connection in this wire just in case i ever needed to pull the coil again.

ctaylor738 · #9 08-18-2003, 08:32 PM

The most common problems in hot start are the fuel accumulator not holding pressure, or the fuel injectors are leaking down. Replacing the injectors cured a long crank when hot in my 380SL.

erubin · #10 08-19-2003, 04:40 PM

Fuel injectors cost $106 each on Fastlane. I definetely don't want to replace them unless I'm sure they are bad. Testing them by running the engine with one injector at a time removed from the maifold to observe leak and spray pattern is scarry. The engine would be running while atomized gasoline is more or less directed into a large see-through container

Maybe I could put a temporary "T" on the fuel line rail and attach a pressure gauge. This way I would at least know how long and how high pressure is maintained after shutting downm the engine. This should give me a quick overview whether or not my system is leaking down. Then i'd need a plan for finding the bad apple.

psfred · #11 08-19-2003, 07:43 PM

Don't run with injectors pulled loose, the engine will run VERY fast with lots of fuel spraying out -- fuel delivery is controlled by manifold pressure.

There is a tester (or used to be) for doing this, but good luck finding one that works!

Peter

afmcorp · #12 08-19-2003, 10:41 PM

elliot

i just did that on the 72.

1. turning the key on will only make pressure for a brief moment. as soon as the relay fails to read the engine turning over or running the pump is shut down. pressure goes to zero

2. during the cranking the relay senses the engine turning over and fires the injectors and a signal to the pump to continue.

3. obviously once the engine starts the pressure continues. mine had 30psi in the rail during the engine running.

4. almost immediately when the engine stops the pressure goes to zero.

not sure if 30 psi is correct cause i've read 2 different pressures in 2 books. but it runs just fine

psfred · #13 08-20-2003, 07:46 PM

28 psi is the spec, so 30 is fine. However, while the pressure regulator is working to control the pressure, the damper is shot (the other canister in front of the pressure regulator in the return line). It should hold pressure overnight -- pressure drop can make for hard starts cold, may take a bit for the pressure to come up.

I need to test mine before winter -- also need to check the cold start system.

Peter