Topic: Why is this? SOLVED!

[Deltarider]

In the first pic: a bank of four carbs (CB500), and their two vent tubes, in Honda speak: TUBE, FUEL, 3.5x330
First question: what's their purpose? They're clearly not for fuel, but for venting. Why? Do the carbs not already vent via their individual overflow tubes?
Second question: Why does that LH tube only vent carb #2 via that hole in its 'palate' (red straw) in the second pic and not carb #1? This is strange, because a short 26mm tube in between carb #1 and carb #2 (first pic) suggests #1 and #2 join that vent tube.
I was already reaching for a can compressed air to clear carb #1's hole in its palate as I suspected it to be blocked.
But... I have the same situation with carb #3 and carb #4. The RH tube only vents carb #3 via that hole (red straw) in its 'palate' and not carb #4. Again, a short tube in between carb #3 and carb #4 (first pic) suggests #3 and #4 join that vent tube.
So... I'd better ask the experts here, before I start blowing air in a hole that maybe is not supposed to be connected at all. Why not? 

[MauiK3]

This should be interesting

[bryanj]

Dont know ash Keihn they designed them.
A pure guess is that the overfow standpipe has too small a hole

[Ozzybud]

Dont know ash Keihn they designed them.
A pure guess is that the overfow standpipe has too small a hole

And where they terminate has very unstable air.  Honda engineers were very sharp. Im sure there is a solid reason.

[HondaMan]

There is a very good reason: the bowl pressure must be the highest pressure in the carb in order for it to push fuel up through the emulsifiers, and those emulsifiers must have lower air pressure than the throat of the carb. Otherwise the fuel will not atomize and absorb oxygen on the way to the engine.

The long rubber hoses on these carbs (and longer is better, see later comments) usually are routed either behind the engine or up and over the airbox to behind it, or in some cases they have a separate canister (mostly on bikes made after 1986 with carbs) somewhere under the seat, which has vents in it to receive still, quiet air. Bernoulli taught us all that the faster air moves, the lower its pressure: still air is at the highest pressure in these carb systems. This still air is fed to the bowls to push the fuel up the emulsifier where it gets mixed with slightly lower pressure air from the carb's mouth (that's those little tubes at the back of the carbs) in the emulsifier's holes. That air pressure is higher than the vacuum between the slide and the engine, so it is responsible to push the aerated mixture up into the carb's throat, downstream of the slide.

This is why, on all these carbs, there is little throttle improvement after 3/4 throttle: the difference in air pressure between the wide-open slide and the bell area of the carb is almost non-existant, and the fuel starts becoming "clumpy" and won't burn as well at those throttle openings.

In cars, this point was almost never reached because the typical carb had a total opening that was less than the size of the multiple intake valves. In theory, if an SOHC4 engine were fitted with a manifold (that in some magic way equally fed all 4 cylinders, like in a car) a single, larger carb could be used - but then it would have to be a CV type carb like cars have. That's pretty expensive tech for a $1000 motorcycle, though.

[Deltarider]

Maybe my English has been insufficient. I'm well aware carbs need to have the ambient air pressure in the float bowls. The overflow tubes can provide that. Honda added two extra vent tubes higher up. You could ask: what for? But stranger is: why do these two only vent carb #2 and #3? The more since small 26mm tubes in between carb 1+2 and between carb 3+4 resp. suggest they share a vent route. But they don't and I find that strange.
BTW, I have no issues. I just wonder why this is? I find it difficult to imagine there is dirt in #1 and #4's upper vent route. So the question remains: why? Could any of you guys maybe check if you have the same situation on your carbs? Personally I hold it for very, very unlikely there is accidently dirt in the #1 and #4's "palate".

[MauiK3]

And folks want to run pods and dispense with all that complicated tubing and stuff. Just ignore the engineering, it will go away:-))

[Deltarider]

Some more clarification maybe. Let's take carburetors 1 and 2 as an example. When I blow into the vent tube, the air only comes out of the bore of 2 and not the bore of carburetor 1, even if I block the bore of 2 with my finger. The same applies to carburetors 3 and 4. This in spite of those ultrashort connecting tubes between 1+2 and 3+4 resp. What made Honda do that? I cannot explain this. My carbs are 100% original.

[Little_Phil]

Delta, Get yourself one of those WD40 cans with the smart straw. If you squirt fluid in the hole in carb 1 (the nearest hole to carb 2) then it will squirt out of the hole in carb 2. Carb 1&2 are connected via a quite restricted pathway. So they do share the vent tube, but can't explain the system.

[scottly]

The long rubber hoses on these carbs (and longer is better, see later comments) usually are routed either behind the engine or up and over the airbox to behind it

What about the K7 PD carbs that had no hoses at all, just a teeny tiny hole which was easily blocked?

