Why does 98 softail blow oil out of breather?

Answer 1

On an Evo Softail the three major causes of oil blowby are overfilling the oil tank, a mis-timed breather gear and stiff, brittle umbrella valves in the rocker covers.

Overfilling the oil tank is fairly common but easy to cure. A mis-timed breather gear isn't as common because it only happens when the breather valve in the gearcase is installed incorrectly, as in its timing mark isn't properly aligned with the corresponding mark on the cam gear.

Over time the umbrella valves harden from the heat they sustain, which keeps them from sealing completely, allowing oil-laden mist to pass through them both ways (in and out).

The umbrella valve (one in each rocker cover) sits in a recess that's designed to accumulate and drain oil from the rocker box back into the crankcase via a small drain hole next to the umbrella valve.

Ideally the oil will drain faster than it can accumulate, thereby minimizing the amount that gets blown out the breather holes. Unfortunately the drain holes are a bit too small, so once the umbrella valves become stiff, the oil accumulates faster than it can drain.

Simply replacing the umbrella valves and slightly enlarging those drain holes will usually solve all blowby problems, assuming the oil tank isn't overfilled.

There are aftermarket kits which include not only a drill bit for enlarging the drain holes but also two little hollow tube-shaped inserts that raise the vent holes on the opposite (right) side of the umbrella valves, the idea being to keep more oil out of those vents.

In the meantime, if your biggest concern is just the oil dripping from the air cleaner and getting all over the right side of the bike, that problem can be solved by simply rerouting the two head breather holes to exit through a single hose near the bottom of the frame instead of having that oil-laden air dumped into the air cleaner.

That won't change the amount of oil coming out of the breather holes, it will simply keep it out of the air cleaner.

I'd post a link with complete illustrations for all the above but WikiAnswers doesn't allow external links.