300 HP Yanmar - black smoke and rpm

[Yearwood]

We have a 300 HP Yanmar with about 1450 hours on it. It has been well maintained. This year we have been noticing a little black smoke commencing at about 3000 rpm. I am certainly aware of the various possibilities that can cause this (including poor fuel) but wonder if anyone has had a similar problem and determined a cause?

Also we run WOT at 3400 rpm. What do other people run?

Thanks

[chiefrcd]

I also have the 300 6lp which is basically the same engine as the 315, just rated a little different. Occasionally I do get some black smoke, not a great deal but looking back on the horizon I can see some. Have you washed your turbo? I had my marina wash mine and i seemed to help. As far as the rpms go, if I have a clean/new bottom I can get about 3850 rpms WOT. This is about 22-23 knots. I usually run mine at 3000 and this seems to be a very good rpm for speed and fuel economy. My boat is a 1999 with 920 hours.

Have you changed out your timing belt and tensioners yet?

[Yearwood]

Our hp is 300. Yes, washed turbo, but I know it could be dirty again. Changed air filter, etc. etc. Also did timing belts. We run about 18 knots +/- at 3400 rpm.

Thanks.

[Jack28]

Have same engine I run @ 3600 18+ knots. Black smoke could come from restriction in fuel system. My full load rpm is 3900. I found out last weekend that my fuel gauge does not go to empty it "ran out" at 1/8, I knew I was low on fuel but I mis read log numbers. 132 gallon tank has usable of 128 not the best way to find out. Not a big deal, was on plane and started to studder so I thought I could just idle to the slip ran at idle couple of hundred yards and quit, had to throw hook and had friend pull me in. Oh well now I know. JRG

[RobS]

Also we run WOT at 3400 rpm. What do other people run?

Thanks

Do you mean the max you run at is 3400rpm or she'll only turn 3400 WOT? If she is only capable of turning 3400 WOT, and the tach is accurate, there is too much load on the engine. Should turn 3800 to 4000rpm WOT & loaded.

[Yearwood]

3400 is what we use WOT. I know it will run up move but was just wondering what others use.