Carb Overflow Tube

[LOUD MOUSE]

The early DREAM engine used the case vent before the top vent as did the CB/CL72/77.
I wonder why the CB/CL72/77 engines didn't vent the top to a air cleaner?. ............................lm

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

[48lesco]

I guess because it wasn't required to control those emissions until the EPA was established in 1970. There's no performance advantage and the clap-trap required to separate the oil and water from unburned fuel takes up quite a bit of space.

[LOUD MOUSE]

So why the DREAM and not the others?????. .............lm

I guess because it wasn't required to control those emissions until the EPA was established in 1970. There's no performance advantage and the clap-trap required to separate the oil and water from unburned fuel takes up quite a bit of space.

[48lesco]

Not sure I understand the question; I believe they all ran the valve guide vents to the air cleaner in one way or another (CB/CA/CL) until the vents were eliminated in all of them. None of them ever connected the crankcase breather to the air cleaner did they?

[G-Man]

Bill

I think 48Lesco has put it quite succinctly for us. The vacuum is created in the inlet manifold and pulls air into the valve guide through the plastic pipes from the air filter. The air pressure in the air filter is greater (nearer atmospheric anyway) than that at the manifold so the air (like the majority of the air in the manifold) moves from the filter to the manifold or the valve guide.

The other "breather" is for a completely different purpose and is there to reduce pressure build up in the engine cases generally. The breather on the Dream has a bigger job to do than the CB or CL because both pistons in the Dream move up and down together causing a rise and fall of pressure at each cycle. The CB and CL piston movements cancel each other pressure-wise, but there will still be a pressure build up due to piston blow-by that must be vented. The plates in the head are not to prevent or restrict the passage of air but are there to separate the oil mist from the air as it leaves the engine.

The reason that we want to reduce pressure build up is to stop oil being forced out of gaskets and seals.

Interesting discussion.

For authenticity connect those manifold barbs to the air filter. For practicality, join them with a pice of clear plastic tube.....

G

G,

For the life of me I can't understand how what you have described would work. I am not saying you are wrong. I just don't see how it would work that way.

The first stumbling block for me is the vacuum. Being a result of the pistons descending, wouldn't it cause all the "flow" to go in only one direction; toward the intake valves. I just can't see how it would flow the other way in the clear tubes to put clean air at the guides. Wouldn't it work like this: Vacuum at carb inlet creates vacuum inside air filter (reduced significantly because of the larger area of the filter media surface); That results in vacuum at that end of the clear tube which results in vacuum at the head side. The "flow" has to go toward the air cleaner, doesn't it?

The next stumbling block for me is that Honda used the barbs on the Dreams and CL/CB's despite the difference of the 180 and 360 degree cranks. The pistons going up and down together in the Dream engine causes alternating pressure and vacuum on the underside of the piston and therefore in the cases and therefore in the head. There is the vent at the top of the head that goes to open air but the plate/gaskets on top of the head restrict its path significantly. It still provides an escape route for some of the pressure. Yet, while that is true, the barbs on the back of the head present slight "negative pressure" if the tubes are connected to the air cleaner. That smaller amount of vacuum is overcome by the pressure created by the down stroke of the pistons but still provides an escape for gases. Likewise it is overcome by the vacuum in the crank case when the pistons go up and ends up reducing it; that is in the clear tubes the net flow would still be toward the carb. Blocking off the barbs could only increase the pressure in Dream cases when the pistons descend (probably contributing to leaks). That is, with the barbs blocked, the restricted venting capability of the barb on the head cover won't keep up and the pressure gets higher than if the clear tubes are connected to the barbs on the back of the head.

Now, Honda's decision to stop using the barbs/tubes argues against what I have said so, I am probably wrong. I just can't see how your explanation works. Help, please.

[LOUD MOUSE]

I have finally been to the shop and was able to look at the DREAM air filter I have.
I have looked at the page in the HONDA parts book and near as I can tell there are 3 holes as I see 3 tubes from somewhere.
Assuming that 2 are from the valve guides I figured the 3rd one was from the top of t he head .
My DREAM filter has 2 holes where I'd expect the valve vent tubes attach.
So where I suspected the DREAM had a tube from the top of the head to the filter is the reason I wondered why a DREAM engine would go to the filter but not the CB/C:L72/77.
Appears the 3 engines did vent to the open air but the DREAM didn't vent through a hole to the right side of the case.
Or did it?. .........lm

Not sure I understand the question; I believe they all ran the valve guide vents to the air cleaner in one way or another (CB/CA/CL) until the vents were eliminated in all of them. None of them ever connected the crankcase breather to the air cleaner did they?

[rrietman]

OK; I have not seen any dreams with the cyl head breather connected to the air cleaner. the CB/CL 's don't either, that's a given. now, I do have some early dream air cleaners with THREE holes, all small diameter. obviously the third hole is not large enough to take the crankcase breather. so now we go back to where to put the carb overflow tube(or no tube). LM?
Randy