Hi all! First time to the forum and was hopping i Someone could help me solve this mistery.
I just bought a 79 merc 7.5 for me and my young son to go fishing. Anyway, I put it on a drum and it Runs great at idle and half throttle, but at wot upper unit starts funking/banging - like a clunk that physically shakes the motor about 2-3 per second. It appears to becoming from the top of the motor, and it actually kicks the motor a bit with the same force as if some lightly thumped their hand against it. It happens in nutral and in forward (didn't try revers). I don't think it's related to the gear shifter (I think I'd recognize that type of clunk) but is it possible it's a bad case of detonation? Or any ideas?
One last clue - I grew up with an old merc 9.9 (est ~1982) it was a great motor that never let me down up until someone walked off with it. Anyway, i seam to remember in later years that it started to develop a internal rattle at wot that sounded that similar to what this 7.5 is doing, and it seemed to get more noticeable as the years went on. (or perhaps I jutted noticed it more as I became older and more aware of engines, etc) The issue I'm experiencing with the 7.5 feels the same except it's really noticeable (louder and with a physical kick to it like someone's actually wacking the upper motor with their hand). It's enough that even a inexperienced non mechanical person would know something's not right. Only think I can think of is detonation, but with the type of sound and kick it's a bit scary to think about whats happening in the motor - had me worried when i was running it without the cowel on.
Anyway, Presuming I'm experiencing the same issues I had with the 9.9, just to a greater extent with this 7.5 would lead me to believe that others exp merc owners must have experienced this too. So any one out there have any experience with this issue, or any ideas?
Any and all help much appreciated!!
I just bought a 79 merc 7.5 for me and my young son to go fishing. Anyway, I put it on a drum and it Runs great at idle and half throttle, but at wot upper unit starts funking/banging - like a clunk that physically shakes the motor about 2-3 per second. It appears to becoming from the top of the motor, and it actually kicks the motor a bit with the same force as if some lightly thumped their hand against it. It happens in nutral and in forward (didn't try revers). I don't think it's related to the gear shifter (I think I'd recognize that type of clunk) but is it possible it's a bad case of detonation? Or any ideas?
One last clue - I grew up with an old merc 9.9 (est ~1982) it was a great motor that never let me down up until someone walked off with it. Anyway, i seam to remember in later years that it started to develop a internal rattle at wot that sounded that similar to what this 7.5 is doing, and it seemed to get more noticeable as the years went on. (or perhaps I jutted noticed it more as I became older and more aware of engines, etc) The issue I'm experiencing with the 7.5 feels the same except it's really noticeable (louder and with a physical kick to it like someone's actually wacking the upper motor with their hand). It's enough that even a inexperienced non mechanical person would know something's not right. Only think I can think of is detonation, but with the type of sound and kick it's a bit scary to think about whats happening in the motor - had me worried when i was running it without the cowel on.
Anyway, Presuming I'm experiencing the same issues I had with the 9.9, just to a greater extent with this 7.5 would lead me to believe that others exp merc owners must have experienced this too. So any one out there have any experience with this issue, or any ideas?
Any and all help much appreciated!!