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Altitude and lumpy idle

Started by raueda1, 15 August 2021, 03:32 PM

raueda1

My cross country road trip is going great.  2800 miles in 4 days with no mishaps.  The car even managed to get 15mpg for a couple tanks of gas!!  [Probably a strong tailwind helped]  The seat-of-the-pants performance is also very noticeably better than at altitude.  Anyway, here's the question.

The car has the high altitude WUR.  It was transplanted from a donor car that was originally set up by the MB dealer for high altitude use.  Above around 750 meters elevation everything is perfect.  However, when I got below about 300M I noticed that the idle got much worse.  Nowhere near stalling, but much rougher and lumpier.  Can this be a result of the high altitude WUR?  I'll be down here in the thick air for several more weeks and unable to do anything about it.  If the idle magically improves when I return home then I guess it really is the WUR.  Is this plausible?  Or should I mess with idle adjustment or mixture or whatever in the meantime to see if I can improve it?  Ideas, comments and thoughts welcome.  Thanks and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

rumb

Yes, the engine will be running richer at lower altitude. You will also feel more power!

The WUR is a fixed operation sort of part.  It opens and it closes depending on temperature, not altitude. It dont think it would be that part.

If you lived at a lower altitude you would adjust the idle mixture screw located in the FD Tower. 

For temporary fix I would try adjusting the idle screw - the white plastic one, not the one in the FD, first to increase idle a bit as that is a lot easier to do. It will still be rich but idle will be higher.



Copied from some website:
The one other part that can be important is the idle mixture adjustment screw. This little screw resides down a tube between the sensor plate and fuel distributor, and uses a long 3mm hex key to turn it. This screw acts directly on the pivot for the sensor plate and raises or lowers the height of the plate. Less is more if you do tweak this, as a small change can make a big difference to the mixture. This screw does not alter warm mixtures at all, only idle.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

raueda1

Quote from: rumb on 15 August 2021, 05:10 PM
Yes, the engine will be running richer at lower altitude. You will also feel more power!

The WUR is a fixed operation sort of part.  It opens and it closes depending on temperature, not altitude. It dont think it would be that part.

If you lived at a lower altitude you would adjust the idle mixture screw located in the FD Tower. 

For temporary fix I would try adjusting the idle screw - the white plastic one, not the one in the FD, first to increase idle a bit as that is a lot easier to do. It will still be rich but idle will be higher.

Copied from some website:
The one other part that can be important is the idle mixture adjustment screw. This little screw resides down a tube between the sensor plate and fuel distributor, and uses a long 3mm hex key to turn it. This screw acts directly on the pivot for the sensor plate and raises or lowers the height of the plate. Less is more if you do tweak this, as a small change can make a big difference to the mixture. This screw does not alter warm mixtures at all, only idle.
Thanks Robert, will try that.  Clearly our thinking wasn't too far apart.  FWIW, idle in drive is about 500 rpm.  In neutral it goes up to 750 or so.  AC on or off doesn't matter, apparently the vac thing on the distributor is working right. 

Here's another question.  I've never, ever heard the engine knock.  I'm tempted to advance the timing a couple degrees to get a little more ooomph maybe.  If I do that and can't hear knocking am I going to OK?
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0