Hi guys,

I'm puzzled by something strange happening to my Westerbeke W21 (1986) engine yesterday. First time like this in 3 years I used it (I run engine every weekend for several hours, change oil every year or around 100 hours).
In the morning, I started an engine, it was running for 5 hours as usual - fair vibration on 800 RPM, some vibration at 1500 RPM and more or less smooth at 2200 RPM. I stopped an engine for 2 hours.
Started it again after 2 hours and it started running smooth as butter - no vibration at all - tachometer went crazy - it showed 1500 RPM on lowest gas setting, and went up to 4000 RPM as I added more gas (not even maximum setting) but I did not feel it was running that fast (max RMP for this engine is 3000RPM and you know it's at maximum if you go to this limit), just running very smooth.
I run it like this for 1-2 min but it did not feel right. I'd be glad to have engine running that smooth but it's probably a wishful thinking for 1986 diesel.
I started it again, it did this again. I stopped it for 5 min or so, started again and it was normal for the rest of the trip (2-3 hours).

any ideas what this can be?