2018 Yamaha 40 HP, Poor top Speed

[Laguna 20]

when the loud noise shows up, does the RPM go up or down?

The noise only happen 1 time, it could have just been the difference from 5,00 Rpm down to about 3,000 Rpm as noting under the cowling is loose.

 

[airshot]

On a 4 stroke the 180 PSI might be normal.

I stand corrected, still thinking in old two stroke mode....

 

[racerone]

Start motor.----Set at idle.-----Remove 1 plug wire at a time.----Post results of this test.