RMPs low at WOT

Peg_O_My_Heart

It has been a tough year for the boat.  Bellows replaced over winter and trim still not working, but that is not the main problem.  Twin merc mpi engines on a 2002 FV 310.  All spark plugs were replaced one month ago.  After that, I could get her up on plane, but 3800 rpm's is as high as the engines would go (with WOT).  

I can't get her up on plane now and rpm's won't go higher than 3000.  The bottom is probably a little fouled, but in the past years that was never really a problem.

What should I tell the mechanics to look at?

My ideas are the throttle cables themselves, fuel filter, fuel pressure. 

Any other ideas?

YYZRC

When you did the plugs, did you do cap and rotor at the same time? They are notoriously short-lived.

When you say trim is not working, are the drives fully down? Are your trim tabs working and fully deployed when trying to get up on plane?

If both engines are behaving the same, it would seem unlikely that you have an issue with something like fuel pressure or throttle cables.

To check the throttle cable adjustment, with the engines off, engage neutral on both engines and go to WOT and then check at the throttle body to see if the throttle plate is fully open.

Are the drives original? Ratio? Are the props original? Pitch?

Peg_O_My_Heart

Caps and rotors were done.  The drives and props are original.

aero3113

Replace the fuel filters.
When was the last time you spark plug wires were replaced?

rmrstlmo

Before my last bottom paint I could only get 3700 ROM at WOT.  I wasn't sure what the problem was but I knew the bottom was pretty furry.  I scrapped the bottom with a plastic puddy knife.  It made enough of a difference that I had a good idea that the bottom paint was the problem.  Sure enough, after new bottom pain I was able to get 4300+ without any other changes. 

PickleRick

Check firing order. If performance issues came after plug swap double check your work.

 Very easy to screw up firing order with that many long plug wires. 

 Did you use OEM wires? Cheap plug wires are notorious for miss fires. 

What brand plugs were used and was the gap set on all of them? Champion, autolite have all been synonymous with miss fires.