Topic: Importance of Breather Tubes?

[KB02]

ON the SOHC 750 engine, there are two vent/breather tubes on the engine: One at the back of the valve cover and one at the back of the case. When removing these tubse from the engine when I pulled it out to rebuild it, they sort of... well, dissappeared. How important is it to replace these tubes?

I know that they both wen down to a collector behind the engine, but what does this collector do? I can never fin that in any manual.

The local Auto parts stores are mostly useless when comes to replacing rubber hoses for various applications (If it's a radiator hose or fuel line they've all got them in stock, if it's anything else they look at you like you've got a horn growing our of your nose or something). And the local bike shops don't even want to help you if your bike is more than ten years old or if the part you want is something they can't just open the first catalog they come to and pick it off the page.

Any suggestions, comments or ideas?

[Gordon]

It seems that almost every model of the 750 had a different arrangement for the breather system.  If yours were routed to a collector, it was there to catch any oil blow-off and would need to be emptied out occasionally.  You really should have the breather tubes on there, otherwise the valve cover breather may spray oil on your carbs, and the rear engine breather could give your rear tire a nice slick coating.  The tubes are not hard to find.  Just measure the diameter of the outlets and buy the corresponding size.  I'm using some large diameter fuel line for mine, because it won't harden as quickly and will stand up to contact with oil. 

[bryanj]

They are there to cut down the "emissions" for those nice American people. All the early ones just had a length of pipe run to somwhere were the oil fumes just didnt hit a tyre/ rider (never worried about passengers/public!!

[TwoTired]

 Their purpose is to relieve any pressure buildup in the crankcase relative to external atmospheric pressure.
This can occur through expanding gasses and vapors from heating and combustion blowby past the piston rings.

At the start of each combustion stroke, some of the air fuel mixture gasses seep past the piston rings into the crankcase.  Some of these gasses haven't burned yet, so they are rich in hydrocarbons.   Modern engine systems (Honda since 1974) route these gasses, along with any oil and water vapor from the crankcase, back to the carb intake where it gets another chance to be burned by combustion.  This helps to keep these compounds out of nearby lungs where it can interfere with normal life cycle.

Earlier models allowed these compounds into the atmosphere.  Modified machines often do this as well.  However, open tubing is an invitation to insects that like to explore and build nests inside these protected areas.  The machine's standard airfilter blocks insect entry into both carburetor and crankcase. And, open tubes can have a foam or gauze filter fitted to them to acheive the insect and other debris blocking effect.