If you have a piece of paper across the inlet port, the flow of intake gas will be restricted and you will get an artificially low compression test reading. A paper funnel which leaves the port itself open, on the other hand, should do no harm.
A slight flow of gas outward from the intake port at the beginning of the compression stroke is unavoidable because the intake valve cannot close instantaneously. It begins to close late in the intake stroke then finishes the job early in the compression stroke. When the engine is running this has little effect because of the momentum of the gas moving through the carburetor and port, but at cranking speed a backward puff can be observed.
I suggest you repeat the test ensuring that the flow of intake gas is not obstructed, then repeat it again with the spoonful of oil in the cylinder (through the spark plug hole). The horizontal cylinder has little effect on this test: the oil is initially at the bottom of the cylinder, but it is swept upward by the piston and rapidly circumnavigates the top piston ring, trying to find a way past the ring.
Do you have the GV400 workshop manual? The only ones I can access on line are the 100, 150 and 200 cc side valve engines. Specified compression pressure for the 100 cc is 30-60 psi, and for the 150 and 200 cc is 85 psi. (The 150 and 200 have 6.5:1 compression ratio, compared with 5.6:1 for the 100 cc).
