To the best of my knowledge the standard heat shield on a B18/B20
is a sheet metal device. I have not seen one equipped with the
plastic separators between carbs and manifold, only the sheet steel
plate, with gaskets on both sides, which probably perform at least
some insulating function.

My problem with grose jets was as evident cold as hot, and could be
seen as almost the same as not having a float valve at all: gasoline
coming out the vent in the top of the float chamber. A friend who is
pretty expert with SUs told me they never wear out, and that soaking in
carburetor cleaner would remove the gum which he thought was causing the
problem. In some cases it helped some, but I still had flooding problems.
I changed back (after about a dozen years) and about 90% of the problem
disappeared. Still have problems at high revs on one carb, which I
attribute to vibration or resonance of the float. The engine revs pretty
high and has not been balanced.

When I first started using Grose Jets they worked pretty well but I suspect
that they are not now made to the quality that marked the early product.
Also the same vibration may have caused some abrasion. Anyway the neoprene
or viton tip seems to work better for me.
--
George Downs Bartlesville, Oklahoma