I came across an unusual problem that can cause wet sumping that you'd never think of but it appears it could be more common than you may think. Basically, if your Matchless smokes or wet sumps despite having an anti wet sumping valve fitted try looking down where the oil filter goes and look for holes in the tube that the filter fits into.
I was working on my own Matchless today, spraying the oil tank that I'd thoroughly cleaned out and taken back to bare metal by hand, all appeared well, the oil tank I was using was a really nice un dented and unpitted one and I was very pleased with it when I took the old paint off and it was only when I had laid the first coat of paint on that I spotted first one rust hole and when I looked closer itere were more tiny pinholes in the tube that the filter sits in. I took out the old oil filter and it appeared to have a bit of a rusty cage but apart from a quick check it appeared ok. It was a shame as the oil tank was absolutely mint apart from that. Fortunately I had another so I used that. I checked the other spare ones that I had and another tank that appeared ok had the same problem. If the holes are below the normal oil level in the tank the oil will drain into the tube until it levels out and it will drain down the oil return pipe and fill the crankcase that way. It appears that the cage of the oil filter must have caused an electrolytic reaction and caused the rust, even though the inside of the tank was absolutely clean and rust free.
