XS650 Breather Exposed!

If you came here expecting to find out where XSMike gets his breather, sorry. This is more to show what the insides look like. :mrgreen:


All apart. I didn't know the reed valve came out. I thought it was glued in or something. I put a little air to the exit port and the reed valve popped out. Two M6 screws, Intake housing, Reed Valve, and Exit housing.


Reed valve back in the Exit housing.


Near the upper right of the reed valve you can see the drain hole I drilled. This will allow any oil that gets pushed up past the reed valve to drain back to the sump.


All assembled. The previous owner had added the barb fitting so I didn't have to.

The barb fitting was probably put in because the original pressed in part fell out, like it did on mine.

The early XS 650 PCV valves didn't have a spigot for the outlet, it was the flange only.

JimC said:

The early XS 650 PCV valves didn't have a spigot for the outlet, it was the flange only.

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That makes sense, it looks like an afterthought compared to inlet.

Just curious, aside form the stock breather hose, why do you need this or what are the benefits of the different breather?

britbike220 said:

Just curious, aside form the stock breather hose, why do you need this or what are the benefits of the different breather?

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Do a search and you'll find volumes of info on the XS 650 PCV valve and other breather valves for the Norton.

Ok, reading about it says more HP, no biggie for me to get a hp or two out of a bike I don't push anyway and helps stop leaks? I guess what I should of asked is what made people start putting these on in the first place, what leaks does it stop and though simple and cheap is there a real benefit to the average person? Oh and are there certain years of commando's this is better for. I just ask because I haven't had any out of the ordinary issues on my 73-850 in the last decade, but if it is something noticeably beneficial I'd put one on. The cnw breather seems like a lot of work and expense if there are minimal street gains. 16.00 bucks is a no brainer for some gain other than it looks cool. The posts I read seemed to talk a lot and not really define why, but I could have missed something in the posts I read, thanks.

I don't think you'll find many claiming extra HP from fitting a breather valve. Most, like me, fit them to cure those persistent irritating small weeps of oil.
Typical ones are the rocker covers, back of the barrel to crankcase and cylinder head pushrod tunnels.

Some engine don't show any of these, other have these and a few more which even the most meticulous rebuilds don't seem to cure 100%. A breather valve gives all the seals and gaskets an easier time saves the hassle of repeated strips and rebuilds chasing down every small weep.

britbike220 said:

Ok, reading about it says more HP, no biggie for me to get a hp or two out of a bike I don't push anyway and helps stop leaks? I guess what I should of asked is what made people start putting these on in the first place, what leaks does it stop and though simple and cheap is there a real benefit to the average person? Oh and are there certain years of commando's this is better for. I just ask because I haven't had any out of the ordinary issues on my 73-850 in the last decade, but if it is something noticeably beneficial I'd put one on. The cnw breather seems like a lot of work and expense if there are minimal street gains. 16.00 bucks is a no brainer for some gain other than it looks cool. The posts I read seemed to talk a lot and not really define why, but I could have missed something in the posts I read, thanks.

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Search "breather" and "comnoz".

Most common benefit is less inside pressure to blow oil out where ever. Less common is extra hp which would only show up at maxed out loads-rpms. To get best of this benefit open up the vents between sump and TS case volume and put in some sort of check valve. To exceed this put in exhaust extractor to pull stronger vacuum and maybe some extra oil smoke out the pipe too. As rpm increase the elastic nature of Brit cycle engines shows up to send waves of metal surf along seams.
Most common crank check valve is made by Krank and cost ~$90. For sure the reed valves look cooler but don't know if any advantage over inline type brake vent valves. Someday I'll put PSI gauge on and see what it shows with various combo's.

swooshdave said:

Search "breather" and "comnoz".

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Those search terms together provided an enormous amount of info, thanks Dave.

That valve is identical to the SAI(secondary air injection) reed valves on my Triumph. A lot of the Triumph guys take those off the bike, replacing them with blank-off plates, due to the 'popping' the SAI's cause on deceleration, especially if the exhaust is replaced or a different map installed on the ECU.

My $6.00 Motormite disk valve is still working fine after 10K miles.

Ya, I thought they looked similar, but the triumph ones do not have the backing plate with intake opening. I have three of them wasting away on the shelf in the garage from my daytona, but unless there is a backing plate they can't be used. I don't have a machine shop so out of luck.

britbike220 said:

swooshdave said:

Search "breather" and "comnoz".

Click to expand...

Those search terms together provided an enormous amount of info, thanks Dave.

Click to expand...

Yeah, I have a feeling this thread will bloat to 5-6 pages before it's done. :mrgreen:

oh ya at least, but this is where I got all of my answers, in the first few pages.........crankcase-breather-t3864.html

To put it quite simply, you must realise that your engine is not just an air pump using the tops of the pistons, but the underneath as well. On the power stroke, the pistons are forcing air out of the breather and then on each of the upstrokes, they are sucking air back into the crankcase. If you think that this is not sapping any power then I suggest you think again. Pump up a tyre using a hand pump and see how much power it takes on every stroke. The one way valve means that the air you are pushing out of the breather on the downstroke cannot be sucked back into the crankcase because the valve closes when the engine tries to do this. After a short run of maybe ten seconds or so, the air is evacuated from the cases. I certainly felt the difference in power when I fitted my one way valve in my breather system. My troublesome tach drive oil leak disappeared as well. Matt Rambow says that it is provable on the dyno and worth a couple of horsepower. I believe him. If/when I do another rebuild, I'll be sending Matt my cases (and probably everything else as well!) for this mod.

If you think about a four cylinder engine or a 180 degree twin, then the pairs of pistons are running in opposite directions, ie, while one is rising, the other is falling, meaning that the volume of air in the crankcase is not changing like it is in our 360 degree twins. I also suspect that this is why our old Brit stuff leaked more oil than anything else, more crankcase pressure. Tell me I'm wrong.

As sure as the earth is round, the one way breather valve is beneficial to the Norton. I hope we have no flat earthers among us.

Well I am going to try one. It looks as though the only way to install the CNW breather is to machine a hole in the case, which would require a tear down or engine removal? If so that would not be an economical solution unless one is rebuilding a motor. I am going to order an xs650 breather if I can get one and I really don't expect outright noticeable results, but if long term effects include less wear anywhere in my motor and less stress it is welcome for the cost.

The most noticeable effect after installing my XS 650 PCV valve was the lack of oil in the primary (I run a belt) and no oil leaks from the usual suspects, such as tach drive and tappet covers. The power gain would have to be measured by something other than seat of the pants. As for less wear and stress, that would be negligible, at best. Although, I suppose pushing air on the piston down stroke does create more stress.

JimC said:

Although, I suppose pushing air on the piston down stroke does create more stress.

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What does this mean? If the valve is working the piston pushes out the pressure for a couple strokes and then it's done (more or less). With the regular breather it's constantly pushing against the pressure. This is most likely negligible pressure on the rotating masses as they are more interested in the combustion pressure on the other side of the piston.

Next time I get a chance I'll post a video of the breather in action. I used clear tubes on either side of the breather and you can easily see the action of the breather. For a couple strokes you can see and hear the valve working to evacuate the crankcase pressure and the oily mist that comes out. Then once the pressure equalizes it's not working as much and everythings happy.

It's pretty cool.

If you have the valve installed and don't have clear hoses you can hear the effects my starting the bike with the oil tank off. In fact I can hear it just when kicking it over.