I recently decided to change my own oil. I drained out the old stuff and poured in new fluid, and the bike ran OK, but about 10 miles later it started smoking quite a bit. That’s when I rechecked the oil and realized it was too full—by a lot. I drained the oil down to the correct level, but the bike still smokes and seems like it might be down on power. What happened, and what can I do to fix it?
When you put too much oil in an engine, it increases the pressure in the crankcase. This pressure rise might be enough to rupture the oil seal at your output shaft if it weren’t for the fact that your engine’s crankcase is vented via a rebreather system. This rebreather plumbing helps balance the pressure inside the crankcase as the air inside it heats and cools during normal operation.
When engine oil is overfilled, however, this rebreather circuit can also serve as an escape path for excess oil. And since the rebreather typically empties into the airbox, you may have pumped a fair amount of oil into your intake system. While it’s not likely to do any major harm, oil in your airbox can saturate your air filter, causing a loss of power, and get sucked into the combustion chamber, causing a smoky exhaust and eventually a fouled spark plug.
Check to see if your air filter is saturated. If it is, you’ll need to replace it. If there’s oil in your airbox, you can wipe it out with a paper towel. You should also inspect your spark plug if it’s easily accessible. But before you do any of that, invest in a service manual for your model (from Haynes, Clymer, or your bike’s manufacturer), and get familiar with the steps involved in your bike’s maintenance procedures so you can count on your next project going smoothly.