Hey, I have a 28 Daytona with twin Ilmor 625's. I have owned the boat for a couple of years and it has about 200 hours on it. My problem is that the engines are running at different RPM's and the more it turns, the farther apart it becomes. They both idle and run great. They are close up to about 2000-2500. At 3000 they are about 200 rpms apart, at 4000 they are about 350-400 apart, and at 5000 to WOT they are about 500 apart. The port engine will hit the limiter and the starboard will only run up to about 5500. I have recently had both engines tuned up and well as the drives gone through. I just got my props back from being worked on from BBLades and I have also tried another set of props. I have had the Livorsi guages checked out as well. I have checked the trim to make sure the outdrives are the same. These are the gen 3 ilmor 625 engines which have a cable to the throttle body. Any ideas or anything I might be overlooking?

Have you done a compression and leak down test?
How is the fuel pressure?

Let's make sure we are talking about the same thing.
The only time you can determine if one engine is "running slower" than the other is when they are under full throttle. Any other time could be something as simple as the throttle cable adjustment.
But since you do say they are off 500 revs at WOT, then that does matter.

You also haven't said whether this is a new problem or if it has been like this since you have owned it - please verify that for us.

#1) Make sure the throttle plate is opening ALL THE WAY UP on both motors when the sticks are wide open. A stretched or slipped cable adjuster can affect rpm the full range.
#2) If they are both fully opening all the way, then make sure you don't have a bent or dinged prop. You can lose revs from a tweaked blade.
#3) A bad plug wire on a 8 or 10 cylinder marine motor will only affect revs by about 75-100 rpm, so you have something more significant than that going on. Still helpful to run the motor in pitch black dark with the hatch open to see if you can spot cross-arcing (or hear a pop from a bad plug wire).

Check those and come back with feedback

Thanks for the ideas, I have not done a compression or leak down test but I am planning on bringing my gauges up there this weekend to test for that. My fuel pressure gauges on the dash show plenty of fuel pressure.
I will also check to make sure the throttle bodies are opening up all the way. I never thought of having a stretched cable, but a buddy of mine had a shifter cable that stretched and wouldnt allow him to shift one of his engines. The props should not be the problem because I have tried a few different sets, all with the same results. I will check all that this coming weekend and report back. Thanks again for the tips...

Isbanshee, I have the same problem, can't find a solution anywhere. still have the Boat? What did you do to fix it?

Docster

I had the same problem with another engine. Seemed to run perfect. Messed with the cables. Checked the butterflies. All good. Still had the problem. Change the caps and rotors, wires and spark plugs. Problem solved.

My Illmors did the same thing. Worried me a great deal. I adjusted and perfected the cables and it did not seem to matter. BTW spark plugs, fuel filters changed. No caps or rotors on these. After a while I just ignored it and synced the RPMS with adjusting the throttle to compensate. If you find out anything on this post it up. I no longer own the boat but would be interested in hearing a solution.

Regarding throttles and opening the butterflies:

Make sure you check them with a scanner so you can ensure the Throttle Position Sensors match. THAT’S ultimately what has to match and could convict a bad TPS potentially.

I have thought the same thing for so long. I have asked about a scanner. Been told u can't hook up to the eco. BS. there is a connection port on it. I haven't been able to get which scanner or brand to buy. Been trying to find for a couple years. Cant get ilmor to talk to me. I get it they don't want to talk end user.

You are the first person to talk. Can you send me in the right dirrection

Docster

Sorry, I'm not in the Ilmor know, but I'm certain it's out there.
Might see if these guys are still around:
https://www.facebook.com/profile.php?id=100057322816096