exhaust valve stuck open

[bigtoe]

hi volvo penta 3.0l GS the exhaust valve for the rear cylinder is stuck open. Ive zero pressure in it, other three are good showing at 175ish. I can hear air blowing out through the exhaust on a leak test. boat has sat most of the year and was fouling the carbs up something rotten when used for a few days last year. Carb is now sorted though so I thought all good to go. But heard it running on three and back fired through the carb a few times when revved. did the comp test and found this.
Is it difficult to free or should I take it to a mechanic for a head off strip down. ? thanks

 

[Scott Danforth]

not difficult if you know how to turn a wrench. first thing to do is pull the exhaust manifold. you will most likely find that the exhaust manifold is leaking into the exhaust port