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  #1  
Old 30th May 2009
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Hopper Hopper is offline
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Default Engine breather 101 / Crankcase vent 101

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?


Pre-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.


[/



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.
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Last edited by IronMick; 18th September 2010 at 07:31..
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  #2  
Old 30th May 2009
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great explaination Hopper.
I do remember another thread that came up with an alternative to venting the air back into the carb.
It would be nice if I could find it and link it, but I am a dummy this early in the morning.
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Old 30th May 2009
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I know this is the Ironhead area but for completeness... the more recent bikes have umbrella valves in the rocker boxes which are one-way valves. These exit through vents in the top of the cylinder head and these are fed into the carb mouth to be burnt. A lot of folks don't like this shite in their carbs and add after-market breather kits to plumb the vents away to a filter, catch-can or onto the road.

Its perhaps also worth noting that when NRHS dyno-ed Kranvents they found no power gain. However they are effective at reducing pressure in the crankcase area which reduces gasket leaks and also they are very useful when the weak stock foo-foo or umbrella valves inevitably fail.
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Old 31st August 2009
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Good explanation for each setup Hopper. One thing I'd love to know and see added is if there's a way to determine if your valve is working correctly and not plugged. Is it as simple as feeling for air being forced out of the valve while the motor is running? Would you feel it at idle or would you need to increase the RPM's?
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Old 31st August 2009
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Quote:
Originally Posted by HardcoreZ28 View Post
... if there's a way to determine if your valve is working correctly and not plugged. Is it as simple as feeling for air being forced out of the valve while the motor is running? Would you feel it at idle or would you need to increase the RPM's?
My experience is that you can feel air pulsing at the 6:00 and/or 11:00 o'clock holes while the engine is idling.
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Old 31st August 2009
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Likewise, pulses of air are readily detectable at the end of the tube on my bike at idle. If you feel her puffing the crankcase is breathing.
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Old 1st September 2009
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Quote:
Originally Posted by HardcoreZ28 View Post
Good explanation for each setup Hopper. One thing I'd love to know and see added is if there's a way to determine if your valve is working correctly and not plugged. Is it as simple as feeling for air being forced out of the valve while the motor is running? Would you feel it at idle or would you need to increase the RPM's?
On mine, 77 model with extrenal foo foo valve, i can hold my finger on the end of the vent tube at idle and little pulses of air are enought to lift it off the end of the tube.

It seems to settle down with more revs. I think the cases develop a partial vacuum and just stay at that pressure with the intervals between up and down stroke being less.

That is on a recently (10,000 miles ago) motor with probably minimal blow-by on the piston rings. A worn motor will blow more out the vent tube as combustion gases leak past worn rings into the bottom end.

If you have doubts, you can easily tear down the external foo foo valve on 77-78 models and clean it with carb cleaner. On later models, you can take off the breather tube and squirt carb cleaner into the internal foo foo valve.
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Old 1st September 2009
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Here are pics of the 77-78 external foo-foo valve. Or more correctly the 1977-1978 crankcase ventilation valve as the manual calls it, ie engine breather.

Located outside the case, below the genny.



Body opened and valve in its correct orientation. Vent pipe below. The top half of the valve body can stay attached to the genny case, just hold it with a wrench and unscrew the bottom nut to get to the actual valve mechanism inside.



And the foo-foo revealed in its glory, showing the spring loaded disc that does the actual foo-fooing. Don't try to pull this bit apart. Just clean it with carb cleaner and make sure it moves freely and the spring is not broken or deformed.

When reassembling make sure the valve goes in the right way. Air must be able to blow OUT of the cases, but must not be able to get sucked back in.
So the spring goes on the bottom. The flat side of the disc valve goes to the top.



This 1977-78 only. Earlier had the timed breather, with just a plain vent pipe in this location; later models had the rubber hose to the air filter with an internal foo foo valve inside the generator drive case. [/QUOTE]
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Old 28th July 2012
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my 77 does'nt have that deal -just a tube like my 76 had-do i have a possibly early 77 or did someone remove this valve,and what are the conseqences,how do I know for sure or how do I check?
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Old 28th July 2012
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Quote:
Originally Posted by shooterj2003 View Post
my 77 does'nt have that deal -just a tube like my 76 had-do i have a possibly early 77 or did someone remove this valve,and what are the conseqences,how do I know for sure or how do I check?
No. Mine is early 77 (built in Oct 76) and it has the foo foo valve. It is essential on this model.

Double check is to look at your oil pump. Earlier models with no foo foo had the square-bodied gear pump. But, 77 has the round-bodied gerotor pump. Gerotor pump has no timed engine breather like the earlier models, so foo foo valve is a must. Post a pic of your pump if you are not sure which you have (or look in a parts book etc.)

Running it without the foo foo will result in 1000cc of air getting sucked in through the breather on every upstroke of the pistons, and 1000cc of air and oil mixture being pumped out the breather on every down stroke, turning the bottom end of your engine into a big air compressor.

This robs horsepower from the engine, increases oil consumption as it is fired out the breather every stroke and increases engine wear as dust is sucked in through the breather pipe every up stroke.

Foo foo allows air to be blown out of the crankcase, but stops any air from being sucked in. So all that comes out is the small amount of blowby from the piston rings.
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