Join Date: Aug 2000
Location: Ft Worth, Tx USA
I think this might be a classic case pertaning to ported/manifold vacuum. I am not being smart the converter is most likely the culpert, if the cam is big enough and the butterflyes are open just to the right place you will lose most of the ported vacuum when you drop it in gear. The problem could be, because of cam size butterflies are open just enough to get enough vacuum at ported port to start vac adv to move and because of converter RPM'S drop enough to loose vacuum at ported port, thus also retarding timing making a progressive problem. Try manifold vacuum.
I will use Holley to explain how I have delt with this problem. This works best on a Holley because they have a rich idle circit(sp) anyway. It is best to remove the carb(and of coarse we all know to pull a bowl screw to drain the carb, and don't turn a Holley upside down) there is a screw in the base plate on the secondaries,(rear butterflies)on the passenger side, that would appear to be an idle screw(you can see it on the top side of the baseplate next to where the linkage hooks from the secondary diapharam). Using a small screwdriver turn the screw 1/2,to3/4 turn clockwise,or to increase rear idle speed.The screw will be hard to turn so use a good screwdriver. Now here is the result the idle speed has been increased from the rear so you can decrease the butterfly opening and the idle speed will be the same as before,except the rich condition is gone,air bleeds function,and and the ported vacuum port is working like it is suppose to if you need it.