Engine dies when in gear
 

This is annoying. My 1969 280 SEL with CIS fuel injection runs great for a while until it is warmed up completely. When hot it will then not hold its idle when put into gear. It just dies. Since the injection pump had been removed a couple of times and the air-fuel mix settings messed with, could this be a fuel mixture problem? Any hints on where to proceed will be appreciated. And no, I don't have an exhaust gas analyzer. Also, is there another device that controls the idle on this car? Many thanks in advance.
Thom