Update it looks like it may have been a muffler mismatch problem. I have a slimline pitts style muffler using a specially machined adapter. It seems like it's too small for this engine. Without the muffler I am getting 7100 on a Mejzlik 22x8.

Edit: The muffler I have is actually a Bisson which has a custom made adapter to fit the BME 50. The output tubes are smaller than the J & A.