well, i have a good one i am dealing with and can use some expertise !

cylinder 3, has 165 compression. it gets spark + fuel.
the plug is always wet.seems like it isnt burning the fuel.

then the starter broke it's plastic internal gear.
we believe it hydro-locked on fuel that went un-burnt and forced the starter to give away.

seems like the exhaust valve . 125 hrs only on motor.

I removed the valve cover , found the rod to have went through he rocker ! How this happen ?

Seen this before, rocker seat hardening not thick enough or lack of oil flow and pushrod wears on it until it goes through. Replace rocker and pushrod reset valve and turn engine over with v/cover off and make sure you have oil flow from lifter to rocker. Check the other ones on same side for pushrod tip wear and if all seem to have wear change all rockers and pushrods and change oil. If all look ok, assume a faulty part and get on with boating. If you have no oil flow, bad lifter or plugged oil gallery.

I have seen it several times !!!!!!! If you remove all of your rockers you will probably find several of your pushrod "balls" broke off. All will probably be on the exhaust valves. There was a not so, vocal recall on the early 496's. the rocker hits the very edge of the pushrod, And it has enough "leverage" to break off the ball. If it is a Merc. motor it may have a white decal or paint spot on the rear,top of the heat exchanger. This tells you it has the updated pushrods. If you look at your pushrods and they are straight cut at the top where the ball is attached, Then they all need to be replaced. You can get some through an auto parts store a whole lot cheaper !! '03 chevy suburban 8.1. also look under the rockers for marks that can break the seat out of your rockers also. Good Luck

Mercruiser has a service bulletin on incorrect push rods installed on certain serial # motor runs. Actually it was GM problem from the start. The jist of the SB was that the push rod tips were the wrong apps and could cause failures. Important is all info and SB's and parts look up go off serial #'s not year of hull or what you think the year of the motor is. You need to post serial # of engine and what model 496 is it. I can tell you if that Merc service bulletin pertains to your app. The SB was revised by Mercruiser 8 / 2003