To me that is too much. I like a lot less timing. I set it to 1.8mm BTDC.
You might be heat soaking the piston causing less power up top.
I put a mark on the flywheel and find where that mark fires. Then mark the case. Find where it is actually firing using a timing tool. Take flywheel off. Set the crank to the right position then line the marks up and reattach the flywheel. Then check again to make sure it is right.