Without having seen the installation.....

If you have the gold muffler drop down to something like an 11x8 or even 11x7 to start with - you are going to have to let the engine turn to get anywhere near making the gold LX muffler work well.

Try the smaller prop. It will not hurt the engine.... its just a big "61" for practicle purposes.

The 12x8 works well on the OS91 combined with the red (longer) muffler.