Surging idle

[mullera]

Hello.
I am a new member and have searched all over this site for info before asking about my problem.
I have a 1970 280sl,totally restored from the bottom up,bought 10 years ago on a distress sale.Have no manual nor papers nor details on the restoration,although its a great job.
Have not been able to enjoy this beauty  ,no more than 300 miles in last 10 years.Now I am ready!The car operates just fine except for the idle.
If idle is set at 1000rpm,(which is where it was when I got it)I can drive it with only an occasional stalling out when stopped at light in gear.If i try to set the idle at 750 to 800,as per specs,the idle surges somewhat up and down and stalls out.
Have triple checked all linkages,adjusted idle screw with vacuum checks,and concurrently the idle speed adjustment on the injector pump many times as described on the "linkages" section here.Also  used my Haynes book.There is no rust problem here as some have experienced.
Any thoughts on why an idle would not stay constant would be very appreciated.Idle will stay constant while not in gear,but only if I get set idle up to 900-1000 rpm . will die in a few seconds if I set to 750 to 800,even in idle.
Thanks in advance.
allan

[ja17]

Hello and welcome,

Your tune up items should be in good adjustment first (timing, spark plugs, points).

There are parts of your injection system which can get stuck from long storage.   The WRD "warm running device" is critical to engine idle cold or hot. The intake "cold starting valve" (CSV)  can leak causing ilde problems. By far the biggest factor is correct linkage adjustment, we will assume you have it adjusted correct for now.

Do a little exploring on these items and get back with us.

 Also you can research the "split linkage test" you will be able to tell us if your engine is running rich or lean at an idle and we can advise.

[Michael C]

On a 70 you should also have a dashpot just below the butterfly on the intake. This activates a slip rod (Linkage) allowing more air while at idle in gear to bring the idle up.  (one of the fun emissions things from the 70's).  You may have to adjust this to get the car to idle correctly while in gear.

Michael C

[mullera]

  Have again checked linkages,,csv,wrd,and the dashpot below  the throttle.
In gear or out of gear this dashpot does not move,and no corresponding movement in the slip linkage.The  nut on the female part of the linkage or the slip part is just touching the main outer linkage rod with no gap in neutral,as I understand it should be.
But no movement anywhere when put into gear.Have checked the vaccum to the dashpot and it measures only 13 hg s  .Dash pot(venturi vauum governer accding to Haynes) adjusting screw is just touching pinchbolt in neutral..
 If the dashpot is supposed to travel between -039 and .059,it is not.
   so if I follow this correctly,I either have inadequate vacuum or a faulty dashpot.How do I check a dashpot?maybe something is just stuck/
Any comments please.
Allan