125-200cc WR125 1999 Oil to Fuel Mix Ratio Question

Hi to all, I recently bought this bike. I downloaded the user manual from the website but could not find the one for my year of make 1999. The new model 2013 WR125 suggests 4% oil to fuel. The guy that I bought this bike told me he used to run it at around 1,2% oil to fuel. He told me that 1,5% oil to fuel will be too much.

I am afraid of destroying the engine with only 1,2% of oil to fuel. On the other hand the previous owner had this bike since 2001 and the bike hasnt even had a piston change yet. Since he is a friend of a friend of mine, he has no reason to lie to me as to this matter. As can be seen from the picture the bike was well looked after. Any feedback would be much appreciated.

With a good fully synthetic oil - running at 40:1 (or 2.5% oil to fuel) is what i would advise. I personally think 50:1 is a bit lean but it can be dependant on the type of riding your doing.

Initially I used 50:1 or (2% oil to fuel) and a lot of black oil was coming out of the exhaust until the spark plug died. When I replaced it, it was full of black oil. At the moment I am using (1,5% oil to fuel) or 50:0.75 and it still seems to be a lot if I judge from the black oil coming off from the back of the exhaust. I usually ride around trees in the mountain.

If its that bad then you have probably got a jetting problem. Idling around can tend to give that black spooge from the exhaust even at leaner ratios. If you are running at 50:1 and fouling plugs under reasonable riding conditions, then it is probably suffering from fuel fouling and not oil. Also, repacking your muffler, checking your reed valves and the blow by on the piston (seen as its never been replaced) might be worthwhile.