Hi,
I own two 91 FX's and the first engine I was able to tune no prob with the baffle in the muffler. The only downside I found with the baffle was that it limited rpm slightly. Maybe 500 rpm or so if I remember correctly.

The second 91 I got just this year and I removed the baffle before I even got the engine mounted on the plane.

Let me add also that the 91 at full throttle can drink an ounce of fuel / minute. So you need at least medium ID fuel tubing for proper fuel flow. Be careful with tanks that come with small ID pick up lines.

Cheers.