Again, good report. Your SOHC VW engine and a Chevrolet pushrod valve V-8 engines are very radically different for combustion chambers. To get the same negative results shows that like always with woodgas motor fuels actual experiences count the most.
I have found very poor results with Robert Bosch new style multiple electrode spark plugs! Failures to constisitly fire off the woodgas - this WAS on a Cherolet V-8 High Energy electronic ignition. Solved going back to a normal wide gapped single electrode spark plugs.
On different small four stoke single cylinder engines now, valve in block and overhead valve types EXCELLANT woodgas results with an American spark plug by an E3 company. These have a ground electrode with a square hollow center over the end of the center hot electrode.