[HondaMan]

The long rubber hoses on these carbs (and longer is better, see later comments) usually are routed either behind the engine or up and over the airbox to behind it

What about the K7 PD carbs that had no hoses at all, just a teeny tiny hole which was easily blocked?

I talk about that sort of thing in my last book on the Mid-Fours. Honda was subjected to a barrage of unscientifically-created regulations regarding "venting" for the 1977 and 1978 Fours that created [what I call] 'emergency' design changes in the carbs and PCV systems on these bikes. Many of the resulting carb changes caused loss of top end speed (noted in my 750 book) in both the 750 and 550 bikes, but few folks noticed because of the 55 MPH speed limit of the time. In the case of the 1977/78 PD42 carbs especially, I would simply drill out those vent holes if the customer got adamant about it, and since the individual bike shop was not [yet] responsible to the EPA nor the DOT regarding emissions, we got away with the fixes. 'We' just don't talk about it a lot, because there's always some hotshot crooked legal type who might want to make $$ from it. The laws are moot now, as 'vintage' class vehicles in most states don't require passing those "venting" rules anymore (I think AZ and CA still do?), and a lot of them were ruled invalid in the Reagan administration as the EPA/DOT did not have the authority to do what they did to Honda and Suzuki in the 1970s and 1980s. I suspect that pretty soon, those old laws may vanish altogether...

I've also noticed, especially in the 1977/78 550) bikes and the 750s with the PD42 carbs, that the vent ports between the carbs (connected by those little rubber hoses)  sometimes have the brass spigot in the sides of the carbs, but the ports were never drilled. This will have the effect of the outer 2 carb bowls filling slower than the inner 2 at speed. But, again, with the 55 MPH speed limits of those days, almost no one ever noticed because they just didn't ride at high enough speed to matter.

Today, this has changed. Drilling the holes yourself is a good idea, at least in the American West.

[Deltarider]

Short interim report. Just received a confirmation from the French forum. He has the same: when he blows in a tube, air only exits from a bore in the central carbs and not from carbs 1+4. Yet, bores do seem interconnected. When I reverse the route and spray carb cleaner in the bores indicated by the red straw, fluid does come out of the bore(s) in carb 1+4. WTF?! Is it then one way only?

[Deltarider]

Sorted it out. The outer bores in both carb 1 and 4 have no function. The inner bore in carb 1 is connected to the left bore in carb 2. The right bore of carb 2 is connected to the LH vent tube and only that tube. And to complete: The inner bore in carb 4 is connected to the right bore in carb 3. The left bore of carb 3 is connected to the RH vent tube and only that tube. So the aeration in the outer carbs is only via the float chambers of the inner carbs.
I have found no dirt in any route, not in the fuel tubes, not in the vent tubes. Slow jets were clean. Float bowls were sterile. Everything is functional. I will replace the O-rings of the float valves. They have served over 140.000 km and never leaked. But I have these original Honda gasket sets lying around. I will replace just one O-ring around a main jet, the rest is good and doesn't show any wear.

If you are interested, here is why I had pulled the bank. Normally I don't do this. I can service all 4 carbs in situ. It was because somebody had suggested that my big crankcase breather exits on the wrong side. That it should exit on the RH side and not on the LH side, where my bike has had it ever since I bought it second hand in 1980. In my attempt to manoevre that big breather tube over to the RH side my hand had accidentely disconnected one of those carb upper vent tubes. It was impossible to press it in again, at least for my hands. In itself that's not a problem. It happened three years ago and that disconnected vent tube never caused a problem. There were some other things however that I wanted to find out. Had I really seen little gauze filters above the float valves decades ago or had I imagined it? The answer is: there are none, at least... not any more. Also I wanted to route the upper vent tubes correctly for once and for all.
The questions I still have: what's the best spot for the upper vent tubes to exit: behind the air plenum, or close to the battery? And... I still don't know what the right side is for the big crankcase breather tube to exit, left or right? I mean now is the time to get it right. First I await your answers, then I have some more that may have your interest. Stay tuned.

[Little_Phil]

Sorted it out. The outer bores in both carb 1 and 4 have no function. The inner bore in carb 1 is connected to the left bore in carb 2. The right bore of carb 2 is connected to the LH vent tube and only that tube. And to complete: The inner bore in carb 4 is connected to the right bore in carb 3. The left bore of carb 3 is connected to the RH vent tube and only that tube. So the aeration in the outer carbs is only via the float chambers of the inner carbs.

That's what I said yesterday.

[Deltarider]

Yes, you did. And also: "but can't explain the system."
With some help from the French forum I cleared it up: So the aeration in the outer carbs is only via the float chambers of the inner carbs.
Aurelia B20, the moderator of the French forum gave me these suggestions for routing the big breather tube and the vent tubes. The latter is what I have always had. But... to have the big breather tube exit in the middle, could be problematic in my opinion: risk of oil spats on the tyre tread. What is your opinion?
Photos: courtesy Aurelia B20

[HondaMan]

The oil breather on the 750K0-1 originally exited in the middle of the frame, in front of the rear tire. Yep, that was a bad idea at USA interstate speeds, too. In mid-year 1970 the hoses got moved to the left side, pointed nearer the chain (not a bad choice?).

