4jhe not reaching full rpm

[Newfiebullet]

I'm baffled, my yanmar, 4 Jhe, is not reaching full rpm. I have had this boat for four years and the yanmar has worked flawlessly. It started last spring. With full throttle, she will only reach 3000 rpm, instead of the normal 3600 rpm. I changed the fuel filters and it did not help. The odd thing is that in neutral the engine will rev to 3600. But it will not go to full rpm under load. Other than that the engine seams to be working normally.

[djmarchand]

Dirty bottom, dirty prop, plugged exhaust elbow, throttle not advancing to stop.

David

[jstevens]

Mostly likely cause is the prop. It may be badly fouled or damaged. Did you hit anything with the prop. Have you hauled the boat since this started and inspected the prop? Do you have a feathering prop? If so the pitch may have changed. Do you have a clean exhaust (no unburned fuel) at 3000 RPM?

I assume you were able to reach 3600 RPM underway before. That is the max RPM rating for the engine. I have a 4JH4AE and 2900 RPM is the max I can achieve with a clean prop. I normally run the engine at 2100-2400.

John

[Oceanride007]

OK, all diesels are governed, if you can get 3600rpm without load, then with a droop setting of say 7% you should get 3300 Rpm, (check with manufacturer what it is, who knows it maybe 10%). By the way I would never rev the engine to design maximum revs without load. You may check the throttle to see if it has capacity for more stroke, in that case if you must, adjust. If on the other hand something has happened to increase the load, such as hull/prop fouling, increase draft, or you are not motor sailing the limitation would probably be either exhaust temperature or Coolant temperature, you should not exceed any design limits imposed by the manufacturer, however just because the maximum revs is 3600rpm it doesn't imply that your engine should do it, many a good installation is deliberately derated so that design power occurs lower in the rev range.
I would suggest that if your vessel is pushing thru the water at good clip, do nothing.

[sww914]

Originally Posted by djmarchand

Dirty bottom, dirty prop, plugged exhaust elbow, throttle not advancing to stop.

David

Yes, always check the basics first.