It could also be that there is a vacuum leak (probably between the top plate and the mid section), so that when you shut off the air to the idle circuit, the mixture improves. There is a great post by dantrcav on how to check and correct top plate problems.

But as Joe alludes, this could also be a synchronization problem.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe