MadMat
05-13-2001, 08:04 AM
Was wondering if anyone on this board could help me shed some light on a problem with my motor. I bought the boat about 3 months ago with 1985 Merc 150 XR2 s/n 6637001. The engine is quite well used, but compression is within 7% across all the cylinders, a bigger variation than I'd like but the budget won't do a rebuild this year (Parts are ridiculously expensive in the UK). This season so far I have replaced the plugs, cleaned & rebuilt carbs, flushed engine with powertune, changed gear oil, new water pump and generally serviced everything.
The engine has been running well, but on Friday while running at WOT engine developed sudden severe misfire. Seems to run OK to about 2500-3000 rpm, then just falls flat and won't rev any more. I can just hear the misfire at 3000rpm with no load but under load it's a dog. Idle seems same as usual. Something must have failed but I'm buggered if I can tell what it is. I have run it on the shore (no load) and checked for sparks on all cylinders, all OK.
I have measured the stator and trigger, resistances all OK and tachometer still works fine. Checked and cleaned all connections to switch boxes. Re-checked compression, no change, still all about the same. Checked overheat switch for short, works fine. I'm completely stuck, what could it possibly be? I have not checked the reads yet, but then I would have thought if I sucked a read on it would misfire on 1 cylinder all the time, which I would have thought would just mean I get 4000-4500 rpm, but it seems to run OK at lower revs. Please can anyone suggest anything which might help me solve this... I've spent the last 2 months working solidly on getting the boat going, first good day this season and bang, broken, so I'm pretty pissed at it.
Thanks
Matt
The engine has been running well, but on Friday while running at WOT engine developed sudden severe misfire. Seems to run OK to about 2500-3000 rpm, then just falls flat and won't rev any more. I can just hear the misfire at 3000rpm with no load but under load it's a dog. Idle seems same as usual. Something must have failed but I'm buggered if I can tell what it is. I have run it on the shore (no load) and checked for sparks on all cylinders, all OK.
I have measured the stator and trigger, resistances all OK and tachometer still works fine. Checked and cleaned all connections to switch boxes. Re-checked compression, no change, still all about the same. Checked overheat switch for short, works fine. I'm completely stuck, what could it possibly be? I have not checked the reads yet, but then I would have thought if I sucked a read on it would misfire on 1 cylinder all the time, which I would have thought would just mean I get 4000-4500 rpm, but it seems to run OK at lower revs. Please can anyone suggest anything which might help me solve this... I've spent the last 2 months working solidly on getting the boat going, first good day this season and bang, broken, so I'm pretty pissed at it.
Thanks
Matt