I read that the reason for heating an intake manifold is to evaporate any pools of fuel that might accumulate on the floor of the manifold. As the fuel/air mixture leaves the carburetor it must make some changes in direction to get to the various cylinders. On your Weber set up there is a 90 right under the carbs. Fuel droplets suspended in the air flow will come out of suspension if critical velocity is not maintained. The spot directly under the carburetor on a "star" shaped manifold tends to allow for some fuel pooling. Heating the manifold is an attempt to revaporize the pooled fuel.

Maybe I am way off base with your Weber set up. I was researching a hot start problem with a carbureted V8 on one of my other cars and came across the heated manifold explanation. Do you have a lot of spark plug fouling?

Best regards,

Joe M