Ignition timing

Thank You,
Honestly you guys were fantastic. Yes I am starting to think that I need to get prepared for the inevitable day (sooner than later) that I need to refurbish the Trigger Points. 1 more question if I may. The service manual states that for a 1973 Mercedes 450 the timing need to be at 5 Degrees ATDC but a lot of people I have been talking to state it should be BTDC (I have it set at 750 rpm, 2 degrees BTDC with a dwell of 28 degrees - supposed to be 30 - 34 but finding hard to adjust to this). The car is surging a little at idle by app. 25 rpm but this could be another issue - possibly T/P's?? It also has an ignition module next to the coil but it is not connected at the bottom so I'm not sure what it does.
Anyway, what is you opinion of the timing mark?
Regards
Peter

Hi Peter,

as we are now talking igntion and no more trigger points, I have moved the subject.

Regarding your ignition timing, it depends on your ignition distributor. You will find infos here under 107 SL workshop manual .in chapter 07.5-500 . Read your distributror no and check there. First adjust dwell, then timing. And do not forget that it needs adjusting and verifying at 1500/min, 3000/min and of vacuum adjustment!

Will do and great reference list.
Thanks You!

Ive got a 1974 450SL and I’ve got my timing dialed in at 30degrees btdc without vacuum. Which ends up being tdc +-2degrees with vacuum line. I’ve been told to keep the vacuum line disconnected and plugged. But it ran a little sluggish at times. So could I keep it timed to 30degrees btdc without vacuum and connect the vacuum line to achieve tdc+-2degrees?

Although my distributor isn’t listed there. Mine anppears in the national version ands ends in 403 007. The standard version for m117 doesn’t show my distributor. Job 7.5-500

Hi Unknown,

please sign ur messages so that we need not guess ur firstname and can greet u.

Your ignition distributor is mentioned in 07.5-500 in chapter B country version US. It shall have 18-22° BTDC at 3000/min with vacuum. And it shall achieve 5° ATDC with vacuum at idle. 

Ur ignition distributor has double vacuum box for advance and retard and it has 2 levels of rpm adjustment. Pay attentin not to mix advance and retard vacuum line,

My apologies,

My distributor doesn’t have but one vacuum port and from the service manual it’s the retard port.

I have it set to 30degrees btdc without vacuum@3000rpms per the chart is 7.5-500.

If I connect the only vacuum line the timing drops to 1 or 2 degrees btdc.

-Joshua

Hi Joshua,

you are right, it has only retard. Bosch Microfiches have wrong information here. Mercedes is right.

However you should set to 18-22° BTDC at 3000 rpm, not 30° BTDC. That is what Mercedes tells in that document on page 2 for Model Year 1974 USA.

Yes I know that is what the charts say. But people on Benz world
Forums claim to set it to 30btdc@3000rpms to have it run better..

They would say since I’m in the USA, that the 5atdc w/vacuum at idle on the decal was just to meet the emissions standards of the time? But I’m still unsure. It seems to run better now that it’s dialed in at 30btdc@3000/rpms vacuum disconnected. Reconnecting the vacuum line and having it at tdc+-2degrees or so instead of 5atdc at idle seems to run it better.


So in summery I have it dialed in at 30degrees btdc@3000/rpms without vacuum. I reconnected the vacuum line and it now reads tdc +/-2 degrees. According to the charts first page I still have the m117 in my 74. So I wasn’t sure if it would matter what distributor was in there. Vs what the spec says.

Hi Joshua,

first u ask and then u claim that ur distributor is not listes and now you have a predetermined opinion from Benzworld.

By increasing the timing at 3000 rpm to 30° BTDC u will shift the timing be -10° . Thus u make it 5 BTDC +/- 2° at idle if u connect vacuum. If u omit it, it will change further to 17 BTDC . Ur car will not run properly with that.

Remember that your car switches off vacuum anyhow if A/C compressor is running. That would normally change idle timing to 5 to 7 BTDC. I do not see a good reason for changing timing overall to 30 BTDC at 3000 rpm. If u want to omit exhaust reduction at idle, dicsonnect vacuum line but leave it to 18-22° BTDC at 3000 rpm.

Yes, it is listed in section B under year 1974 in the national version chart and not in section A for the standard version. So which chart should I follow for timing spec of the job 7.5-500. 

Right now the car runs and sounds fine at the setting I have it at with vacuum connected. At idle it shows tdc+-2degrees now. 

