G,
I wonder if Soichiro wasn't just a little ahead of his time... When the original clear plastic hoses were connected between the barbs on the head and the air cleaner, there was a slight vacuum pulling crankcase gases out of the head and into the motor to be combusted (while slowly making a mess of the air cleaner). Without those hoses the gases end up in the atmosphere. Though there was no PCV valve in there (and another vent hose to boot), it would be sort of performing the function that is now part of every modern internal combustion engines.
Those hoses weren't needed and may not have worked perfectly but, I for one, plan to keep them in place to honor Soichiro Honda's forward thinking on the environment (unless someone on this site explains how I am full of crap - any takers? (you won't be the first)).
honda305.com Forum
Vintage Honda Owners, Restorers, Riders and Admirers
Carb Overflow Tube
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nevernotbusy
- honda305.com Member
- Posts: 60
- Joined: Thu Apr 03, 2014 10:49 pm
- Location: Baltimore
Gentlemen
The 'vacuum' is actually created in the inlet port by the pistons descending so the 'flow' in those pipes is a tiny amount clean air from the filter into the valve guide to prevent oil being pulled down the guide into the cylinder under vacuum on the inlet stroke.
While your intentions are honorable, I think the great Soichiro and his men realised it was pointless....
If you want to save the planet, better to lop a couple of cylinders of your favourite V8 at your next car purchase or run, like us Brits, a 60mpg diesel which can happily top 110mph with ease........ :-)
G
The 'vacuum' is actually created in the inlet port by the pistons descending so the 'flow' in those pipes is a tiny amount clean air from the filter into the valve guide to prevent oil being pulled down the guide into the cylinder under vacuum on the inlet stroke.
While your intentions are honorable, I think the great Soichiro and his men realised it was pointless....
If you want to save the planet, better to lop a couple of cylinders of your favourite V8 at your next car purchase or run, like us Brits, a 60mpg diesel which can happily top 110mph with ease........ :-)
G
'60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160
'66 Matchless 350 '67 CL77
'67 S90 '77 CB400F
'61 CB72 '64 CB77 '65 CB160
'66 Matchless 350 '67 CL77
'67 S90 '77 CB400F
Hose Barbs on Head
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.
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.
The barbed fittings screw into the cylinder head, connecting to a passage that leads to the mid-point of the intake valve guide. It does not see any crankcase pressure. The intake valve guides have an annular groove on their OD that intersects with this passage. There is a hole drilled in the groove that goes through to the ID of the valve guide. When vented to the atmosphere, this passage provides a vacuum break to prevent oil being drawn down from the top of the valve guide when there is vacuum in the intake port during the intake cycle. There are no valve guide seals on these engines as we have nowadays to prevent this oil migration. This was a rudimentary "valve seal" technique used in many early engine applications. The mid-1950's Pontiac V8's even used this, having small holes drilled from the outboard side of the cylinder head (in the exhaust flange area), intersecting with the intake valve guides. These had no filters, just the open holes. Regards, Chase
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
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