Bypass ports

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mikenixon
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Bypass ports

#1

Post by mikenixon »

The first carburetors were pretty crude. Someone has called them equal to an empty peaches can with a couple of holes in it. And that's not far off. With basically two circuits: idle and main, if it wasn't for the decades-ago extremely low-rpm engines and the gross overlap of the main circuit, there wouldn't have been any midrange at all. In fact, blubbery richness of the main circuit meant the circuit extended almost down to idle. But as carbs got more sophisticated their circuits shrank in size and multiplied in number. They became more like sprinklers and less like fire hoses. Circuits were tightened up, made smaller, more precise, more efficient. And the idle circuit had to fend for itself. Out of these changes came the bypass port.

The idle circuit has a tiny opening (or "port") ahead of the throttle. This port is always exposed to the engine because the engine is expected to idle with the throttle virtually closed. Slowly open the throttle however and you'll uncover two to three additional ports, similar in size to the idle port and very close to it, and usually staggered in their placement so the throttle exposes only one at a time. These auxiliary ports supplement the idle port and fill the fuel discharge gap between the idle and the next circuit--the needle jet, providing transition between the two. The bypass port is also called in some carbs a "transfer" or "transition" port, a nicely descriptive term. The more familiar "bypass" name comes from the fact that these ports bypass the idle mixture (pilot) screw, despite being connected to the idle jet, just as the coolant bypass in an engine bypasses the thermostat.

Because the bypass ports are fed by the idle jet, they are part of the idle circuit. But unlike the idle port, bypass ports are not controlled by the Gold Wing's idle mixture (pilot) screw. They circumvent it, bypass it, remember. This means that if for some reason a too-large idle jet is installed in the carburetor, the fallout will be mainly due to the bypass ports. While the pilot screw can mitigate the resulting excessive discharge at the idle port, it can't do anything about the overly-rich bypass port discharge. This is just one of a number of reasons increasing idle jet size is not recommended.
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Re: Bypass ports

#2

Post by flyin900 »

Mike thank you for that description of that circuit. I have been having poor idle issues on one of my CB DOHC bikes this summer. I finally removed the carbs and totally broke them down and ordered OEM Honda gasket kits and a new set of OEM #35 correct idle jets. The originals I believe were clean, yet I want to be sure and decided to go new. The hole in those jets are so tiny that this will ensure that there are no issues there.
I have gone through each carb body and meticulously cleaned and checked every circuit and feel confident that all is well in there now.
The carbs were also suffering from the discharge of the oil gases into the air box connection to the PVC hoses from the motor. I picked up your discourse on that subject from your website. I refer quite often to the excellent articles written there for clues and information.
Just in the throws of winter up here now, so next spring will be the test on how well the cleaning and rebuild came out on this carb bank.
Thank you for your input and support on the forum.
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Re: Bypass ports

#3

Post by mikenixon »

:)
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Re: Bypass ports

#4

Post by gltriker »

Thank you, Very Much, Mike for writing this article! action1
I respectfully hope this contribution (my epiphany?) causes you very little angst as you read it.

Combining information from at least 5 of your technical articles, and eventually correctly understanding the dynamic interaction of the Air Cut Valve and the Pilot fuel discharge circuit during a rapid, closed throttle, engine deceleration scenario,
those precise discussions finally clarified in my old man's inquisitive mind what is actually happening in the entire "idle" fuel delivery circuit in my 1976 GL1000 Keihin carburetors, downstream from the slow fuel jet, which you have called the "idle" fuel jet. tumb2
And, importantly note, I set and leave each carburetor's pilot adjustment screw at 3 turns out from seated as you had recommended in another of your technical articles. (Valves' lash clearance are set at 0.006", too.)

Around 2 years ago, I had suggested to set the pilot adjustment screws even richer than 3 turns from seated might be beneficial in providing a little additional fuel in greater than normal, engine output demand scenarios. Climbing a long steep highway incline in 5th gear at highway speed was my example. It seemed to be working if I could maintain at least 4,000 ish RPM on my 1975 GL1000 engine during the ascent. 4,000 RPM equals ~ 60 mph . Trike would also continue to slowly accelerate with widely opened throttle as we went along. ;)

Now, with my much better understanding of the slow air jet's exact purpose in the "idle" fuel delivery circuit , I believe there may be no benefit in that suggestion, after all.

I was always aware that with the edge of throttle plate covering them, all 3 bypass ports were not discharging fuel because the Bernoulli's Principle was not active when the engine idle speed adjustment was set in a procedurally correct, carburetors' idle speed synchronization session.
Studying the GL1000 carburetion Theory Of Operation illustrations in the Factory service manual had continually caused me to process the pilot fuel adjustment screw was intercepting liquid fuel, only, in the idle discharge circuit on its way to the little brass idle discharge tube standing downstream of the throttle plate.
Several weeks ago, I had pm questioned your identification of the Pilot screw's purpose in the 'Pilot Screws Revisited' article was not a liquid Fuel adjustment. you stated it was an Air/Fuel adjustment. :-?

