Suddenly black smoke!!

[pwilletts]

I have a 1 cyl diesel that is old but runs well..starts every time..recent valves/rings/injector rebuild ....I was running 30 wt oil but switched to 10-40,,,,suddenly at full throttle I was getting significant black smoke and visible fuel residue in the water..at half throttle it was less as might be expected,,,,i drained 10% of the oil and smoke decreased but still have too much at full throttle,,could it be that the thicker oil is overloading the engine??I plan to go back to 30wt and see...any thoughts....

[a64pilot]

10W-40 isn’t thicker. Why did you switch?
Anyway black isn’t oil.
You may have a bad injector, if it’s making full RPM

[pwilletts]

my understanding is that 10/40 at operating temp is in fact 40wt?? yes I said the black is fuel....

[Nicholson58]

When you did the rings did I you check the cylinder? Round? Tapered? Glazed? You may have trouble seating the new rings. Certainly injectors could also be an issue and stick with the oil that works.

[jkishel]

Prop fouled?

[ATC]

If it's an Yanmar, look at the mixing-elbow on the exhaust.

[jamhass]

Originally Posted by pwilletts

my understanding is that 10/40 at operating temp is in fact 40wt?? yes I said the black is fuel....

Single viscosity oils thin substantially as the temperature increases.

Multigrade oils have a relatively flat viscosity vs temperature profile. So a 10/40 oil has the viscosity of a 10wt cold, but a viscosity of a 40wt hot. This makes for better oiling and easier starting when cold, and higher viscosity (relative to a single wt) when hot, avoiding too much thinning of the oil film on the bearings, etc.

[Wotname]

Originally Posted by pwilletts

my understanding is that 10/40 at operating temp is in fact 40wt?? yes I said the black is fuel....

Be aware that it can take awhile for the oil to reach a stable temperature. I was surprised to find my 2GM20 took ~40 minutes under near full load for the oil temp to stabilise.

However I'd be surprised if it is the oil that is causing your problem; it could be it but I would look at the other suggestions in this thread first; prop, bottom, exhaust etc.

What engine is it (some engines have better injector pumps than others)?