Hi Howard,
you wrote the airplane porpoised the whole journey. How was this movement ?
Only a vertical oscillation?
The installation at yoke is very suitable and neatly arranged but I am not sure
in case the extra weight is the problem I would it remove.
Thomas
It only oscillated vertically…the plane dropped…then rose again…then dropped then rose again, with quite slow oscillations. 20-30 seconds from memory.
I still think there was something strange/marginal going on… for example a lack of lube in the pitch control linkage, and that includes the yoke tube, could account for this, coupled with the extra weight on the yoke pushing the yoke down against the sleeve bearing.
Most people don’t lube the yoke tube properly (it should be done with some sort of clear silicone) because they don’t want to get it on their clothes if they accidentally touch it.
Traditionally, on the King autopilots especially, you get oscillation if the tachometer in the servo is shagged, or a wire has come off it. Details here The design is one which Brunel would have been proud of but a 20th century design would work better 
I would suggest, @thomas, that you remove the said GPS and do a flight test where you just push down on the yoke to simulate its weight, without its mass being attached.
