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.

 

[Laguna 20]

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

no more noise, this only happened once, i think talking about the noise has misled this post. When referring to the noise difference is when the engine was running at 5,000 RPM and then dropped top 3500 RPM. Now I can't get back to top speed.

 

[99yam40]

do you have a tach or are you just guessing at RPM?
you keep saying speed, so I have no idea if you mean boat speed across the water or RPM.

there is no reason I can think of that a motor would make a loud noise like a muffler fell off when it just lowered RPM.
motors make more noise at higher RPM
I would hook up a timing light to see what the spark timing is doing at low and high speed to see if it is stays in spec
maybe if an intake valve stuck open it would fire back thru the intake making a loud noise.
and make sure all the timing marks still line up on crank and cam