I pulled the intake manifolds off last week to have them ported. Today I was preparing the engines to put them back on. In the intake runners for cylinders 7&8 on both engines (with the intake manifolds off) there is a strange heavy residue - the consistancy of Vaseline. It does not smell like gas and it doesnt smell like oil. My belts are old and there is a good bit of belt dust on the back of the seat. Could this be build up of belt dust mixing with the air/fuel and creating a slimy paste...

What is this? What is it from? Why is it only in the back two runners?