Finally, after reading this 'Bypass Ports' article several times, the realization hit me (EUREKA!) that the slow fuel jet orifice is so small of internal diameter, that the total volume of liquid fuel it could provide to the idle discharge port and the 3 bypass ports in all engine running conditions, above off idle engine speeds, that it would be impossible to fill the carburetion demands of the engine without ,first, creating a precisely slow air jet metered, pre-aerified mixture ratio of the liquid fuel being delivered to the idle discharge port ! :shock:

When you regain your composure Mike, ;) I may have another question, or two.

Thank you, Sir!
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Re: Bypass ports

#5

Post by mikenixon »

:) For years I contemplated ways of adding an accelerator pump to the GL1000 carburetors. The GL1100 carbs have that as a significant advantage. But I didn't think replacing the carbs was the right way. First, I thought about adding an accelerator pump completely dynamically, that is without adding extra parts. It is possible, the standing idle outlet makes this a more than slight possibility. A shift of pressure would have to be engineered in, timed the rapid throttle movement. Lot of daydreaming about it. Later, I thought maybe I could adapt the Honda car accelerator pump, whose parts more or less bolt up to the Wing carbs. Some fiddling, sure, external hoses, sure, but very do-able. But then I realized that the effect of an accelerator pump can be duplicated without actually having an accelerator pump, and the three turns on the pilots is a large part of that solution, though not all. That and careful tuning achieves the kind of throttle response everyone wishes their GL1000 exhibited, as you can see in the video found at the second link in my signature. Check it out.

Yes, the idle circuit is important, and what with the bike's big, torquey engine and long-legged gearing, it stays important on the road. It's too bad Keihin made the idle circuit the way they did on the GL1000 carburetor--the hole so small blockage is inevitable, and the jet body as tiny as a gnat's behind and buried in the center of the carb casting. It's arguably the most important part of a carburetor rebuild.
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Re: Bypass ports

#6

Post by 5speed »

So the release date of the book "all things wing" by Mike Nixon will be? :mrgreen:
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Re: Bypass ports

#7

Post by gltriker »

mikenixon wrote: Wed Nov 18, 2020 8:17 am :) For years I contemplated ways of adding an accelerator pump to the GL1000 carburetors. The GL1100 carbs have that as a significant advantage. But I didn't think replacing the carbs was the right way. First, I thought about adding an accelerator pump completely dynamically, that is without adding extra parts. It is possible, the standing idle outlet makes this a more than slight possibility. A shift of pressure would have to be engineered in, timed the rapid throttle movement. Lot of daydreaming about it. Later, I thought maybe I could adapt the Honda car accelerator pump, whose parts more or less bolt up to the Wing carbs. Some fiddling, sure, external hoses, sure, but very do-able. But then I realized that the effect of an accelerator pump can be duplicated without actually having an accelerator pump, and the three turns on the pilots is a large part of that solution, though not all. That and careful tuning achieves the kind of throttle response everyone wishes their GL1000 exhibited, as you can see in the video found at the second link in my signature. Check it out.

Yes, the idle circuit is important, and what with the bike's big, torquey engine and long-legged gearing, it stays important on the road. It's too bad Keihin made the idle circuit the way they did on the GL1000 carburetor--the hole so small blockage is inevitable, and the jet body as tiny as a gnat's behind and buried in the center of the carb casting. It's arguably the most important part of a carburetor rebuild.
Occasionally I had wondered, if the complete carburetor was reassembled "ultrasonically" cleaned, where might a small particle of trash come from to eventually wind up plugging the slow fuel jet.
This photo of a carburetor I was disassembling to inspect and clean from my trike, shows a very likely point of entry.
The fuel bowl's, atmospheric vent passage.
20201011_171347.jpg
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Re: Bypass ports

#8

Post by mikenixon »

Hi, gltriker!

Well, sure, but don't forget that vent is filtered by the air filter. While it can route dirt, an even more likely path of contamination is from the fuel supply. Not only can the fuel supply carry contaminants, at some point the fuel itself is a contaminant, inasmuch as it gels very quickly. The idle jet is constantly in jeopardy.
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Re: Bypass ports

#9

Post by gltriker »

:) tumb2
Yes, I had a blocked slow(idle) fuel jet occur 2 1/2 years ago while trike and I were at a CVMG Rally in Canada. At the time it developed though, I lived with the engine aggravatingly stumbling off idle.
The next day I rode 250 miles back home , and broke my bad leg 20 minutes after getting off trike in my garage. :cry:
C'est la vie.
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