1978 Mercruiser 260

Question

I've got a 1978 searay with a 350 mercruiser had it running last summer. The ( towards the bow ) forward right cylinder had a funky mixture of water and gas that came out when the sparkplug was removed. What exactly would cause this being it only in that one cylinder. I suspect the water came back through the manifolds. Theres no water in the oil and it runs great. It has a pre alpha outdrive. Don't give any rediculous answers, I dont have no million dollars to replace everything.

Answer 1

This was probably caused by hydro-locking, water entering through the exhaust.  Water shutters are located at the TOP of the exhaust y pipe, and can be examined by removing the riser. If they are missing, you will have to remove the outdrive, they will be trapped in the bottom of the y pipe, you can remove them with long needlenose pliers.  Leave the leakdown test alone, do a simple compression test on that cylinder, 100 PSI + is good.

Answer 2

its quite common, couls also be just the moisture frome the air, a hot engin could acumulate moisture when cooling

Answer 3

You need to do a leak down test. This is where you put the piston on top dead center with all the valves closed. Next apply compressed air to the spark plug hole  (I use an old compression tester hose with an air chuck clamped to it.) Listen at the oil filler (ring condition) The exhaust (Ex valve) and Carb intake (intake valve ) and in your case the  largest water line near your problem.(head gasket) Which ever one is  blowing the loudest is your primary problem. Rotted manifolds are harder to figure with out visual inspection. If the leak down shows nothing- and the problem will not repeat- do the flappers.

Answer 4

You probably did have some water in there.I have seen this many times

before.This is your most likely cause.Your exhaust flappers are either worn out or missing completely.What happens is one of two things,you

either get water in from reversing to fast or stopping fast.The water is forced back in through the manifolds and then goes into witch ever cylinder the valve is open on.I am sorry to say but to replace the flappers on your boat,you must remove the engine to be able to get access to them,because if I remember right they are in the bottom of the Y pipe.Sorry but I hope this helps.