I have an MCM 120 in a 19' Renken Bowrider.
Well, out on the water today, I was cruising around at 4200 RPM WOT, and suddenly lost quite a bit of power over maybe 5 seconds, then couldn't get back over 3k RPM at WOT. I also developed an engine knock (doesn't sound like detonation; more like a deep throbbing knock). I can hear it at idle, but of course it gets louder and faster when I open the throttle.
Sound seems to come from the center right of the engine, just below the exhaust manifold. I pulled the valve cover, and everything *appears* ok, but I guess you can't tell if a spring fails until you remove a bunch of stuff. Oil looks good, plugs have carbon deposits from some carb issues I had earlier this year, but gap and timing look right.
A compression test is next, but on the water I pulled each plug in turn and saw the expected loss of power/RPM for each cylinder, but the knock stayed exactly the same. I also didn't hear a difference in cylinder cadence; just the new knock and the loss of power.
I've been thinking about rebuilding the engine or repowering the boat with a 3.0 (or please recommend any engine that'll bolt into the same motor mounts?) anyway, and my comp test was on the low side last year, but what do you guys think? Can I fix this myself? Not even sure how to troubleshoot this.
Well, out on the water today, I was cruising around at 4200 RPM WOT, and suddenly lost quite a bit of power over maybe 5 seconds, then couldn't get back over 3k RPM at WOT. I also developed an engine knock (doesn't sound like detonation; more like a deep throbbing knock). I can hear it at idle, but of course it gets louder and faster when I open the throttle.
Sound seems to come from the center right of the engine, just below the exhaust manifold. I pulled the valve cover, and everything *appears* ok, but I guess you can't tell if a spring fails until you remove a bunch of stuff. Oil looks good, plugs have carbon deposits from some carb issues I had earlier this year, but gap and timing look right.
A compression test is next, but on the water I pulled each plug in turn and saw the expected loss of power/RPM for each cylinder, but the knock stayed exactly the same. I also didn't hear a difference in cylinder cadence; just the new knock and the loss of power.
I've been thinking about rebuilding the engine or repowering the boat with a 3.0 (or please recommend any engine that'll bolt into the same motor mounts?) anyway, and my comp test was on the low side last year, but what do you guys think? Can I fix this myself? Not even sure how to troubleshoot this.
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