Joshua1974/450SL wrote:

Yes, it is listed in section B under year 1974 in the national version chart and not in section A for the standard version. So which chart should I follow for timing spec of the job 7.5-500. 

Right now the car runs and sounds fine at the setting I have it at with vacuum connected. At idle it shows tdc+-2degrees now.

Some of the people on Benz world forum know these cars too and they claim these things. I’m siding with the factory manual. 

Hi Joshua,

standard for Germans is Europe version and not US version as we are middle of the world   SO in Mercedes-Benz documents u always have to check for US version.

Remember that a running A/C will further advance your timing. I would set to 20° BTDC at 3000 rpm and disconnect and seal vacuum if u like to eliminate exhaust gas reduction at idle.

The timing you’re talking about setting it to at 20degrees btdc @3000/rpms is with vacuum connected. Thus making it around 30 or so without vacuum connected.

Hi Joshua,

no it is not. Retard does not work at 3000 rpm. It only operates around idle. Nozzle in throttle is built like that.

So in principle u shall set ignition timing to 20° BTDC at 3000 rpm and itshall not change if you remove vacuum.

No, I know that vacuum isn’t applied at 3000rpms. I think we misunderstood each other. Or I’m just terrible at explaining it. Haha.

I was saying that with the vacuum line connected in general. If I set the timing without vacuum connected at 30btdc@3000/rpms and then check it at idle still without the vacuum line connected it reads about 12-15degrees btdc. When I stop the engine to reconnect the vacuum line to the distributor and start the engine it reads TDC give or take a degree or two at idle speed. It seems to be running better with the vacuum line connected at that setting.

I understand initial timing but total timing is what I’m after.

Joshua1974/450SL wrote:

The timing you’re talking about setting it to at 20degrees btdc @3000/rpms is with vacuum connected. Thus making it around 30 or so without vacuum connected.
 

Hi Joshua,

this statement is wrong. Setting it to 20° BTDC @ 3000 rpm will not be affected by vacuum or not. It will not come to 30° BTDC at all.

Vaccum only applies around idle. U can connect it there to come to 5° ATDC or leave it off (but close line from throttle) and run engine always in mode "Give me more power as A/C is on" with timing BTDC.

And for the last time: If u set to 30° BTDC @ 3000 rpm u will have terrible timing when idle and A/C compressor runs. I recommend not to do so. That is what u have nticed when u disconnect vacuum.

Now u have to decive whether u want to follow Benzworld or Mercedes-Benz and my recommendation.

Sure, from what I’ve gathered reading about ignition timing in general is you remove the vacuum line and plug it. And that allows you to set total timing which occurs around 3000rpms, correct? And these are mechanics and engineers who have told this. Setting timing is no secret. There is really only one good way to do it on older cars.

Turning the A/C on increases rpm’s slightly though.

So keep vacuum line on my distributor connected when setting the timing to 20degrees btdc@3000rpms? Is that what you’re saying? Which implies 5atdc@idle 700-800 with vacuum connected right?

My car has the black plaque with the information. Saying it’s supposed to be 5atdc at idle 700-800rpms without A/C w/vacuum 750rpms in N. Setting it to that timing makes the engine run sluggish even despite adjusting the idle screw to achieve 700-800.

I have noticed though, having owned two of these 74, 450SL’s that the throttle valve housing had two intake port nipples for vacuum lines and the one closest to the front of the engine was always plugged. It’s currently plugged so no vacuum leak. I need to find something better. I know it’s irrelevant but I think that port was for the vacuum advance line on the 72-73 450/350SL?

We’re looking at the same chart from 7.5-500 right? Standard version/National version, etc? Haha
Are we able to attach pictures here on this website?

Hi Joshua,

I have a ‘72 450 SL (US spec) and have recently decided to plug the retard pipe on the throttle housing, but otherwise kept the timing unchanged. Before it was set at 5 degrees ATDC at idle and 20 degrees BTDC at 3,000 rpm. After plugging the pipe the total timing at 3,000 rpm is unchanged (as described by Volker), but timing at idle is now around 5 degrees BTDC which makes it idle better. My CO is 3,2%.

best, Peter

Hi Joshua,

do you notice that everyone else greets and signs with his name?

If you have a throttle housing with 2 nozzles for vacuum then your original throttle housing was replaced. Then the advance connection must be closed.

Gotcha Peter,
the service manual says the CO should be between .5-2% I think.

Josh