What would cause oil to blow out the breather on a Harley Davidson Sporster 1200 motor?

Answer 1

Sportsters vent their rockerboxes into the airbox. Why? It gives a chance for the oil vapors to be combusted along with the gasoline instead of being vented to the atmosphere.

Quick rundown on pressures. High pressure likes to flow to low pressure. That's what makes sucking soda through a straw work. Well, the crank case of a sportster is pretty high pressure. When combustion happens in the cylinder, a small amount of the combustion gases leak past the rings into the lower parts of the engine, increasing the pressures there. These over pressures are moved up the pushrod tubes to the rocker boxes. Now you have higher pressures in the rockerboxes.

Where do the rockerboxes vent into? The air box. The air box has the engine sucking air out of it. This makes a low pressure.

Boom: High pressure to low pressure.

When you rev the throttle or even let it run steady on the interstate for a while, you allow the pressures to pull more oil out of your rockerboxes into the air box, saturating your air cleaner, and later in really kickin' engines, your pants leg.

How do you fix this? Well, going with a K&N filter set up (aka Screamin' Eagle Air Cleaner) is a good first start, 'cause those air elements can be cleaned. There are other solutions that have been tried like boring out (slightly!!!) the oil return port on the rockerbox. You can also put on a "horseshoe" that plugs the ports into the airbox and vents the rockerboxes to a small hose with a filter added to the end.

AnswerAn overfull oil tank will do it. Even if not overfull just about all harley's will blow oil out the air filter. Unless there is a bunch of oil coming out, it's not a problem

What I've found is that the air filter eventually gets saturated and then starts to leak through. The level in the oil tank doesn't seem to matter. If you change the air filter the problem goes away, until the new one gets saturated and then it starts all over again. My plan is to get a K&N air filter, which you can clean yourself and save having to buy a new air filter every 4,000 miles or so.

AnswerI had a sportster 1200 custom and now have a 85 wide glide and i can tell you keep from Twisting the throttle to much and you will notice less oil in the filter. The answer takes the fun out of Harley's but it blows oil when your wrapping it out for fun. If a person stays off the throttle you will notice that the filter will not have near as much oil in it.

A clogged tappet screen.

Buy a oil breather kit if you dont burnt oil will build up on top of the piston fauling out plugs and giving bad gas mileage and if its carberated it will clog up the carb !