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1972 350SL 4.5, car #917
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Discussion Starter #1
So my Haynes manual arrived and I'm just full of questions now. For example - I had to adjust my idle up to deal with vapor lock but I did not adjust the CO2 setting. The CO2 adjustment is essentially mixture, right? Should I baseline or tweak that setting? I'm not hearing detonation so I'm not lean at this point...
 

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1972 350SL 4.5, car #917
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Discussion Starter #2
timing advance question

Hi guys - on an early car that is otherwise stock is there a specific timing advance these cars like (if other than factory)?
 

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It Is What It Is, Dude
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.


Do some advance searches of threads started by MBGraham. He's posted a ton about the D-Jet in general, it's vapor lock proclivity, and the myriad of related issues. Got to be something about initial timing in there somewhere.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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tdskip,
As Dave said, there is a ton of stuff here on DJets and luckily Proctologist summarized a lot of it and it is in the stickies.

Yes, there is a sweet spot for the timing. The factory figures are aimed at reducing emissions at idle. But for best performance use the Euro settings in the manual. Based on advice from an 'expert' (?), I set my car to idle at 700-800rpm and timing at TDC, then rev up to 3000rpm and see how much advance you get. It should be in the 27-30 BTDC range. If not, tweak it a bit. The car runs well there. At this point, the distributor will be turned CCW almost to end of the adjustment slot. Another way, is to hook up a manifold vacuum gauge, set idle at 700-800rpm, then advance timing until you see maximum vacuum. Then check advance at 3000rpm. It should be close to being right.

In adjusting mixture, the figures MB specifies are %CO (not CO2). The figures are in the stickies. There are two adjustments - one for idle ( a screw on ECU) and for load (actuated as soon as pedal moves even slightly) the adjustment is on the MPS. In order to make these adjustments, you need a good quality exhaust analyzer. On my car I have a permanently installed wideband AFR meter that uses an O2 sensor. Without an analyzer, you can't really get the mixture right.

Hope you found the manual in the stickies. Haynes, I hear, is not too good and Chilton (which I have) probably worse.
 

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1972 350SL 4.5, car #917
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Discussion Starter #5
Yes, there is a sweet spot for the timing. The factory figures are aimed at reducing emissions at idle. But for best performance use the Euro settings in the manual. Based on advice from an 'expert' (?), I set my car to idle at 700-800rpm and timing at TDC, then rev up to 3000rpm and see how much advance you get. It should be in the 27-30 BTDC range. If not, tweak it a bit. The car runs well there. At this point, the distributor will be turned CCW almost to end of the adjustment slot. Another way, is to hook up a manifold vacuum gauge, set idle at 700-800rpm, then advance timing until you see maximum vacuum. Then check advance at 3000rpm. It should be close to being right.
Well holy cow - I think I found the sweet spot. The distributor was at the full counter-clockwise adjustment, in other words way-way-way off the sweet spot. This is especially annoying because I drove it like as I paid for a PPI and once-over before my trip and assumed that basic stuff like this would have been checked. Grrrr. Lesson learned to give the shop a punch list of items rather than assuming anything.

In any case she is a different car now - thanks.
 

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Well holy cow - I think I found the sweet spot. The distributor was at the full counter-clockwise adjustment, in other words way-way-way off the sweet spot. This is especially annoying because I drove it like as I paid for a PPI and once-over before my trip and assumed that basic stuff like this would have been checked. Grrrr. Lesson learned to give the shop a punch list of items rather than assuming anything.

In any case she is a different car now - thanks.
Perhaps your mechanic was using the Haynes manual. :)

Proctologist is one of the few members here who I've spoke to on the phone, and I believe we both agreed that these cars can get some serious pep at the low RPMs by advancing the timing, but the higher RPM power seems to suffer a bit. I am not sure why this is, but I have taken the conservative approach of setting the timing at the factory spec. There are a few reasons behind that:
- Spec is spec for some reason. I may not know exactly why, but there sure is some reason.
- I want my car to perform really well a high RPMs. If I had to chose an RPM where my car was running perfectly knowing other RPM bands would potentially be sacrificed, trust me, it would be high RPMs. I know I think MBGraham tuned his to 30 BTDC at 3000 RPMs, but I don't know if our low compression brothers were designed to run that far advanced. It might require you run higher octane fuel to limit pre-ignition and (power killing) knock at those higher RPMs. I run regular. I am not sure the timing advance works the same way on our North American cars as it did on the Euro 3.5. (When was the Euro 4.5 high compression engine introduced? Was it after 1973 when the D-jet changed a bit?)
- I think the exhaust is cleanest with the late timing. I get a little more stink in the exhaust when it is more advanced (or at least that is my perception). I have heard that later timing will make the engine run hotter. If that is from hotter ignition, then the fuel will burn more completely at higher temps. I am not sure this is true, but it might be possible.

