Hi all,
I need some engine detective help here. Odd series of events: Hauled the boat, did the bottom, aligned (inboard) shaft coupler to engine. Put boat back in the water and things were cool. One week later (after a week of just sitting) engine started to randomly drop RPM's and come back up again, along with clunk noise around transmission (Velvet Drive). Thought this might be engine misalignment after boat sat, so re-aligned shaft (needed it). Problem still there -- so far I've disconnected the shaft and run it and even pulled the transmission. Running with no load at initial start engine sounds very happy. After a couple of minutes there is a random quick drop in revs along with a clunk sort of noise. Using the old dowel to the ear trick I've listened to all the add-ons (water pump, generator...) and the bearings sound cool and the thunk isn't coming from there. The thunk was louder with the trans still in place (thought it was the trans at first) but is still there without it. Suspect that the increase in noise with trans was gear backlash at rpm drop. My suspicion is this: Something is binding briefly, probably a bearing, then freeing-up and coming back. Don't know total hours on engine -- did an oil and filter change about a year ago after first getting boat -- she's due for another. I'm thinking it may be rebuild time, but I'm also thinking that it might be worth flushing the engine out with part clean oil, part kerosene and then putting in fresh oil just in case there's a blocked oil channel somewhere -- this might give me a little more time pre-rebuild. Any thoughts about what other causes there might be for the noise/rpm drop? Compression tested good and even on all cylinders when last done. Problem start after warming up a bit but probably before thermostats open -- thus I'm thinking oil...
Any help much appreciated!
I need some engine detective help here. Odd series of events: Hauled the boat, did the bottom, aligned (inboard) shaft coupler to engine. Put boat back in the water and things were cool. One week later (after a week of just sitting) engine started to randomly drop RPM's and come back up again, along with clunk noise around transmission (Velvet Drive). Thought this might be engine misalignment after boat sat, so re-aligned shaft (needed it). Problem still there -- so far I've disconnected the shaft and run it and even pulled the transmission. Running with no load at initial start engine sounds very happy. After a couple of minutes there is a random quick drop in revs along with a clunk sort of noise. Using the old dowel to the ear trick I've listened to all the add-ons (water pump, generator...) and the bearings sound cool and the thunk isn't coming from there. The thunk was louder with the trans still in place (thought it was the trans at first) but is still there without it. Suspect that the increase in noise with trans was gear backlash at rpm drop. My suspicion is this: Something is binding briefly, probably a bearing, then freeing-up and coming back. Don't know total hours on engine -- did an oil and filter change about a year ago after first getting boat -- she's due for another. I'm thinking it may be rebuild time, but I'm also thinking that it might be worth flushing the engine out with part clean oil, part kerosene and then putting in fresh oil just in case there's a blocked oil channel somewhere -- this might give me a little more time pre-rebuild. Any thoughts about what other causes there might be for the noise/rpm drop? Compression tested good and even on all cylinders when last done. Problem start after warming up a bit but probably before thermostats open -- thus I'm thinking oil...
Any help much appreciated!