Basil:

Break-in methods are dependent on cylinder/piston type, two or four stroke has nothing to do with it.

There are three schools of thought on vibration and balancing:

1) Balance everything, including the spinner. This is the way I do things, and I don't have vibration problems, new or well worn engines.

2) Find the heavy blade of the prop, and install it with the heavy blade down when the piston is up. Some people claim excellent results this way.

3) Forget balance, bolt the prop on and go. This is the method most people use. And they seldom have trouble caused by imbalance.

But I would suspect your TT 91 has a balance problem.

Bill.