75 65hp mercury sounds like it's snapping or pounding

[caribooal]

This old 74 mercury 65 hp runs fine but it has a hard snap or knock type sound when working. There are white marks around the head. What are your thoughts as to what is wrong. I don't want to wreck a a motor that doesn't have many hours. This is a fresh water situation.

 

[rickdb1boat]

Re: 75 65hp mercury sounds like it's snapping or pounding

Could be head gasket, although I'm not sure what you mean about the white marks around the head. Can you give more info?? Is the noise coming from the head area? Can you isolate it?

 

[caribooal]

Re: 75 65hp mercury sounds like it's snapping or pounding

The white mark appears to be calcium type matter.<br />It extends along the head gasket on the bottom piston from a couple of inches up the left side across the the back of the gasket area and up the the right side of the that piston. Then on the right side of motor there is a small pin hole (maybe a vent )ahead of what looks like some cover. It appears water has come out of this hole and run along the bottom of the above cover leaving this residue again. My repair manual is still a couple days away and I'm just trying to get as many thoughts on this problem as I can before I take anything apart. One opinion was that I had preignition. This boat and motor we bought from the inlaws a few years ago. It had been primarily used a fishing boat. We used it for two or three two week sessions mostly for skiing etc. It would get maybe 7 hours or three tanks of fuel put thru it each day at this time more less at full throttle. I figure we may have over heated it. It still runs great except for that snapping or coming up hard sound coming from the head area. I have not put a shaft on the block yet and listened to the area of the sound as I seem to get it mostly under load.

 

[caribooal]

Re: 75 65hp mercury sounds like it's snapping or pounding

Originally posted by caribooal:<br /> The white mark appears to be calcium type matter.<br />It extends along the head gasket on the bottom piston from a couple of inches up the left side across the the back of the gasket area and up the the right side of the that piston. Then on the right side of motor there is a small pin hole (maybe a vent )ahead of what looks like some cover. It appears water has come out of this hole and run along the bottom of the above cover leaving this residue again. My repair manual is still a couple days away and I'm just trying to get as many thoughts on this problem as I can before I take anything apart. One opinion was that I had preignition. This boat and motor we bought from the inlaws a few years ago. It had been primarily used a fishing boat. We used it for two or three two week sessions mostly for skiing etc. It would get maybe 7 hours or three tanks of fuel put thru it each day at this time more less at full throttle. I figure we may have over heated it. It still runs great except for that snapping or coming up hard sound coming from the head area. I have not put a shaft on the block yet and listened to the area of the sound as I seem to get it mostly under load.