Where did my performance / top end go?

[Mordekai]

So after a few outtings this season, I've noticed that the power plant package of my boat is not doing what it's supposed to.

Standard merc cruiser 3.0L w/o alpha 1 gen 2 here. Last season I was able to do about 30mph @4000rpm and about 40mph WOT @ 4800 RPM, speed is measured with GPS so it's pretty accurate.

This season, I'm doing about 25mph @ 4000rpm and WOT I cannot get above 33mph @ 4800RPM.

I've ruled out the prop/hub slipping since I switched to an identical back up prop / hub and it's still doing the same thing.

I've noticed no obvious leaks or anything wrong anywhere. I did the standard winterizing last year, changed the impeller at the beginning of this season and everything went back fine and dandy.
Fuel filter was changed last season.

The load is the same in terms of weight, 2 adults, 20 gallons of fuel.

So I'm at the end of the line here in terms of my ability to diagnose issues.

Help please!

 

[gtochris]

Re: Where did my performance / top end go?

I'd check the cap/ roter, plugs and wires, just cleaning my cap/ roter helped my performance.

 

[Philster]

Re: Where did my performance / top end go?

Must be asked: The prop is the same prop? The prop, if it is the same, is in the same exact condition? Do you run on a river or water with tides, currents, etc?

If you sure the hub is not slipping, and you have the same prop in the same condition, then there is no place else for slip to occur. Then we turn to the tach.

Is the tach correct? You need to ensure the tach is correct. If the engine is really turning 4800, the something is slipping, so back to the hub. If nothing is slipping and you lost speed, then the engine isn't turning as fast as it did last year or you are running against the current. A GPS speedo will reflect this. Run against a 7 MPH current... if speed over water is 40, then GPS speed is... 33, and the RPMs will match up, too.

 

[Mordekai]

Re: Where did my performance / top end go?

Must be asked: The prop is the same prop? The prop, if it is the same, is in the same exact condition? Do you run on a river or water with tides, currents, etc?

If you sure the hub is not slipping, and you have the same prop in the same condition, then there is no place else for slip to occur. Then we turn to the tach.

Is the tach correct? You need to ensure the tach is correct. If the engine is really turning 4800, the something is slipping, so back to the hub. If nothing is slipping and you lost speed, then the engine isn't turning as fast as it did last year or you are running against the current. A GPS speedo will reflect this. Run against a 7 MPH current... if speed over water is 40, then GPS speed is... 33, and the RPMs will match up, too.

The props/hubs are identical since I bought 2 last year and kept 1 for back up. I have not hit anything and the props are pretty much in the same shape.
I run in a lake and there's very little current, bimini was down to reduce drag, either way, with the bimini up i'd lose 2mph tops, not the 7 I'm seeing now.
I am using a GPS speedo verified against my iphone speedo also.
How would I check if the tach is right?

Also, I did change the spark plugs at the end of the last season for the first time (2 year owning the boat). I've checked the spark plug wires and they are all tight.
Could a lose plug be responsible for this? I can retighten it down but I didn't think they need a lot of torque, usually high tighten if i understood correctly?