Power not revs
TONY REPLIES... Don’t dismiss a fuel problem. The amount of fuel required relates to the power the engine is developing, not the revs. The power required to rev an engine in neutral is very little more than at tickover. An engine with a fouled prop running flat out at, say, 900rpm will use more fuel than one driving the boat normally at 1,500 or 2,000rpm.
I assume you have checked your prop is clear and tried giving it a momentary rev in reverse in case fallen leaves or weed have packed into the prop disc.
As long as you have adequate oil and oil pressure, and there are no signs of overheating, it points to a fuel system problem, but first check the gearbox oil is at the correct level and nice and clean. Make sure, too, your stern gland is not running hot.
Then replace/clean all your fuel filters and water traps. Drain the old ones into a tub so you can inspect the fuel. Take the old filters apart so you can inspect what they might have caught. If the fuel is discoloured, there is any sign of jelly like substances or there are more than a few drops of water, you have a fuel issue.
If you have an older engine such as a BMC, Perkins 4-10x, or older Lister, check the strainer and water trap under the fuel liftpump cap. Try blowing back down the supply pipe into the tank in case it’s blocked. You might have an air leak in the fuel system but, as you don’t mention difficult starting or the need to bleed the system, this is less likely.
The symptoms might indicate a faulty liftpump but check the filters first. Mitsubishibased engines (mainly Vetus and more modern Thornycroft) often use a cylindrical electric liftpump with a small filter under an end cap. These are known to cause problems when they are not changed during a service. When cruising, my boat loses power intermittently. The revs drop towards tickover and then pick up again. With the engine in neutral, even when the engine appears to be struggling in gear, I can immediately rev it up to or beyond 2,000rpm. That makes me suspect that the issue isn’t fuel-related; I only lose power when in gear and it seems to happen more at higher revs. I’m currently on a month-long holiday and if this continues it’s going to ruin it because I can’t rely on the engine.
PLEIADES, from the CB website