Looking for help. I have a 2003 merc 496ho mag in my wellcraft excalibur, never started untill 2006. Ran like a top untill this spring with 80 hours on it. It had the aluminum water rails and they corroded off in that short of time. I found this was a service or recall issue and merc. shipped new manifolds and they were installed by a local merc. service marina. The problem is every since they did the work my boat runs terrible with missfires, backfiring and no power. I took it back to the marina that did the work, they put new platinum tipped plugs as required for the boat and labor at 600.00, boy did i get ripped. The boat ran fine for about 4 hours of run time and is now back to running terrible again. I have added yamaha ring away to the fuel to unclog any carbon, new fuel and seperator, had a mechaninc check all 8 coils, wires and plugs which all check out ok. On a compression test one cylinder is about 20psi less than the others. Totally stumpped here have a nice boat but cant use it and cannot diagnose the problem. Any help would be greatly appreciated

Is the transom getting really black? Do you have any alarms sounding? If they didn't replace the temp sensors in the manifolds or you have a problem with one of them, It is maybe going into service/limp mode.

get it scanned,,,or yull throw money at it,,for nothing

as stated get it scaned . you may also find no codes but yet still runs crappy.i have found a rah of bad injectors this season due to the fuel. doesnt show up on scan. and if checked with the injectors pulled out there is a spray evident, but it is not sufficient. check fuel amount with a beacon or adequate measureing cup. then swap injectors.