sunman496mag

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




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