You should be able to blow through the pressure fitting. Some brand new mufflers are like yours, with the hole blocked. I've had 2 like this in recent years, and here's what I did:

Remove the muffler, and gently drill through the pressure fitting with a 1/16 inch drill until it goes through into the muffler bolt hole (won't be much). Be sure to clean out the debris after drilling. I wash them in the sink under high pressure hot water, towel dry and finish drying with a hair dryer or heat gun.

You should be able to turn that 7.4 x 8 at 24,500 or better on a new motor, but don't burn it up hunting for peak RPM on the ground, let it run rich about 600 RPM down from peak, and pinch the pressure line for 1/2 second to peak it to check the RPM but don't hold it. (You can pinch the fuel line too but it's harder to see the actual peak RPM.)

Whatever you do, NEVER run it lean and it will last a long time.

Good luck,

Drew