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Discussion Starter · #1 ·
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
 

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Discussion Starter · #3 ·
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.
 

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Air intake, and yeah I wasn't particular fond of them either. I still got the airbox at home I'll snap a picture when I get there, from what I remember it didn't look like much else than 4 boots connected to a plastic box
 

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Discussion Starter · #5 ·
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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Yeah you're right, at first glance I was thinking they were held on by the adjustable clamps but nope there is a trumpet shape to them
Automotive tire Gas Bumper Synthetic rubber Automotive wheel system


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
Automotive tire Motor vehicle Automotive lighting Automotive design Automotive fuel system
 

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Discussion Starter · #7 ·
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.
 

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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.
Oh hell that's a good idea, of course due to the angle of the carbs it would be difficult to set them in the box right as they are. I'd have to extend it back a bit with radiator hose and fit spacers for them to clamp onto, probably shouldn't affect it too much though
 

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Discussion Starter · #9 ·
...
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.
 

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Well the other option is to form the front of it with a very Obtuse v shape. This is the only modified bike I'm ever going to make so there's going to be a lot of experimenting and years of work. Probably something I'll never say is "done" so I don't mind trial and error
 

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Discussion Starter · #12 ·
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.
 

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to pass tech we have to have a hose going from the crankcase to a catch bottle, then you need another hose for overflow going from catch bottle to the bellypan,
 
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Is that a pair of Murray's carbs on the motor? They have a great reputation for making more power than stock and being easier to tune. if you are worried about the crankcase breather, you might look at building a still air box and venting into that.

And there are no good velocity stacks for Mikuni VM carbs from a gas flow or resonance perspective. Terry Naughton at Hansen Honda had a bunch designed and flow and dyno tested until he finally came up with a design similar to the old Shemozzles used on the works BSA/Triumph triples all those years ago.
 

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Discussion Starter · #16 · (Edited)
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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The intake length is the total length from valve head to atmosphere.
The carb placement in that length is the 'performance' part.
Having carbs close to valve helps prevent fuel 'drop out' but can also limit performance when hot air is being 'blown' over carbs.
Evaporating fuel may not not provide enough cooling effect or may cause fuel in float bowl to get 'forced' through jets causing over rich conditions.
Carb temperatures of 200f are not uncommon in the 'middle' of an inline 4 or even on all carbs when tight fairings are fitted.
On air cooled motors, even having properly designed ducting can cause different problems, the 'outer' side of engine can be over cooled leading to distortions of block and head.
Personally, I wouldn't fit dual carbs to an inline 4, may as well go for a single large carb with a better designed intake manifold, information from automotive designs in the 1950's would be a good starting point (or just convert to fuel injection which could be a major performance upgrade)
 
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