carb question from a newbie

[mhorning]

Hi, I am new to this list. I currently have two Chevys- a 63 stepside and a 86 Silverado. The stepside is going to be a long project, as I get the money to restore. However, I wondered if you guys could give me some suggestions for the 86 SIlverado. It has a Rochester Quadrajet M4MED carb on it and a 350 Engine. Summit Header with 224 duration and 245 lift and exhaust. I bought the truck recently as something that I thought I could fix and drive a little more quickly than the 63. The guy before me took out the old 305 and put in the 350. I got the truck in less than perfect condition. It would run, and then stall, the timing was off so it wouldn't start, etc. I did a compression check and found one of the valves to be a little tight, so I corrected that. Also he had failed to connect the idle stop solenoid on the carb. Now the truck starts and runs pretty well. However, when I put it in drive it dies. I already have the rpms at about 900 which I'd like to lower, so I don't think that is the issue. Also, I checked for vacuum leaks. I noticed that this diaphram on the back of the carb called a vacuum break assembly doesn't seem to pull back the linkage when vacuum is applied to it. Could this be the problem? I thought I would ask those in the group who were more knowledgable on the qudrajet if they had any suggestions. Thanks.