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Adjusting the fuel rich screw 280 SE

Started by Dazbot99, 16 July 2018, 04:50 AM

Dazbot99

Hello all

Hope all is well!!

Ok, how do I adjust fuel rich or lean screw on my 1977 280 SE?

I know where the screw is, 3 mm hex screw etc. which way do I turn it to make car lean or rich ? Do I do this white car is running and how do I know when it is right?

Thank you

TJ 450

Hi Dazbot99,

The screw is for setting the idle mixture, but it does have an effect when you open the throttle as well.

From memory, clockwise is rich anticlockwise is Lean. Easiest way to confirm is by lifting or pressing the plate. If it dies or runs rough when pressing the plate a little, it is too rich. If revs pick up you are lean. It doesn't take much to deviate from the sweet spot and this will alter your idle speed.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

TJ 450

Also, this is done when the car is running, correct.

Any other mixture adjustments will require measuring fuel pressures and testing the warm up regulator full throttle enrichment function.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

Randys01

Bring to full operating temp. Clockwise to enrich.  Adjust one third of a rotation at a time. Allow to settle after each movement. Remove the hex key after each adjustment. If you have to adjust half a turn one way or the other to get it to idle smoothly, there is a greater issue afoot.
Before fiddling, ask yourself why you think it needs any adjustment.?  If all things are equal, the idle mixture should almost be set nearly forever. If the idle becomes "wooly' look to other reasons first. The obvious one is the air filter. If this become crudgy it will impede the airflow and the mixture ratio becomes confused conspiring with a lower idle speed. If it is obviously a crummy idle, look for an air leak in the inlet tract.
Use a magnet to lift the air flap up and down a millimetre or so to determine any variation better or worse in the idle. Lifting it leans the mixture...pushing it down richens.
And of course as Tim has just posted, everything else has to be in its sweet spot too.

rumb

Make sure your ignition timing is correct first.  If your AAV idle speed screw is all the way turned in, then that device needs rebuilding. When you start the engine the rpms should be high and then drop to correct idle in about 4 minutes.  if not then your WUR needs rebuilt.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

Dazbot99

Thanks everyone, think I might leave to the experts!!

beagle2022

I think yours is a K jet system.  They run crazy rich when cold, so only fiddle with this when it is truly warmed up.
Sydney, Australia

Dazbot99