Hummm...I know that some 66mm commercial engine are putting 25lbs trust. You have about 2 things to get more trust. Optimized diffusor or optimized combustion chamber.

You played with diffusor...That can be the answer..can you post a pic of you diffusor??

140K with 66mm engine seems to be quit high isnt it??? Since we run 54mm at 160000...