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Crankcase Ventilation is Vital to a 4-stroke engine

1195 Views 16 Replies 5 Participants Last post by  crazypj
I think that about says it all.

Don't be sticking tiny paper air filter pods on your crankcase vent and think that will work, it won't.

If you doubt that then research 'Crankcase Ventilation Pump' and why they even add a vacuum pump on the crankcase of a racing engines.

Why did you post this Trials, everybody knows what happens to a car when the PCV valve is clogged ? <- Because of the pathetic number of motorcycles we see come through here with stupid little paper or felt air filters stuck on the engines. It's like watching lemmings jump off a cliff, it looked like a good idea at the time
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The Murray's carb kit for my cb750 DOHC came with pods. I was planning on building a filter box to replace them and weld 2 pieces of pipe on the bottom of the box for the engine breather hoses to connect to
Crankcase ventilation pods? or Air intake pods? Huge difference.
Air intake pods is a whole different bunch of lemmings, your motorcycle (most if not all of them) also came equipped with velocity stack shaped air intakes. Those can not be removed or the manufacturer would not have put them on there to begin with.
They are never just boots, case in point:
Map Auto part Automotive design Drawing Diagram

Bunch of things to note here, the intake boots are stacks and the crankcase ventilation goes to a place where there is a vacuum, plus a place to discharge water and waste oil and intake only filtered air.
... and that's not even a vacuum carb system but it does have intake stacks just like all the rest of the engineer designed engines, be that 2-stroke or 4-stroke.
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Mikuni sells intake stacks for their carburetors right out of their catalogue.

It will never work as good without them as when they are on there, simple as that, you have more turbulence and any flow back of fuel is going to soak your filters instead of being sucked in on the next intake stroke.
Of course these were designed to work with the original 4 rack CV carbs, the new ones are mechanical slide 😎. I want to make an enclosed box around a filter element and put an air scoop on bottom View attachment 107015
Perfect would be if you could shorten the intakes, that's not really an optimal setup. 4 carbs on 4 intakes is a performance consideration because it shortens the intakes and improves flow. Long intake manifolds are not a problem on a diesel engine.
Getting back on point.. you think the breather hoses would be best built into the same unit or into their own filter?
Breather hose is best drawing from and spewing into the airbox.
4-stroke Trials bikes have an airbox. If it wasn't needed they wouldn't have one just to save weight. You have to clean the engine oil out of the bottom of the air box regularly. If that oil is clean you are good to go and it it is black you got problems.
Velocity stack straight off a Mikuni VM carburetor:
Wood Bicycle part Bumper Electric blue Auto part

:geek: stacks can have a convoluted shape and still have a velocity stack shape, that is how you can make it clear a frame member etc. Doesn't even need to be round in cross section, it can be oval, square or even flat on one side and it is still an air funnel.
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