Paul - You did nothing wrong. We have had similar experiences on a number of older engines, my last being with a Fox .29/tongue muffler.
The one thing you might try is a conventional muffler, which, due to its volume, will allow the exhaust gasses to expand more, dissipating some energy.
Also, what Randy said about fuel makes a lot of sense but just be aware of possible pre-ignition or detonation.
Bob Z.