How to correctly set timing on 107?? - PeachParts Mercedes-Benz Forum

Bondavi · #1 09-30-2006, 07:32 PM

My car keeps having too far advance when under revs. I changed out the ignition distributor to rule out the springs, but no luck.

Do I set timing at idle WITH or WITHOUT vacuum connected?

Assuming vacuum connected, I set at 16 degrees BTDC, but when I rev up to 2800 rpm it goes to 35-40 degrees, not the spec'd 25 degrees!

What the heck?! What could be the culprit? Vacuum routing? No vacuum leans as far as I see, idle is nice and steady.

Bondavi · #2 09-30-2006, 08:43 PM

I tested spring lash in the ignition distributor by disconnecting vacuum advance, setting to TDC at idle, and at 3000 rpm's I'm seeing 25 BTDC.

So, with vacuum connected, I get 16BTDC and 40 BTDC at 3K rpm. The advance diaphragm under vacuum load is supposed to narrow the gap and allow 25BTDC under 3K rpms?

Assuming this is normal, is this something that the advance diaphragm regulates? There should only be 9 degrees difference in advance between idle and 3000 rpm.

David

Roncallo · #3 09-30-2006, 09:24 PM

Check your timing at idel With Vacume, but remove vacume for checks at 1500 and 3000 RPM. It sounds like everything is perfect from your 2 posts.

John Roncallo

Bondavi · #4 09-30-2006, 11:51 PM

So timing is spot on if it is 16 BTDC connected vacuum at idle, but 25 degrees at 1500-3000 rpms disconnected??

So I'm getting 40 BTDC at 3K rpm vacuum connected, that is correct, no?

page62 · #5 10-01-2006, 12:00 PM

Here's a technique that works on Chevys, Fords...and Mercedes too!

Disconnect your vacuum advance. You want your timing to be about 36 degrees at full advance (the rpm at which it advances no more, probably 3000 rpm). Reconnect your vacuum advance and test drive. If there is pinging, retard the timing a couple of degrees. Repeat as necessary. Don't worry about initial timing.

This procedure assumes you don't have smog testing. (Factory settings are based on emissions requirements.) And it's only for non-computer controlled ignition systems.

Here's a link to a tech paper. Although it's written for Chevys, it works just as well on Mercedes (I wouldn't suggest something I haven't done myself...)

http://www.corvettefaq.com/c3/HowToSetTiming2.doc

Bondavi · #6 10-01-2006, 12:39 PM

You bet I am going to do that AFTER the smog test. The shop nitpicked and said my timing was set at 20 BTDC, too high, should be 16. Whatever.

Got too high NOx readings (they test NOx here), so I adjusted the air/fuel mixture 1/2 turn clockwise and am running the fuel additive to remove carbons from the system. Yes EGR works fine.

Does the mixture screw apply ONLY for idle, or does it apply for the rest of the rpm range except WOT?

David

page62 · #7 10-01-2006, 03:36 PM

The mixture screw is for idle only.

ctaylor738 · #8 10-02-2006, 11:30 PM

You set the mixture at idle, but it affects the basic mixture throughout the RPM range, but the system will add addtional enrichment on acceleration.

If you have a working lambda setup, it probably compensated for your adjustment.

Bondavi · #9 10-03-2006, 12:04 AM

No lambda control on this one.

Roncallo · #10 10-03-2006, 07:26 AM

Quote:

Originally Posted by Bondavi

So timing is spot on if it is 16 BTDC connected vacuum at idle, but 25 degrees at 1500-3000 rpms disconnected??

So I'm getting 40 BTDC at 3K rpm vacuum connected, that is correct, no?

You dident post the year, model (country) of your car so I dont know the correct values. If you give me the year and model preferably the engine code I can get you the specs. But every thing sounds about right.

