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280s first start after engine swap - Idle extremely high!

Started by Steveg75, 22 January 2022, 11:35 PM


Hi all,

I finally got my 76 280s started last night. It fired easily but idles really high! It's the original 4a1 carb with new gaskets. One thing I've noticed is the choke valve that connects to the fast idle cam seems to flop open as soon as you crank the engine, there does not appear to be any resistance or control. If I then close the choke valve the idle drops to a reasonable level.

Any ideas? should the choke valve be closed during cold idle? Should the choke be keeping the choke valve closed? Any  other suggestions? I'm new to Mercedes and don't know a great deal about carbs. The section in the manual doesn't seem to help. I'm thinking in reassembling the carb I messed up the linkage from the choke to the chole valve.




Good luck Steve,
im no help with carb 280s.  I had the same issue with my 280te 123 wagen i just did up, but its the 280 with K jet fuel injection. 
did auxiliary air valve which helped, but could be warm up regulator and cold start switch perhaps.  but i dont think these are on you car with the carb!
regards from WA
6.9 79 #6436
6.9 79 #6290
6.9 79 #6181
6.9 78 #4764
6.9 77 #3096
6.9 76 #1741
6.9 76 #0902
6.9 75 #0018


The section in the factory manual on the Solex 4A1 carburetor is nearly 116 pages long.  It shows diagrams and has explicit instructions on how to assemble, test, and set up the carburetor.  When giving advice, most of us will echo and/or refer you to some part of this section of the factory manual.  While the manual is written with the assumption that the reader has intimate knowledge of the car an all of its workings, the carburetor section is a little more forgiving, and is somewhat easy to follow.  There are lots of pictures, and there are even diagrams for special tools in the event that you have to make your own.  Essentially, there isn't much that we can tell you in any more simplistic of a way than the manual does.  Especially on the choke and/or carburetor's pretty black and white, in a manner of speaking.  My suggestion it to read and reread the pertinent area as many times as it takes to understand.  The choke is all mechanical linkages with no magic, unseen power holding anything together.  In addition, the adjustments are also mechanical with set screws, spring pre-tension adjustments, and not much else.  There is also the high idle adjustment for when the choke is on, but that, too, is nothing but a screw to be turned.  If all of this is too daunting or nerve wracking to you then have someone else set it up.  There is no disgrace in asking for help if/when something is over our heads. 

All of the aforementioned is assuming your carburetor hasn't suffered the ravages of warping between the three main housings of the body.....which can cause many other issues as well.  Unfortunately, this is a very common problem amongst all 4A1 carburetors. 
1966 250se coupe`, black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3, papyrus white/dark red leather
1971 280sel Black/black leather
1973 300sel 4.5, silver blue metallic/blue leather
1979 450sel 516 red/bamboo


Have you had the car running at all beforehand?

I've had my carby apart a few times (I don't have a service manual so just winging it), put all new gaskets through it and checked the carby for warping (what UTN_boy is talking about, they can warp and cause internal vacuum leaks - too warped and you basically need a new carby).

On a cold start, my choke valve is closed and the fast idle mechanism is pulled. It will keep a fast idle until I touch the accelerator and the engine has warmed a little. The choke plate shouldn't flop at start up.
Any pics of your choke mechanism?
1975 280S - Australian delivered


Thanks all. I think I've partially solved the issue. The red and clear vac line from the switchover valve to the carb were mixed up. That said, I'm not convinced the choke is working correctly i.e. the fast idle cam that controls the choke valve doesn't appear to do anything or offer any sort of resistance.

The carb body is warped, although it does appear to have sealed with new gaskets.

it's the first time I've had the car running. It had been sat for almost 10 years!