working on my son's yz125..fixed the carb but their is black oil blasting out of the exhaust all over the rear wheel..he says its 40 to 1 mixture..any ideas what would cause this..?? Gotta be the mixture somehow i would think..?? Thanks, Mike

bad rings would cause oil blow by

I would start by checking compression pressure, should be 120 or so, and then a leak-down test.

This engine is a 2-stroke with a wet sump crankcase with it's own oil supply, I think, in which case it has a crankseal that may be leaking, allowing more oil into the process.

Either one of those problems can cause partially combusted (black) oil problem you described.

The racers use anywhere from 30:1 to 40:1 so your son is accurate there.

Just my best guess, hope it helps.

If you have the oil injection system that a lot of the two strokes have check that out. Also when you were working on the carb you might have missed a linkage to the oil injection. You can run the synthetic oils at 50:1 or even higher. They run 100:1 in model aircraft and it works fine for them with out all of the oil problems. In any case you have too much oil going in. If you have a separate tank for the oil you have the oil injection.

Nightstar,

Not real familiar with the motocross bikes, but spent a lot of time wrenching on my Skidoo, which was also a two stroke.

Is the 125 oil injected? If so there may be a second throttle cable or other type of linkage running to the oil injection pump that will allow for some adjustment of the oil ratio. With the sleds, tightening the cable meant you were increasing the amount of oil being injected.

When set too oil rich you will get a good amount of black crap out the exhaust. With our sleds we would, fill up both oil and gas and then measure how much oil was required to refill and compare that to the amount of gas that we put in. It was just an average ratio for the ride since the amount of oil injected varies with throttle position. At idle the sleds could run as low as 100:1, but at WOT they would be about 40:1.

One word of caution...if you do decide to lean out the oil injection ratio remember that injection oil is cheaper than crank bearings.

Hopefully you can find a service manual on line...that would be helpfull too.

Best of luck.

definately not oil injected.. i appreciate the input..i will check compression but it runs like a bat out of hell so im guessing that should come up normal..he does run synthetic m/c oil in it..golden spectrum i think.. the only oil source is the mixture..

Are you sure the oil was mixed correctly? Ya might want to start with some fresh mixed gas.

like everything else Davej,you cant be sure unless you do it yourself..

Make sure reed valve and cage are ingood shape

I ran a YZ 250 for years....never had to replace rings or piston. I broke in with 30 to 1 mix and that was a lot. Eventually went to a 50 to 1 with high quality oil...don't skimp on oil quality especially on a 125 they make lots of horsepower for such a little piston and need to be well lubricated.

one of the weird things is how adding oil to the fuel can actually change the mixture because the oil actually takes the place of the fuel which leans out the mixture (someone correct me if I'm wrong)because there is only so much that will flow through the jet and if oil and fuel are mixed together too much oil leans out your mixture.

Check that you air cleaner is clean.
check compression
check head gasket
make sure the choke isn't some how closing

if compression is good, its gotta be another factor that influences the mixture...black smoke is rich mixture due to not enough air, too much fuel which usually doesn't just happen unless you change the jets or possible the needle in the carb is stuck somehow letting too much fuel in. My gut tells me its a dirty air cleaner or poor compression if nothing else has changed.