Crankcase vent using a reed valve.

As far as I can see, that 67 Euro reed valve housing that replaces the OEM breather, has one advantage, at least in principle.
It adds close to zero volume to the crankcase volume, where any other breather check valve would add the volume of the connecting hose, plus the volume of the OEM breather itself. Which in turn should help lower crankcase pressure a fraction.
On my 2 aircooled/ 2valve Ducatis, the OEM breathers are in fact a reed valve. However, they are crankcase mounted, as the only passageways between heads and crankcase are the oil drain bores. Anyway, those breathers work flawlessly, and the Ducati engines are 99.9 % oil tight, despite having vertically split cases, just like old British twins.

My 2cents, just get the crankcase breather valve of an ol F150 truck, doesn't get any better besides no weight to it therefore no inertia issues.
I always prefer to mount the valves further away to not trap reciprocating oil fumes out of the crankcase, but then i use on my race bikes really low piston ring pressure with in unisono with gas ports.

Kind regards

Got the ebay AIR valves in. The one for a GS500 was NOS never been installed.
Branded Mikuni.
Sawed open the 4 cylinder gixxer unit. It had 2 sets of reed valves on one housing. So had to be split anyways.
Just from blow suck they will work with the vacuum valve in place, increases resistance slightly but it will still function as a one way valve.
The vacuum valve is an integral part of the aluminum housing. So no simple way to remove it. I think it could be gutted out without cutting the housing, unscrew the vacuum line cover. grind off the swage on the end of the shaft in the center of the metal disk. the larger "steel inlet tube and cover are just a tight push fit into the aluminum. Think a snug bolt pushed in the shaft and some determined wiggling would loosen, let you remove it . Then the guts of the valve could be pushed out. You could replace covers and be good to go, the housing is a bit bulky. Will get an XS on the lift and explore room, possible mounting options.
Need to shop fleabay some more, one of these reeds from a single cylinder bike would be an easier repurpose.
That's all for now.
Guess I need to look into a vacuum transducer for testing purposes. may start by tapping a vacuum port in a valve cover take a look with a vacuum gauge. There's some talk that a simple gauge starts getting confused as RPM goes up past a few thousand.
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Looking forward to seeing how this turns out. Those valves have been removed from a lot of bikes so should be easy enough to come by. My Honda 500 single has one hanging off the side of the engine, and looks like an after thought just stuck on there. But it is staying for now, need to have it to qualify for BC's collector plate program. The XS has one from Mikes, like in the pic above. Mounted low on the engine side cover, so it's not too obvious. My engine is fairly oil tight, just the usual suspects, starter seal and one cam seal have a little weep, no big deal, wipe a drip or two twice a year. Before I start it up, kick only, I kick through to clear the clutch plates, it sounds like it is farting, as the engine turns over creating a vacuum in the crankcase. Been on for years now, and recommended.


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That's helpful @kiwi thanks!
looking at the vacuum valve the normal position IS open with a vacuum signal CLOSING the port.
since the shaft's been released here the plunger is against the housing but it is held back by disks on the other end.
the seal is against the housing to the right of the disk. So again you could just leave the vacuum diaphragm in place.


This is a 79
Not a lot of room for more stuff right there.
next photos are from a 72 head
72 CCV.jpg
Top hole connects via passages to cavity above the intake valves
Bottom hole joins with passages from intake valve area and drains to base of cylinder where that oft forgotten little sheet metal shroud lives.

72 CCV Disassembled, the guts are a snug push fit.
Contains a coarse metal screen/scrubbie this one was not moving much air, all plugged up.
Last edited: pump check valves.pdf

Okay this deal was a bit different.
With a crossover in a dual exhaust (headers) these were placed just ahead of the crossover and "Tee'd" into the PCV.
This could be so effective that you had to keep a vacuum gauge on the crankcase just to make sure you weren't pulling pre-combustion mixture past the rings and turning the crankcase into a bomb waiting for a spark.
@gggGary , I once tried to adapt two smoke reeds to a XS crankcase. Once I found the right material and thickness it worked.
Now I want you to imagine a Coleman stove fuel can attached to the breather outlet. Now imagine the can being crushed by the vacuum. There is such a thing as too much of a good thing.
Keep in mind the swept volume of a 360 degree, 650cc crankcase is what it is. If you make it too efficient you'll be sucking pebbles, pets and small children into the crankcase :)

Monitor with a vacuum gauge...

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Gary, you are correct in thinking the diaphragm valve only closes when throttle is rolled off, allowing no air movement through the reed valve. At least that is how it works on my Honda. When I installed the Mikes valve on the xs650, how much pressure and then vacuum is created in the crankcase had me thinking, then my head started to hurt, so quit. I was going to tap in a 1/8th npt nipple into the dipstick top, then run a gauge out of it, see what actually happens when the reed valve is in play, so to speak. Got put on the back burner, do it another time, then the flame went out. Be interesting to see what you come up with here.


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I looked at this mod some time ago, ordered a reed valve, which I still have, but haven't done anything with it yet, just too big to fit in the allotted space. Unfortunately, oil will still blow by, which was my reason for making one, so the idea was shelved.


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Breathless first report.
I topped up the oil before I left, 90 miles, mix of rural, city stop n go, about 20 miles of 70MPH plus freeway.
catch bottle BONE dry, not a drop. Single petal reed valves in a compact housing can be found.
filed the top fitting down a bit to better fit the stock breather hose.
Drilled the exit out to 15/32 and deepened it a bit. Tapped it 1/4 IPT, threaded in a 1/4 IP X 3/8" barb brass connector, used 3/8" fuel hose down to the catch can, done.
Ole Madness has been using a bit of oil, y'know 750, high comp, lots of miles, unknown history, ridden hard. Tends to have a drool trail on the sidecovers. For sure need to put more miles on but this looks REALLY encouraging.
Thread revival, I haven't forgotten about this project even if you have, but madness didn't see a lot of miles last year.
Now back from a run to the Ozarks rally, 1700 miles in a week.
HiComp 750 motor, a little tired, compression is dropping off some. Used 2.5 quarts of oil on that trip, so a good test of the reed type crankcase valve that's been on her.
She was previously making an oil mess with a late model factory crankcase vent, plumbed down then up into UNI air filters similar to the way the factory did it.
Then I installed a reed valve, close behind the factory CC vent box with a rubber hose down to an open catch bottle.

Pics show how oil free the area remained.
I cut away the bottle to show the inside, this has not been drained since installed in Aug 2022
A 2oz chainsaw oil bottle, guess I will make up something a little more elegant now.
The reed valve prevents outside air going back into the engine so don't see a need for a filter on the set up.
I'm calling this a success.
"elegant" way to eliminate the oil mess from the factory vent tube.
Norton guys say keeping the valve up near the motor as compared to at the other end of a long tube a good idea, so cold gummy oil doesn't make the reed stick on it's seat.
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I think the power brake check valve has a rubber flapper. PCV valves routinely use the ball-spring or a spring-loaded valve but it's said they can't react fast enough to our engine pulses. Basically, I think they would just stick open. Here's another I ran across on eBay that looks like it may do. I may order a couple .....
Did this solution work for you? looking at these myself.
I'm afraid I haven't ordered any to try yet. I'm still using the automotive power brake check valves, which seem to work just fine. These are cheap and don't take up much space. There's not much room in this area so that's a good thing, lol.

Power Brake Check Valve.jpg

Uni Pod.jpg