Hi There!

I have just bought a '73 280 and i would like to do her up a bit. but she is running a bit rough and I am sure its something simple?

I spent today having a look and i changed the points and plugs and did the timing but it didn't really help much.

It idles roughly but it revs nice and smoothly in neutral or park, then when you go for a drive it just doesnt want to go consistently, everytime you step fully on the accelerator it will surge and then nothing. consequently when on a flat or incline i have to basically pump the accelerator like a drummer on a bass pedal to move along. The car revs fine out of gear so.. (not that i know anything) i assume that the carby is working well enough? as i mentioned i did the timing and plugs... etc, could this possibly be a vacuum problem?

any help would be appreciated as i really want to make it look nice but this would seem a waste of time if i can't get her running nice

Thanks

Good news is that the 73 is a fine car, however not collectable. It was way ahead of its time when it was new. Great independent rear suspension, rear shoulder safety belts, nice size body. Yours being a 73 has the smaller bumpers which is much more attractive than the ugly ones that were added in 74 in North America.
The M110 engine is a wonderful engine, very smooth and powerful.
Bad news is that it has one of the worst carbs ever made (this may be an exaggeration). The Solex 4A1 carb is very tempermental and prone to warpage due mainly to someone over-torquing the 4 mounting nuts and possibly the air cleaner cover.
There are other causes of rough idle that should be checked before blaming the carb however. Check for vacuum leaks, check spark plug connectors, rotor, distibutor cap, coil, do a compression test.
You could have more than one problem as well. Your hesitation could be due to the secondary flaps (the larger ones) on the carb sticking or opening too easily. There is a tension adjustment that has a set screw to lock it - loosens sometimes or someone might have been messing with it. Press the flap down with your finger & check for smoothness and that there is resistance (from a spring). There is a procedure for adjusting it but I don't know if it is in the manual (using a weight and checking the flap displacement).
My first Mercedes was a used 74 280 close to 30 years ago. Learned a lot from doing my own work on that car and it gave me many years or enjoyment.

Thanks a lot for your help rick, i will check out those things.

A compression check would be in order followed by a valve adjust and then a vacuum leak check.

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I found the service information that describes how to check and adjust the secondary stage throttle valve. This is an easy check - all that has to be disassembled is remove the air cleaner. This could very well be the cause of your hesitation but not the rough idle. Autozen had a good point about checking the valve adjustment - you'll find posts on how to do that. Buy yourself a 17 mm crowsfoot and quality metric feeler gauges.
Basically the procedure for checking the throttle valve is to place a weight on the valve near the rear edge (closest to the firewall) and the valve should open 1.5 mm. The weight is 112 plus or minus 2 grams for 73 and 74 Federal, 143 grams 74 California, 170 grams Federal and Cal. for 75,76. I made a crewd test weight out of an old wheel weight, trimming it to the desired value. Mercedes had a test fixture for the test, but I doubt if anyone has one now.

The quote from the SI is:
"Check upper throttle valve for free movement.
Carefully place the correct test weight on the upper throttle valve (do not drop weight).
The throttle valve should be moved downwards slightly (approximately 1.5mm/.06").
If the throttle valve does not open at all or it moves more than 1.5mm, the tension on the return spring must be corrected accordingly with the adjusting screw. Loosen clamping screw prior to turning the adjusting screw. Recheck movement of the throttle valve with test weight. Retighten clamping screw."

I do have the test fixture with the three different weights, but I would still chck compression and adjust valves first, but that's just a professional mechanic talking.

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