2002 SANTE 5.7l Excalibur Backfiring, lacking power under load

[Freeheel720]

I just got a 2002 SANTE and it's not quite running right. It has 813 hours on the Excalibur 330 5.7l. When I got the boat, it was stalling when going back to neutral, so I replaced the IAC, but that didn't help. So then I adjusted the butterfly valve to make it run. Now, when I take off under load (when surfing), it backfires and has no power. Also, it can't seem to get out of its own way when it is loaded with some ballast. We have the hard tanks in the rear, and today we put bags on top of them, for a total of about 1200 lbs in the boat. It didn't have enough power to go fast enough to surf, at times. When the boat is empty, it cruises well and tops out at around 40mph. I do not have the maintenance records, but what has been done is:

IAC from 95 Chevy K2500
New cap and rotor
New Fuel Filter
Checked Fuel pressure, and I have 57psi at the rail
New Iridium plugs
New LPP
Checked compression, all are good and equal.
Fuel is good

What could cause the backfiring? I have been searching around on this forum, and I see mention of the cam retard service bulletin, but I don't know how to tell if it has been done. Also, is the IAC from automotive any different than marine? Could that be hurting my performance? I'm trying to get into a shop to check my timing, but they are all backed up for weeks here.

Thanks for any advice!

-Arthur

[functionoverfashion]

It sure sounds like ignition to me. Since you've done cap, rotor, and plugs, did you do plug wires also? At this point, I feel like it would be an easy thing to try next.

[Jonny Quest]

I would start with Ignition timing.

JQ

[Freeheel720]

I would start with Ignition timing.

JQ

How do I do that? I'm very mechanically inclined, but have never done timing. Can you please point me in the right direction?

[Jonny Quest]

Take a look at this thread

https://www.planetnautique.com/vb5/f...bur-330-mefi-4

[Freeheel720]

Take a look at this thread

https://www.planetnautique.com/vb5/f...bur-330-mefi-4

Thanks! I had seen that thread and I read it mean that I need access to Diacom (or Rinda) to set the timing properly, correct? What about the old "turn the distributor while I'm driving" trick? Any thoughts on tuning by feel?

[Fast351]

Check to make sure when you changed your cap you didn't reverse a couple of the wires. That would behave exactly like this...

[homer12]

What Fast351 said. Double and triple check you plug wire routing in order of cylinders. It’s in the PCM manual for reference.


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[Freeheel720]

It was the Throttle Position Sensor. Replaced the TPS and all runs great now.