Hard Start Problem
 

2000 Scarab w/ twin 350 Mag MPI

I've asked this once before but got no suggestions so I'll try again. I've got a hard start problem when the boat warms up. Both engines are experiencing the same issue. Engines start right off when cold, but just turn over when hot. In order to get them to start I have to turn the key on and off numerous times and throttle up all the way when cranking and it might start, along with some backfiring and then it takes a few minutes before it will idle correctly.

What I've tried:
Replaced temp sensor for computer
Swapped in known working IAC, no change so went back to original
Installed Holley in line booster pump (thinking vapor lock and this is a known possible solution) Pump is a 7psi pump.
Changed fuel filters
Filled up at different gas stations, including an ethanol free station.

Edit: Anyone think this could be ignition related? Got to thinking when the boat is warmed up is is smoking just a tad like its running rich. Also already changed the plugs but nothing else on the ignition side.

Edit#2: Think I might have found the issue. Fuel pressure is 30psi at best at the end of the rail, tested with two different testers and same result. Even after engines crank and are running. Shouldn't this be ~43 psi? They hold the pressure for a while so I wouldn't think pressure reg, maybe bad pumps?

I'm at a loss here and out of ideas, I really thought the booster pump would work. Any ideas are welcome.

Wow, 70 views and not one response. So no one has ever experienced this issue with as many mercruiser mag engines are out there? I am thinking the pumps but would at least like some second opinions before spending the $$$.

I would say that your low fuel pressure is most likely the problem. It should be 43psi, key on engine not running. If it does not come up to that point then check inlet screens, service or replace your cool fuel system pumps.

Best Regards,
Ray @ Raylar