That's a neat trick. Because it works there must be an explanation as to why it works.

If the engine is mounted horizontally, so that the wing and the cylinder are parallel, fuel could not pool up on the glow plug.
This would be true for an engine that is mounted vertically, with the glow plug up.

The only time you would experience fuel pooling on the glow plug is if the engine is mounted inverted.
Fine tuning and changing fuel usually corrects this.

I have a O.S. FL-70 and I think it is the most fussy engine I own. I think a lot of the problem is that the engine is non ringed and built inexpensively, not cheep, so that it is very susceptible to changes in temperature and humidity.

The engine is now on it's 3rd season and is working much better. It's had a long time to break in.

If the washer method can be explained I too would love to know why.