See also in the Sportsterpedia:

• Breather Venting / Relocation (especially the sub documents at the top of the page)
• Using / Diagnosing with a Slack Tube (Manometer) (to test crankcase pressure)

• 79-85 cam covers can be modded to accept a secondary breather vent hose.
  • There is a plugged hole at 6 o'clock below the 10:00 vent line (to A/C) in the cover (when looking at the cover installed).
  • Older bikes were originally plumbed into the A/C from a vent in the 6:00 position with a metal tube as a vent dumping to atmosphere. ¹⁾
    It would mist a small amount of oil while riding but not a major oil slick.
  • 79-up Ironheads can benefit from removing the 6 o'clock plug from the cover and adding an additional breather hose there.
    It is said to allow the crankcase pressure to evacuate faster, resulting in the motor being a little more free-revving.
    A side benefit is that it will largely eliminate the “oil in the air cleaner” syndrome, assuming the original breather is left in place.
    You can terminate the extra breather hose under the engine. However, it will drip a spot or two of oil there when parked or after a “spirited” ride.
  • In the pic below, you can see where the oil normally drains back into the engine.
    Air and oil mist (oil that gets past the oil slinger on the generator) gets blown into the red valve.
    In the cavity behind the reed, oil drops out of suspension and drains back.
    At the same time, the air is purged out the vent hose.

• Considerations:
  • With the cam cover off, you can see a tiny hole drilled at the bottom of the cavity that by-passes the breather vent.
    This hole drains separated oil that collects in the cavity back into the motor.
    Be aware that opening the bottom port in the breather defeats the oil drain-back feature.
    A hose connected to that position can still dribble oil out in small droplets from time to time.
    If the motor is pushing oil out the breather, you may have a ring seal or other problem.
    Adding a second breather hose will not fix that.
  • Engine crankcase pressure is designed to have a slight vacuum with the engine running.
    That is a little confusing as it is a variable (mean vacuum) pressure measurement based on ring seal and RPM.
    You'll have the most vacuum at idle. As the RPM rises, you'll have less vacuum.
    Around 5000-5500 RPM, ring flutter will turn the vacuum into a positive (pushing) charge.
  • All you are affectively doing with this mod is enlarging the vent outlet size past the breather valve.
    Note: if your spark plugs get real black and cruddy looking, you might want to remove the internal breather valve and install and external breather valve.
  • It's never a bad idea to periodically test crankcase vacuum with a slack tube (manometer). That way you know what the mod is doing to the inside pressure.
    See the link at the top of the page.
  • If installing an external breather valve.
    If you decide to go this route, remove the internal valve as it will also change crankcase pressure with 2 valves in line.
    Then, you might want to consider installing something like the external Krankvent or others.
    Click here to see the Converting Head Breathers to Cam Chest Breather page in the Sportsterpedia.
    That page is for an Evo conversion to cam chest breathing but the Krankvent and plumbing is shown.

• Installing the extra hose:
  • You do not need to remove the cam cover to do this. The 6:00 o'clock position has a 1/8“ NPT Allen head plug.
    (same plug as on the corners of the rocker covers)
    
  • Simply remove the plug and replace it with an 1/8” NPT hose fitting and hose.

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• Moving the line is relatively easy.
  • The 10:00 o'clock position has 1/2“ NPT threads in the cover.
    You can pull the plug, install a fitting and lower hose, remove the upper hose and install a 1/2” NPT plug in it's place.
  • Both upper and lower vent holes access the internal valve equally and function the same way.
• Considerations.
  • However, plugging the 1/2“ NPT outlet and using the smaller 1/8” NPT outlet may reduce crankcase vacuum.
    You also may get a little more oil drips simply because it is at the bottom (gravity). ⁴⁾
  • All you are affectively doing with this mod is reducing the vent outlet size past the breather valve, reducing mean vacuum.
    This WILL affect crankcase pressure.
    Reducing the air escape area past the vent creates less mean vacuum in the crankcase (more positive pressure).
    This is good for oil scavenging but not as good for ring seal.