There is an access port in the center of the boat (between the passenger and drive seats) that allows access to the fill location and vent. I will see about tapping into one of those lines.

Does anyone know the volume of fuel that is returned to the water separator?

There is actually quit a bit of fuel returned when there isn't much load, so don't go with a really small line. 3/8 would be the smallest that I would use. CP Performance makes a Fuel Line Return To Tank Adapter, if that will work
http://www.cpperformance.com/c-500-f...s-returns.aspx

Excellent, I will find out what the filler tube size is and pick one of those up.

I stopped the vapor lock problem by wiring a Holley Blue Marine fuel pump prior to the fuel/water separator to force feed the high pressure pump at the cool fuel.

BUT, I still have the stumble after a long run.

Id did it again yesterday, I shut it down and immediately pulled the codes. Damn thing flashes 12.

So far I have done the plugs, wires, cap/rotor. What is next? Should I throw a ICM at it?

First person to suggest the "fixing solution" will get a 6 pack of Coors mailed to them!!!

Just something else to try. Make sure 5 and 7 plug wires are not run tied and loomed together from distributor cap to the plugs. This was a common misfire issue on automotive applications.

check your anti siphon valve coming out of tank, replace it with barb to npt fitting. put metal body fuel filter in line to prevent trash from getting to fuel pump. had similar problem during mtbe and ethanol change over, they make silicon pellets, running underload they clog inlet to antisiphon, stop boat and they fall back in to tank for next run, repeat.

I already checked the anti siphon valves, they move freely. I can pound out the stainless ball but why? If they haven't give me any problems I would like to keep them as they do serve a purpose.

I will check the routing of the #5 and #7 wires.

This is definitely heat related. It was cold on Friday (77 degrees) and I did not experience the stumble/misfire once. On Saturday it was hot (97 degrees) and stumble/misfire was apparent after 30 to 45 mins of run time.

Still happening. Taking longer to manifest itself with the weather getting colder.

I am to the point of throwing parts at it. I have a new ignition control module coming, so we will see if that fixes it after this weekend.

ICM makes some weird symptoms. Mine looks used up after 100 hours. Normally not heat related tho.

Weird is an understatement... When the problem occurs, I can snatch the throttle from where it has the hiccup to WFO and it will run WO all day. I thought it could be a tps, so I may end up throwing one of those at it as well.

I am going to break down and get rinda diag software if this doesn't fix it. I have a few other buddies with 454 Mag's and 502's so I may split it with them.