Here is where I messed with mine, and settled into this "philosophy".
 

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Discussion Starter #7
Hi Fonzi - hope you had a good weekend. One thing the MG and TRs have taught me is that when our cars get this old factory timing is more of a suggestion than hard rule. I've had some cars be in the sweet spot at factory spec, others take way more advance to be happy. Sticking close to spec certainly is a good and prudent way to go.

I haven't really gotten on her to see top end power, partially bevause she is so much stronger down low.

The PPI was done my a factory trained guy, so he was either lazy or just sloppy. Kinda ubcool in either case. Once nore may I say GRRRR...
 

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Hi Fonzi - hope you had a good weekend. One thing the MG and TRs have taught me is that when our cars get this old factory timing is more of a suggestion than hard rule. I've had some cars be in the sweet spot at factory spec, others take way more advance to be happy. Sticking close to spec certainly is a good and prudent way to go.
To be at factory spec, the rest of the engine would also need to be as new. For example, cam timing. I think my car is at about 27btdc. My cam timing marks are off a bit. At 40+ yrs old, this is no racing machine.
 

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Discussion Starter #9 (Edited)
Too add to what Graham said I'd just add that advance to pinging and then backling off a touch is a low tech but effective approach that is commonly used. Drives my inner geek crazy but it works.

Fonzi - hope you got the PM saying Thanks!
 

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It took me a long time to get through typing this. I started long ago. You'll see that I try to understand things that I might never fully understand. My inner geek is strong.


To be at factory spec, the rest of the engine would also need to be as new. For example, cam timing. I think my car is at about 27btdc. My cam timing marks are off a bit. At 40+ yrs old, this is no racing machine.
When the chain stretches the cams will lag behind. Of course it is possible that various things like offset woodruff keys could cause the Cams to be advanced with respect to the crank. But in most cases, I think that to match the cams, as the chain stretches and gears wear, more retarded spark timing (with respect to the crank where timing is measured) would make more sense. The 27 btdc is more advanced than book spec by any measure in the below chart (expect for Euro/J spec & m116 70/71 specs). So, while I'm not trying to get into a pissing match with MBGraham (who clearly has been working on these much longer than me) I don't understand the logic of a newer engine getting more advanced timing, at least not without offset wooddruff keys to advance the cams.

EDIT: [ Also, as points wear, they also cause a later spark. The spark signal is sent when the tweezers in the points open up, which is later and later as the rider wears down. So the spark timing will naturally get later and later as points wear. So maybe this is a reason that advanced timing would be good, giving you longer life on your timing before you have to reset it. Pertronix would fix this though.]




I set my 73 at spec of what seems ridicuolously retarded in comparison, 5 ATDC at idle. I also check to make sure the vacuum retard at idle is the correct 10-14 degrees, meaning the idle timing goes up to about 5-9 BTDC when I remove the vacuum from the dizzy. Or as the chart redundantly says 7 BTDC without vacuum. It is pretty difficult to set the timing at multiple values, but if setting it at 1500 (with vacuum) it should be 10-15 BTDC (pretty big advance from 5 ATDC at idle 500-7050 rpm), and 18-22 BTDC at 3000 RPM (not nearly as big of an advance over those extra 1500 RPMs).

Vacuum should have little-to-no affect at 3000 RPM. (Interesting to me since 3000 RPM with no load is hardly WOT).

So it looks to me like it would be pretty tough to set the timing at different RPMs. I think you just have to pick one, and make sure you like how it runs at that one.

If you hardly ever let your early 4.5L 107 hit the high revs, then I agree that you'll like how it runs at the lower RPMs with more advanced timing. You'll spin a lot more tire at the stop signs with advanced timing. But if you like to feel gobs and gobs of power running at 80 MPH on the highway (at what? 4000+ RPM?) then you will probably find the factory spec is pretty nice.

My two cents. :)
 

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Discussion Starter #11
My inner geek is screaming "time for an experiment"!

I'll break out the iPad dyno program and instrument how she responds, measured by acceleration, this weekend.
 

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Too add to what Graham said I'd just add that advance to pinging and then backling off a touch is a low tech but effective approach that is commonly used. Drives my inner geek crazy but it works.

Fonzi - hope you got the PM saying Thanks!
No PM, but I get the message. And I downloaded an iPhone dyno app. :) Thanks for the reminder. I can't wait to try it on the various vehicles.
 

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My inner geek is screaming "time for an experiment"!

I'll break out the iPad dyno program and instrument how she responds, measured by acceleration, this weekend.
When I tested my car, I mainly used digital stopwatch. But also used iPod app - I think it was called Dynolicious - Posted results here, I think.
 

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So, Fonzi, MBGraham, Procto...

I am assuming that this is written for D-Jet and not K-Jet either because of the differences in the systems or because of the low compression?
 
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