OK, today I've been back down to Zara to try and sort the problems out with lack of speed and the engine revving up and down on its own accord.
I have changed the exhaust manifold for a known good one (the original had 3 or 4 small pinhole perforations possibly causing seawater to enter the exhaust port?)
I have renewed the fuel lift pump for a known good one.
Taking her out, at about 1/2 throttle (appx 2000RPM) no problems, she behaved quite happily, no over revving, no soot/black smoke, all happy.
At about 2/3rds-3/4 throttle (about 2300RPM) problem manifested itself again. The engine speed would reduce by about 400-500 RPM and then after 20-30 seconds would speed up and then settle at the original 2400RPM for about 2- 3 minutes, before repeating the cycle. I didn't notice any black smoke from the exhaust at this stage.
The maximum RPM I can achieve at the moment is 2600, should be nearer to 3400 I believe?
Returning the throttle to about 1/2 way gave us a comfortable engine speed, no issues and a speed of appx 2.5 Kts. I took a sample of the exhaust water and it was fairly clear, no particles, just a very mild grey colour (caused more by aeration of the water I guess)
We then powered up to full throttle, let the engine run at that speed for a few minutes (before the engine revs dropped) and took a second exhaust water sample. This time the water was a very dirty black colour, with particles of soot contained within the sample and floating on top.
So conclusion is that we are either severly overpropped, or we have a build up on the prop or the hull.
So I took my underwater digital video camera and shot the following video:
Houston, looks like we have some fouling on the prop!!
Can you have a look at the video and give us your opinion please? The hull looks fairly clean, but judging from what I can see ( at about 23 seconds in ) we have a build up of weed and several barnacles on the prop.
Would this be a potential cause for the revving up and down of the engine on its own?
I understand that we are swinging a 14x12 prop, through a 3.26:1 ratio gearbox, a waterline length of 19 ft and a weight of apprx 4500 Lbs. Propcalc suggests a 14 x 10 prop I think, so looks like a re-pitching of the existing prop will be called for? Can we also reduce the diameter of the prop to increase the speed it rotates at, thus allowing the engine to reach its max RPM of 3400?
Any advice sought.
Thanks for any input.