The too long; didn't read version of this is, why wouldn't the inline valve shown below work on a 250/350 a single if it works on a 90 degree twin of larger displacment?
The long version;
It'd be nice to find some sort of inline reed valve to install, but so far I haven't seen anything that would work well for what I want. I did find a Moto Guzzi part that is an inline valve with a ball in it. I wasn't sure what size it was before I ordered it but for around $15 I figured I'd order one and see if it would work.
It's smaller than I was expecting, the hole on the inlet is about 9mm diameter as compared to the 14mm ID of the fitting in the case. The fitting has about 2.5 times the open area of the inlet to the valve, and the open area between the ball and the cylinder in the valve is even less I would assume.
The valve will be more restrictive than the open hose and many comments here about breathers warn against anything restrictive. That seems to make sense, but in thinking further on it...
This valve is from a bike with at least twice the displacement of the engines I'm looking at. Yes, it is for a twin cylinder engine but at some point in the cycle both of the those pistons are moving down in the cylinders so they aren't compensating for each other with any change in crankcase volume. A 900cc Ducati I was recently working on flows it's breather through a smallish hole also, and again both it's pistons are moving down at some point in the cycle.
Why does a 250cc single need such a large hose in comparison?
The single doesn't produce more blowby than the twin does. It doesn't move more air due to the piston movement than the twin does, not when both pistons are moving down anyway.
Looking at the restriction caused by a valve like this - The first time down the piston will move air out of the case through the breather hose. Pressure in the case will go over atmospheric if it needs to to get rid of the air. The seals will also see this pressure and that doesn't help with oil leaks. But when the piston goes back up, it won't be drawing air in. Not only is the pressure trying to refill it limited to atmospeheirc, but the valve is blocking flow in the hose. So where does the air come from to build pressure on it's later trips down the cylinder? It's expelling it faster than it can take it in, and blow by can't be any worse than a twin.
So yeah - why wouldn't this valve from a larger engine work on a 250 or 350?
I'll probably end up going with a shorter hose with a filter on the end, but thought I'd ask about the valve...