You have just described the dreaded "Balsa Effect".

When an otherwise perfectly adjusted engine is placed within 50-75 cm of a balsa object (model plane), AND this object is placed at a distance greater than it's gliding distance from the field...this "BALSA EFFECT" is at it's maximum......It has been shown to cause all sorts of dreadful effects. Many a Master Builder and Expert mechanic have been created in this fashion...

<GGGGG>

In all seriousness....

Make sure you're airplane's fuel system is a good approximation of your test stand set up. Ie: Fuel tank c/l on a level with the engine'e needle valve. Fuel lines as short as possible, tank adequately padded to prvent foaming, Needle set for 2-300 rpm below peak on the ground, etc, et al.

As for the "transition delay"....understand that the load the engine sees in flight is different than that on the ground. In flight, the prop is unloaded somewhat (the blades unstall) and the mixture needs to be "pre-adjusted" on the ground to compensate. It's almost like the engine is over-running the throttle opening. Maybe DAR or someone has a better insight into remedying this issue. Could be as simple as a little more/less lean on the low end. Easier to fix when you're standing right there to see it.

Good Luck.

Can't wait to visit the UK again too!!!