A friend recently described a problem that they were having with their Volvo Penta, the symptoms he described were that their Volvo Penta D1-30 won’t rev it would struggle to reach 2000 rpm under load but it would freely rev without load, he also said that he was concerned with the black staining on the hull in the vicinity of the exhaust, he attributed this to the black smoke emanating from the exhaust when the engine was under load.
Most of the research he had done implied that the problem was with:
Dirty or blocked fuel filter(s).
Water of other contaminant in the fuel.
Even a dirty prop.
It occurred to me that I had seen this before on other boats, including G-whizz. The problem there was a blockage in the elbow at the back of the exhaust manifold where raw water is mixed with the exhaust gases. The offending component is circled in the accompanying picture.
The mechanic who rectified the issue on G-whizz where it’s Volvo Penta D1-30 won’t Rev by replacing the elbow explained to me that the problem is a build up of carbon inside the elbow restricting the flow of exhaust gasses negatively affecting the engine’s performance.
He went onto explain that he sees the problem quite regularly, especially on boats that are raced locally; where the boat is cruised or used for off-shore racing the problem is more with corrosion than with a build up of carbon. His conclusion to this was that the locally raced boat’s engines are run for a short time to leave the mooring, get to the start area and then turned off, then again started post race for a short time to return to the mooring. Conversely boats that are raced off-shore or cruised have engines that are started and then run for a considerable time, allowing for all components to get to proper operating temperatures reducing the potential for carbon build up. Going on to say that this issue is not unique to Volvos but also prevalent in Yanmars as well as other engines that have a mixing (Raw water and exhaust gas) elbow.
The conclusion to all this was that our friend had the exhaust elbow replaced and their problem was solved, he said that the mechanic that did the job told him that it was one of many that he had done recently, it would appear that this issue is well known amongst those that fix boats, less known amongst those that own boats. If your Volvo Penta D1-30 won’t Rev this could be the simple, if not exactly an inexpensive solution.