In the 1972 750K2 versions it was moved again and made longer, to hang down in between the chain guard and the left of the rear wheel. The 1972 CB500 first had this same arrangement on the left side I had one of those). The 550 in the USA versions has the engine breather going into the bottom left side of the air plenum, due to DOT emissions laws, where it helps to make a mess inside the air plenum and cylinders #1 and #2 can smoke from it at extended redline speeds (racetrack conditions).

But, for the Euro versions I cannot say where that hose exits after 1973.

[Deltarider]

In practice this bracket https://www.cmsnl.com/honda-cb500k2-general-export_model50441/guide-breather_90514323610/ , fitted exclusively on the CB500K2 [-ED, -F, -G], will result in a slight underpressure (riding wind), provided you will fit it right.
The CB550K2 - and only that K2! - also had a bracket in that position https://www.cmsnl.com/products/guidebreather_90514374700/ , but... it is different - smaller with a 9mm internal diameter - and serves another purpose: Nurse Julie in the UK site has suggested "it's for the skinny tube that goes from the bottom of the air box chanber to atmosphere"[...]. Owners of a CB550K2, please chime in.
Phew! p. 170 in the Shop Manual Honda CB500-550 has puzzled me for many years. In spite of the headline there: CB550K2 ('76), this page lists the modifications both the CB500K2 [-ED, -F, -G] and the CB550K2 had. The CB550K2 was for the US market, the CB500K2 [-ED, -F, -G] (renamed CB500K2 GENERAL EXPORT by CMSNL) was for continental Europe.
1. BREATHER TUBE
Fig. K2-1 seems to be about the CB500K2 [-ED, -F, -G] breather only. Confusing, as the other mods: 2. FRONT WHEEL, 3. FORK TOP BRIDGE and 4. REAR FORK apply to both the 500 and 550. The next page 171. (specifications) is about the 550 only.

[denward17]

In practice this bracket https://www.cmsnl.com/honda-cb500k2-general-export_model50441/guide-breather_90514323610/ , fitted exclusively on the CB500K2 [-ED, -F, -G], will result in a slight underpressure (riding wind), provided you will fit it right.
The CB550K2 - and only that K2! - also had a bracket in that position https://www.cmsnl.com/products/guidebreather_90514374700/ , but... it is different - smaller with a 9mm internal diameter - and serves another purpose: Nurse Julie in the UK site has suggested "it's for the skinny tube that goes from the bottom of the air box chanber to atmosphere"[...]. Owners of a CB550K2, please chime in.
Phew! p. 170 in the Shop Manual Honda CB500-550 has puzzled me for many years. In spite of the headline there: CB550K2 ('76), this page lists the modifications both the CB500K2 [-ED, -F, -G] and the CB550K2 had. The CB550K2 was for the US market, the CB500K2 [-ED, -F, -G] (renamed CB500K2 GENERAL EXPORT by CMSNL) was for continental Europe.
1. BREATHER TUBE
Fig. K2-1 seems to be about the CB500K2 [-ED, -F, -G] breather only. Confusing, as the other mods: 2. FRONT WHEEL, 3. FORK TOP BRIDGE and 4. REAR FORK apply to both the 500 and 550. The next page 171. (specifications) is about the 550 only.

I bolded the part above, my K2 has this bracket as well and from my research that's how the drain tube is attached on my bike. See pic attached....
I didn't have the tube installed in this picture yet....

[Deltarider]

Thank you for posting that photo. It's obvious a crankcase breather tube would not fit there. So... yours is a CB550K2? And... which drain tube? From the airfilter case or from the plenum?

[denward17]

Thank you for posting that photo. It's obvious a crankcase breather tube would not fit there. So... yours is a CB550K2? And... which drain tube? From the airfilter case or from the plenum?

yes, 1976 CB550K, K2.

The tube connects to the bottom of airbox/air filter to this bracket, it is a smaller tube than the one that connects to the plenum.  If I recall correctly the plenum hose just goes to atmosphere.

[Deltarider]

Thank you for posting that photo. It's obvious a crankcase breather tube would not fit there. So... yours is a CB550K2? And... which drain tube? From the airfilter case or from the plenum?

yes, 1976 CB550K, K2.

The tube connects to the bottom of airbox/air filter to this bracket, it is a smaller tube than the one that connects to the plenum.  If I recall correctly the plenum hose just goes to atmosphere.

Yes it does and I think, but am not sure, it routes toward the same guide the battery vent tube has, RH side.