Proceerure is like this

Set static timing key on engine off as spscified usually about 10°
Disconnect vacume check at 1500 and 3000 RPM usually about 27° to 30°.
Connect advance vacume only and recheck at 1500 and 3000 again there should be a gain of about 8° to 12° (total 35° to 42°). Adjust vacume advance to correct. It I recall that is done with springs and shims.
If I recall the idel vacume chack is done with the retard vacume line connected and advance removed. Change of timing from no lines connected is about 5° to 10° retarded.

John Roncallo

ctaylor738 · #11 10-03-2006, 11:42 AM

Do you have an O2 sensor? If so, what is it hooked up to if you have no lambda?

Could you post your CO, HC, and NOx results from the test?

Thanks,

Bondavi · #12 10-03-2006, 03:48 PM

Oh, sorry about not specifying the year. I thought my signature made it obvious.

The car is a '83 500SL, M117.962 motor. Had frequency valve system (lambda), but system was disconnected and removed. Does have cats (newly replaced ones).

15mph (1402rpm) HC max 118 meas 63 CO max 0.89 meas 0.00 NOx max 1060 meas 1514 (FAIL)

25mph (1361rpm) HC max 92 meas 56 CO max 0.69 meas 0.00 NOx max 890 meas 1042 (FAIL)

What I have done since the test:

Reset distributor timing at idle from 20 degrees to factory spec 14 (16+- 2)
Adjusted fuel mixture screw half a turn CW
Rechecked EGR lines

At this point I am bound to run a fuel system de-carboner (pass smog guaranteed kind of thing), run thru the whole tank, then refill. Drive the car on freeway for 20-30 miles, then straight up to the smog tester.

Could be the idle timing at 20 degrees would have caused hotter than normal conditions causing NOx to go up the roof. I'm hoping the extra 6 degrees will help it cool off.

If this does not pass, I don't know what else would!

ctaylor738 · #13 10-03-2006, 04:28 PM

Because you have new cats, you have some headroom on the COThe problem is that you don't know how much.

Can you get a voltage reading from the O2 sensor at hot idle (after driiving)?
Positive lead to the wire from the sensor, negative to ground. I would try for something around .6-.7v.

BTW - make it look like the wire from the sensor goes somewhere. If the inspector sees the wire dangling, then you flunk, at least in VA.

david s poole · #14 10-09-2006, 12:59 PM

based on your post with the failing #'s the system was running too lean.[you had too much spare o2 which is needed for the formation of nox] one good trick to lower the hc is to back the timing off to about 8 degrees.this can easily be accomplished by disconnecting the vacuum to the distributor and plugging it.half a turn clockwise with the screw is too severe 1/8th turn is more like it.because of age you ought to consider rebuilding the intake manifold.all the rubber is petrified and subject to cracking.start with all the injector seals and cups and the air hoses fitted to them.good luck.

Roncallo · #15 10-09-2006, 05:49 PM

This attachment should help yo with the timing.

Check EGR valve and tube is not only working but is clear. Remove EGR valve and make sure the port from exhaust to valve is clear. Apply vacume to valve and make sure you can blow through it.

John Roncallo

Quote:

Originally Posted by Bondavi

Oh, sorry about not specifying the year. I thought my signature made it obvious.

The car is a '83 500SL, M117.962 motor. Had frequency valve system (lambda), but system was disconnected and removed. Does have cats (newly replaced ones).

15mph (1402rpm) HC max 118 meas 63 CO max 0.89 meas 0.00 NOx max 1060 meas 1514 (FAIL)

25mph (1361rpm) HC max 92 meas 56 CO max 0.69 meas 0.00 NOx max 890 meas 1042 (FAIL)

What I have done since the test:

Reset distributor timing at idle from 20 degrees to factory spec 14 (16+- 2)
Adjusted fuel mixture screw half a turn CW
Rechecked EGR lines

At this point I am bound to run a fuel system de-carboner (pass smog guaranteed kind of thing), run thru the whole tank, then refill. Drive the car on freeway for 20-30 miles, then straight up to the smog tester.

Could be the idle timing at 20 degrees would have caused hotter than normal conditions causing NOx to go up the roof. I'm hoping the extra 6 degrees will help it cool off.

If this does not pass, I don't know what else would!