Hey people, let me know if this topic is in the wrong place or has been covered previously as I'm new to this (only my second post).
What are the current thoughts on Sportster (2004) crankcase vents?
Background: I run a 2004 883 roadster fairly hard in mostly reasonably warm conditions (Australia). I have an NRHS 1250 kit (the one with iron lined alloy cylinders and total seal rings) on the shelf beside a Mikuni HSR42 carb, which I am planning on installing soon(ish), but I keep reading threads about people with 1200/1250's ect spewing oil out of the breathers into the air filters, especially on hard runs. I will not installed the kit or carb until I have a reasonable chance of doing so without excess oil breathing problems (and yes, I have read everything I can on the subject but am still a little confused as there are so many conflicting opinions; maybe I am missing something).
With the 883 I do not get any breathing problems, and am currently running a brass 'T'in the cam chest breather to oil tank line (called the vent line in the manual) to an automotive PCV valve and then to atmosphere. Have never had any oil drops/mist out of this line.
My question is this: Has anyone tried running the original umbrella seals and an aftermarket horseshoe pipe on the head breathers to a 'T' fitting at the top of the cam chest to oil tank line (the manual calls it the 'vent' line) instead of venting the horseshoe pipe to atmosphere (or near the rear tyre)?
My feeling is that it can vent as much as it likes in there, it is still a closed system without any chance of ingesting outside contaminants, the umbrella seals only let oil mist out of the heads and no mess.
Thoughts people?

 

What will it accomplish to vent the crankcase to the crankcase?

Where will the blow-by go?

 

I'm not sure where that oil tank vent line goes but if it is attached to the crankcase then aswracing is correct, all you will be doing is pressuring up the crankcase and oil tank with blow-by which is no good, you have to allow that pressure from the breathers to get to atmosphere.

 

Excuse the crude drawing, no cad on my laptop.

I'm guessing that this may be what he's trying to describe.

 

There is a bit more to this than seems to be suggested above. The breathing referred to is to allow air to be admitted into and then expelled from the crankcases. Every time the pair of pistons go up the cylinders air has to be admitted into the crankcases, then expelled when the pistons go back down. So a breather has to allow air to flow in both directions, which is why they are normally connected to the air filter housing. On most road bikes that will continue to work after mods. Worked fine for many years on my 89" Evo.

 

This is what I found when I did my 1250 change over. This pretty good layer of carbon was caked on top of my pistons with just 5000 miles of running with the breather connected to the air breather. I no longer run that set up. I have a horse shoe connected to a small external breather. No more oil mist going into my engine.

 

You may be correct, but if that's what he's proposing, then it has other issues.

For one thing, the oil tank is no longer vented. That hose at the back of the cam box cover leads to the oil tank and provides it's vent.

There's another problem with this approach, too. Since there's no check valve in that hose that comes off the back of the cam box, opening it too the atmosphere allows the crankcase to suck in air as the pistons go up. That causes an inhalation/exhalation cycle that tends to pump oil out the breathers. This is why you see so much oil come out of breathers when the umbrella valves are bad, the inhalation/exhalation pumps it out. He's basically removed the check valves from the picture and opened up the crankcase to the atmosphere in both directions.


The function of the umbrella valves is to keep the crankcase from inhaling air when the pistons go up.

The way the system is designed, if you had perfect ring seal and perfect umbrella valve seal, the crankcase would exhale on the first down stroke, and then the crankcase pressure would cycle back and forth between a vacuum (pistons up) and atmospheric (pistons down).

In reality, though, some of the combustion gases get past the rings. Therefore what you get out the breathers in a properly functioning system is a small net outflow equal to the engine's blow-by.

You get very little inhalation, though, not if the umbrella valves are functioning properly.

 

Thank you everyone for your responses. Unfortunately I do not seem to have described the situation very well and being a new member am not able to post photographs to provide clarity. I will try to make it clearer.

ASWracing: Lines 8 & 9 of my original post described the present venting system which will be retained in addition to the proposed new line from the horseshoe pipe. It is a line vented to atmosphere with a PCV valve stopping the ingestion of air and foreign matter to the engine.

Hotlap: That Harley vent line goes from the cam chest to the bottom of the oil tank but an internal tube takes it further up to above the oil level in the tank. This is the line that my present venting and PCV valve is plumbed into with the 'T' union.

Hexnut: That carbon on the pistons situation is the one I would like to avoid.

cHarley: It is a little more complicated than your drawing, which is an excellent idea, by the way. Unfortunately I am not able to include a drawing either. If you imagine your drawing with the inclusion of an oil tank and a line going from the cam chest to the oil tank (the original Harley vent line) and a line plumbed into that with a 'T' union that goes up under the top of the frame backbone higher than the engine (to allow any oil mist that gets in there to drain back to the cam chest to be scavenged) then turning down to exit under the bike with a PCV valve in line. Any blow by should vent out this line. I took a photo of the end of this rubber tube today and it is completely dry - no oil mist at all. Then, from the horseshoe pipe in your drawing, take that line to the Harley vent line/my present vent junction (the 'T' union).

grbrown: I will be retaining the Harley umbrella valves (or seals, as some call them, breathers according to the Harley manual) which are a one way valve/seal allowing pressure out but nothing into the motor. In addition to these there is the PCV valve in yhe system thus doubling the protection.

Hope this explanation clears up any confusion for you good folk. When I am allowed to post photographs I will in an attempt to make it even clearer.

Cheers, Will

 

OK, I see what you're getting at now.

When you take that line that vents the oil tank to the cam box, and you vent it to the atmosphere, you've basically bypassed the whole top end breathing arrangement. The crankcase pressure, which comes from blow by, no longer needs to go up the pushrod tubes and out the vents in the heads. It can just go straight out the back of the cam box.

And worse, it has no check valve in that path. So it can suck in air, which is something you'd like to avoid. Now I realize you're talking about putting a PCV valve between that line and your breather filter. But a PCV valve is not a check valve. Google it and read up on how a PCV valve works and what it does. There are actual check valves readily available that would be a much better choice than a PCV valve. You can even buy a breather filter that has an integrated check valve.

But assuming you use an actual check valve instead of a PCV valve, basically all you're accomplishing with this is you're venting off the cam box instead of off the breathers in the heads. The blow by has no incentive to come out the head vents anymore. The umbrellas are going to do nothing. You might as well block the head vents.

Also, be aware that 4-speeds all vented this way, off the cam box. They had an integrated check valve in the cam box itself, and no vents in the heads. HD moved the vent to the heads in 1991 to reduce the amount of oil coming out of the vent. There's a lot of oil flying around in the cam box, more than in the rocker box.

So in the end, assuming you use an actual check valve in your vent, you're basically undoing a 1991 design change.

 

As I cannot yet download pictures to show what I mean can i direct you to this website which may provide a little insight:http://www.thunderpress.net/editoria...2012/04/12.htm
Cheers, Will