The 103 engine has what MB techs call a twitch at idle. I don't know how the bad the viration is. I would take a look at the injectors. If they haven't been replaced, they should be. Also check the compression in all cylinders (should be approx. 95 psiG). You can get at the mixture adjustment by removing an aluminium plug in the top of the fuel distributor/flowmeter housing. I recall that you will need a 3 mm allen wrench (it's been awhile since I have done this). You should get between 20-25 mpg with this engine.