Hi Peter, just in case you haven't solved this one yet - I also had trouble with my Princeton probe. It was a while back so I can't remember what the output was but it didn't change when I added fuel. Princeton were very good to deal with. They sent me a replacement which solved all the problems - well, except for the fuel leak that developed from the tank fitting which took another month to plug.
See
http://sonexaus.wikispaces.com/Peter+Anson+-+Sonex+894 Peter Anson
Sonex 894
185 hours