This seems to be a recurring question, and we have various threads addressing it, so here is a summary and links.
WHAT IS IT?
The engine breather, or crankcase vent as Harley sometimes calls it, is to allow air out of the lower crankcase, but not in, as the pistons rise and fall. Without some kind of controlled breather, the lower end would become a 1,000cc air compressor, robbing the engine of several horsepower. Old time tuners like Jerry Branch, Tom Sifton and Dick O’Brien paid much attention to the engine breather because they knew it could give them extra horsepower if set up right.
One thing not to do with an engine breather is to simply plumb a hose to the crankcase without some kind of one-way valve or timed breather valve. It is commonly done, but it wastes power and is not good for your engine.
WHERE IS IT?
P
re-1977
There is a timed breather valve built into the oil pump drive, which vents crankcase pressure into the cam timing chest. A six-inch metal tube hanging down from the timing cover near the generator drive, at the 6 o’clock position vents that controlled pressure to atmosphere. A metal disc on the end of the generator drive gear centrifugally separates oil from the air as it is discharged overboard.
1977-78 only
The timed breather on the oil pump drive was dropped. An external non-return valve was plumbed into that vent tube sticking down from the timing cover at the generator drive. This allows air out, but not in. It is sometimes referred to as the foo-foo valve. Searching this site for foo foo or foo-foo will lead to extensive discussion of this mystical device.
1979 onwards.
The external foo-foo valve and the six-inch metal vent tube at the front of the timing cover were done away with.
Instead, a one-way foo-foo valve was built in inside the timing cover. A rubber breather hose then ran from the generator drive area of the timing cover, at the 9 o’clock position. It connected to the stock air filter so that any oil mist was fed back through the engine, making the EPA pollutocrats more happier than they were with the idea of engine oil spraying out into the atmosphere.
Many of these bikes with custom air filters simply run that hose down to the bottom of the frame and let the oil mist blow out in the time honored manner. That is fine too, as long as you are not an EPA man.
Just to add to the knowledge base, here is a pic of IronMick's internal foo-foo in his post-79 model. You can see the 9 oclock fitting and the 6 oclock fitting both enter the same cavity.
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ALTERNATIVES
There is a product called a Krankvent that can be plumbed into the lower, 6 o’clock position as an alternative to a stock foo-foo valve. But they are not cheap.
Automotive PCV valves are not really made to handle the revs or air volumes of a Harley. While a car engine is bigger, it has one piston coming down while one goes up, so not much change in internal crankcase volume, so not much breathing to be done. A Harley has two pistons and rods on one crankpin, so is one giant air compressor.
Some guys have found that plumbing in a 77-78 foo-foo valve on the later model engines improves breathing.
LINKS
Discussion on foo-foo valves and engine breathers pics etc here:
http://xlforum.net/forums/s...d.php?t=213630
I know there are other threads but cant find them right now. This pretty much covers it all anyhow. Pretty simple but seems to cause repeated headaches for such a pesky little thing.
OIL PUKING OUT ENGINE BREATHER
These seems to be a common question too. The most common causes of oil puking out the engine breather are listed below, in order of how common they are.
1. WET SUMPING
This is simply oil from the tank draining slowly back down into the crankcase while the bike is parked for any length of time. When you start the engine, the excess oil in the crankcase is fired out the breather, onto the floor, (or into your air-filter on post-79 models).
It is normal, if the bike has been parked for a week or more unused. Don't worry about it. Put a catch tray under the breather pipe before start up.
If it is too bad, you can put a new ball and spring in the check valve in the oil pump. See the workshop manual for that. Quite often though, it can just be tiny debris in the oil holding the ball valve ever so slightly off its seat.
The oil should stop puking after the engine runs a few minutes and pumps the excess oil back up into the tank.
2. OVER-FILLED OIL TANK
If the puking starts after you top you oil tank, this is probably the problem. If you fill the oil tank to the Full mark while some oil has wet-sumped down into the engine, you have too much oil in the system. The oil from the sump will be pumped back up to the tank, dribble down the vent tube to the timing cover, from where it is fired out the engine breather.
This puking will continue after initial start up until all the excess oil has been fired out, which can take a while.
The cure is to drain a quart or so out of the oil tank, run the engine for five minutes til the puking stops, then top up the oil tank to the full mark.
DO NOT be tempted to drain oil out of the sump by taking out the threaded drain plug under the front of the engine. These are notorious for stripping the threads and are very difficult to repair properly. In most cases the plug is factory installed and is not intended to ever be removed. Leave it alone.
3. WORN ENGINE
If your engine breather continues to puke oil or blow smoke after the above two things have been eliminated, your problem is most likely wear in the cylinders and heads.
Worn rings and even valve guides, can allow blowby of combustion gasses into the crankcase area, which then comes out the breather.
Usually this will be accompanied by smoke or oil coming out the exhaust pipes too.
A compression test will give some indication of top-end condition. Anything below 120psi is suspect, according to the factory manual. These bike will still run ok at even 100psi, but they will be down on power and consume oil, and blow fog out the breather pipe.
Engine breather 101 / Crankcase vent 101
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