1973 Other Volkswagen Models Running

I'M having a problem with the running of my car, newly rebuilt 1966 engine and it runs great for about 5-7 min. After I start it the it starts to run on about two of the 4 cyc. And condesation starts to form on the intake manifold which I am sure is on the inside of the manifold as well, which make it run bad. I am missing some of the sheet metal for the heat down by the tappet covers, I'm not sure what it looks like or if this car ever had it.