ok, Andy, 734406pk has steered me to the probable solution.
I have on the carb a main body to throttle body gasket from Summit,
part 108-5. I should have used part 108-7, which is a slightly larger bore
and which does not cover up some idle circuits. When I bought the gasket, it said
for 4150s, and was the only one I saw at the time, and it seemed to fit up real nice,
but, I missed some of the small details. A painful 3 month leasson!
will order new gasket and see what happens.