Yanmar 3GMF: Mystery Drop in RPMs
Fellow Sailors, I am perplexed by a drop in rpms on my old (but well maintained) Hunter. Any advice would be appreciated!
I noted a drop in the past year of rpms on the Yanmar 3GMF in my 1985 Hunter 34. After purchasing the boat in 2008 (and during summer 2009 sailing), I noted the engine would rev to 3400-3600 in neutral. Under load and WOT, I reached rpms of 3200+. This would allow us to cruise at about 7 knots.
In summers 2010 and 2011, the boat would still rev to 3400-3600 in neutral, but under load and WOT, rpms would max at 2600-2700. We would still attain speeds of 6.3-6.5 knots. I've had a technician come out and do periodic mantainance (fluids, filters, impellors, hoses, etc.). He noted that the true tach reading was about 500 rpms higher than indicated on the tach.
I have the bottom cleaned several times a year, noted no excess black smoke in the exhaust, and don't have unusual vibration, noise, or difficulty at start-up. I tried to inspect the exhaust elbow, but could not get the hose off easily - i did take the smaller hose off the 90 degree raw water fitting and saw it was clean inside. The elbows and fittings look in new shape (as if they were replaced several years ago).
Given the previous performance - higher rpms and speed - I don't think the cause could be the prop pitch. My current thinking is carbonized injectors, but I would have expected to see more symptoms listed above. Before I make the service call, I would appreciate any comments or similar experiences. Given the options on the boat...no fuel gauge, no engine hours meter...I don't have some of the metrics (fuel usage, etc) that would be helpful!
Thanks very much!