Hi Toddt, I sail on a friends Hunter
. We had noticed that the eng (Yanmar 3gm) Has not been running properly. It appeared to be missing on one cylinder. At times it seemed OK, then it would show white smoke at times at others blue smoke. I fig'd he had a bad injector (white smoke) and perhaps a bad valve seal (blue smoke) He took it into a marine mechanic
. Turned out he had two bent rods one considerably more than the other. The eng was pulled While doing the job We had him hone the cylinders reseat valves, 3 new conrods and conrod bearings. Crank checked out OK. Next question was, how did water
get into the cylinder. First thing (I still can't believe it but have the part to look at) The connecter from exhaust
manifold to the exhaust
pipe which has a water prevention loop (inverted U) had an amazing feature. In that the pipe putting raw water
back into the exhaust is on the wrong side of the preventor. IE the water went in on the manifold side of the preventer instead on the exhaust pipe side. Then there was an anti syphoning loop with a valve at the top On the raw water line leading to the exhaust pipe. So now when you stop the eng. The raw water in the raw water line bleeds into the exhaust pipe (Enough water to fill a 1/2" hose about 18in in length) and because the induction pt is on the wrong side of the preventor and there is no outflowing exhaust (the eng has been stopped) the water can run back into the exhaust manifold and down into a cylinder. Stupidest system i ever saw. This is on a Hunter Production model that had not been messed with. To prevent this from reoccuring A much longer exhaust Prevention loop was installed About 10 in high as opposed to the original one about 2 inches high. The Raw water is introduced into the exhaust line well onto the exhaust side of the elbow
. The nipple is pointed in a downward direction not just at 90 deg to the pipe. This longer pipe requires Heat tape and lagging on the dry side of the line. We also chnged out the muffler
to a muffler
with a one way valve re water. Nothing comes back up the pipe esspecially when involved in some rigorous sailing. The eng is running like a sweetheart. It was an expensive fix. The engine had about 1000 hours on it.
The mech. had recently done another Yanmar
with the same problem It was another Hunter from our Club. The main cause here in my minds eye was the introduction
of raw water on the wrong side of the exhaust elbow
. So basic that you can't believe it when you see it. The elbow can only go on one way as the manifold side is threaded and the exhaust end is designed to take a rubber exhaust hose. as this part i believe is one supplied by Hunter I wonde how many more Hunter owners are out there with bent conrods caused by a mistake in design of the exhaust elbow?? Should launch a class action on this one.