You cut the end cap off and remove the internal baffle. It will also make it louder so I have heard.
I just opened up the tail pipe by cutting off the 5/8 tip and welding a 3/4 inside diameter 2 inch long pipe to a washer and welding it to the muffler. This was done after I drilled out and grinded a 1 1/4 opening and found it was stupid loud and by not jetting it up it got hot and the intake valve seat came out and bent the valve, end of motor. At least the motor was 3 years old. The performance was worse due to no back pressure.
So I came up with the 3/4 tail pipe when I found that a old set of bmx handle bars sitting in the garage was the perfect tail pipe size to get a little more flow and keep back pressure.
On my 6t8 chevelle with my built 350 I went from dual 2 1/2 to dual 3 inch flowmasters and it sounded awesome but killed the overall performance. Put 2 1/2 tail pipes on and it came back. On nitrous it liked the 3 inch but on the motor it liked the 2 1/2.
Also if you ride in your neighborhood you might not want to open the muffler too much. These little 150cc motors can be crazy loud with a straight pipe.
Just my opinion but if you do go bigger jet your carb up because lean is mean.
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