I wonder why the CB/CL72/77 engines didn't vent the top to a air cleaner?. ............................lm
48lesco wrote:The breather and intake valve vents are two completely separate systems that I don't think were designed to interact in any way although they probably do to a degree. On the intake stroke, the greatest negative pressure occurs in the vicinity of the intake valve. The valve guide therefore has this negative pressure on the intake tract side, and very nearly atmospheric pressure on the rocker arm side in the head (which is hopefully very oily). The intake valve guides have a groove and a small hole through them to allow atmospheric pressure to bleed into the clearance between the valve guide and stem, therefore reducing the pressure differential and oil migration through that clearance. I think that's the thought behind those.
Who knows why they were eliminated, but there could be a couple reasons. They may not have been very effective, or they may have been slightly too effective and led to premature valve guide wear due to lack of oil. At any rate, less parts and machining steps was also probably a consideration. I don't think it matters much whether you cap them off, or run the tubes back to the air filter(s). I just wouldn't leave them open to moisture, debris, etc. Also a good idea to plug the holes in the air filter(s).
The breather is just meant to maintain near or slightly less than atmospheric pressure in the crankcase to help the rings keep oil from being sucked up the cylinder walls during the intake stroke. During the compression stroke, some fuel/air can blow by the rings and wind up in the crankcase, as well as some combustion products on the power stroke. These gasses make their way up into the head and come out the breather hose, into the American atmosphere. Later Honda models had the breather tube not open to atmosphere, but attached back to a complex and wonderful system attached to the air cleaner box and therefore a slight negative pressure. This system separated the unburned fuel from any oil and combustion products (water) and routed the unburned fuel-air mixture back into the intake tract inside the air filter.
These are my beliefs...
-48