RE: Giant scale Ultimate Biplane
I just checked this, and you are using the C.G. for the 1.20 Biplane.
The C.G. for the 50CC Ultimate is approximately 1.5-2" in front of the rear top carbane mount.
What you are using would make the plane far too nose heavy and accounts for all of the added weight you believed you had to add.
You do not have to add the weight, which is also why I mentioned that I had no problem with it...
Not to sound disparaging, but I would have assumed that a 25 yr expert builder would have checked this first... I do so and I have far less experience than you.
C.G. calculators help a lot.
However if you've never worked with Biplanes this might not have been obvious.
If you've had other Ultimates, the C.G. tends to be in about the same rough position ( adjusting for the scale ) on all of them.