Black oil

A lean burn engine keeps its oil very clean for long periods.  So the answer is get your mixture as clean as practical ,so as to not produce too much carbon . I believe one of the Castrol oils meets the C4 specification for low ash production so that could be helpfull too.

I would think it is the metal particles from your newly rebored engine.

What I would do is buy 10 litres of cheap 20-50 and do some more oil changes until it is run in.

Also I have found the Mapp oil filter from the USA that uses a T160 oil filter element to be very good. 

You didn't say what piston and rings were used. I put JP in my ES2 and it smoked for ages. I eventually changed the JP to GMP and with the Mapp filter I was surprised how clean the oil was, is.

Did you remove and completely clean the inside of the oil tank? If the bike needed a rebore, I would guess the oil tank would be pretty manky as well.

I also concur with Robert, if it is running rich that will blacken the oil pretty quickly. 

Don Anson

Melbourne 

Hi Richard,

   Did you thoroughly wash out your oil tank before re-filling it? The old pre detergent oils turned any contaminants into sludge which collects in the bottom of the oil tank and with only the standard wire gauze filter this can be disolved by new oil and circulated around the engine.

Hi Don,

   I think our posts crossed over, but I will leave it on anyway.

I agree with what has been said, so let's keep it clean. (No pun intended)

I uses a Fiat car piston in my 530cc ES2, old bore completely worn out @ +60. Fact 1.

Fact 2 is that it took over 2,000 miles of running to get the oil nice and clean.

FACT 3 ,with almost 4,000 miles on  clock the oil has remained clean.since running in ended.

FACT 4, finally, I could have used cheaper oil more frequently but stuck with Castrol SEA 50..

Our engines' combustion efficiency is not as great as modern computer controlled motor vehicles.  Soot (carbon) is an inevitable product of combustion, and some will get into the oil.  Soot particles are extremely small to begin with, and do little if any harm until they clump together. Modern oils have dispersants which prevent this.  The particles remain in suspension because they are too small to be removed by filters (assuming you have a filter...most Nortons don't). So the oil goes black.

Our modern machines have virtually perfect carburation at all times, so their oil stays cleaner because there is less soot getting into the sump.

I think we need to be careful to manage our expectations.