1986 560SL Start problem...

We happened to get a good deal on a 1986 560SL that sat in a barn for a few years... After cleaning it up, everything was running fine... and a beautiful car at that. Anyways, we have been having problems starting the car... We've replaced the battery so far. The car never starts unless you pop off the air cleaner cover and hold the throttle plate(?) down with a screw driver. As soon as the car is started, pull the screw driver out and pop the lid back on, it works like a charm... Any suggestions why this might be caused? I'm not too great w/ mechanics... but anything would be appreciated. Thanks in advance :)