Topic: Airbox breather tube question

[HondanutRider]

On my CB350F, there seems to be missing a tube that would attach to bottom of the rubber box between the filter chamber and the carbs.  Should this just be an open hose section or is it supposed to be plugged?  If it were open, wouldn't unfiltered air be sucked-in?

[HondanutRider]

Bump...I've connected a tube with a plugged end up to this opening, as it looks like a drain spot to me, where moisture might collect and be able to be removed.

[tomkimberly]

It is just a drain tube and it is OK to leave it open.


Tom

[OldSchool_IsCool]

I'm not familiar with the particulars of the 350 (I have a 550).  It sounds similar to my set up though.  A breather tube from the top of the engine to the bottom of the filter box.  The filter box has a "pinch tube" that collects gunk which then requires periodic emptying with a simple pinch.

The second chamber, the air plenum, also has a drain.  You can look on a parts fiche to see if a pinch tube is needed here too.  I have an open tube here to the swingarm.  You really want the plenum to drain as any carb overflow that gets in here needs to be wisked away quickly.  A flooded plenum could lead to a flooded carb throat which could lead to liquid fuel in the cylinder.  If your piston attempts to compress a liquid, it will NOT win!!

[HondanutRider]

The air plenum I'm talking about is on the carb entrance side, just after the air filter box.  If a drain tube located there was open, wouldn't it suck unfiltered air?

[OldSchool_IsCool]

Yes, you are right, it would suck unfiltered air.  I'm not real good at paradox, can anyone explain what the Honda engineers were thinking with this.... apparent flaw?

[IHWillys]

I don't know.  Perhaps with the correct hose(ID/length) no significant amount of air is pulled through this tube.  The intake is not experiencing a smooth constant flow of air just as the exhaust is not.  Perhaps the dynamics are such that this tube contains a column of air that is pulsing back and forth but not really flowing as long as the filter is properly flowing.  Perhaps the area where the drain is located experiences a higher pressure versus where the tube terminates and there is actually a flow out of the tube.  I don't know.

I would think that if the tube went down behind the engine and was flowing air from down there up into the intake, unfiltered, that riding on a dusty dirt road would be murder on the rings in short order.   But sometimes one's intuition and even understanding as to how things work is completely wrong.

A simple test for it may be to install the tube with a film of oil on the inside and then also wipe a film of oil inside the chamber and inspect it periodically.  Dust will show up in the oil film if it's in the air.

Ken

[TwoTired]

I call the box between filter and carbs a plenum chamber.  There is a drain tube for that in the event the carbs overflow into that chamber.  On the 550 there is a reduced diameter tube that connect there.  The outlet diameter of the tube is much smaller than the nipple size on the plenum.  Yes, it can suck in a small amount of unfiltered air.  Probably a better alternative than an air plenum explosion/fire so close to the nether regions.

[The Mayor]

I feel like the more topics I read on this the more I'm confused. So I have a 350F too, a breather tube from the top of the engine that runs and dumps right behind the swingarm, and nothing attached to the air cleaner chamber (box between the carbs and the element seal box).

Does the original design have the breather tube running back into this chamber? Does anyone know for sure?! If so, did it have some sort of drain?

[TwoTired]

It wasn't until 1974 that Honda started controlling engine crankcase vapors on the 550.

http://www.cmsnl.com/honda-cb350ffour-usa_model435/partslist/F++14.html

The "Chamber, Cleaner" has a drain tube on it so gasoline can't collect in there.

The "Case, Air Cleaner" has no connection to the engine breather on the 350F.