GL1000 Carb Button Design

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cfairweather
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GL1000 Carb Button Design

#1

Post by cfairweather »

I recently asked a man to give me some feedback on my carb buttons and he mentioned the originals had a concave bottom and mine were flat. I decided to look into this and discovered he was right. After looking deeper into the design, I came to the conclusion I have been making these wrong and needed to change the design to get optimal performance.

1. If you use a flat bottom button, the slide and needle jet will not be able to reach the maximum top position. The original concave tops allow the rod to travel an additional 4.5 millimeters (when compared to using flat bottom buttons).
2. I have come to the conclusion that Honda designed the button for the sole purpose of making an air tight seal. I previously believed that the button was also a "stop" for the piston, but now I am almost certain Honda designed the button to avoid being contacted by the rod. At the maximum top position, the slide rod should come very close to the button and maybe even barely touch it.

I am in the process of redesigning my button and I think I have just about perfected it. Here are a couple of pictures.
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pidjones
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Re: GL1000 Carb Button Design

#2

Post by pidjones »

I seem to remember Pistol Pete (who supplied and installed beautiful replacements) noting several design points about the buttons. Have you checked his posts on them?
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Re: GL1000 Carb Button Design

#3

Post by cfairweather »

I just looked at some of his posts and his buttons are absolutely beautiful. I can tell he used injection molding to create them but unfortunately, I don't have an injection machine. I may buy one or make one in the near future. I would like to see a picture of the bottom of one of his buttons.

Like me, Pistol believed the purpose of the button was to provide a stop for the rod. After careful examination of the design, I don't believe this anymore.

Here is a post made by Pistol:

The plastic buttons job is to limit the upstroke of the vacuum piston. It also made it possible to drill a small bleed hole in the cap.
Pistol

viewtopic.php?f=1&t=52744&p=567136&hili ... ns#p567136
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Re: GL1000 Carb Button Design

#4

Post by redglbx »

CF, when I redid the caps originally for my 76 I used copper plumbing caps (your buttons are really nice) and got educated on all this. Yes the cap button is what controls the piston travel, they wanted it to stop on the button instead of the banging on the piston which likely would’ve cracked & broken.

It also as you say provided access to add the communication port between the vacuum chamber on top of the piston and the rod allowing it to move freely. I spent a lot of time getting the slides to travel the same amount.
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cfairweather
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Re: GL1000 Carb Button Design

#5

Post by cfairweather »

Take a look at this picture closely. This is a 1975 755A carb without a button installed. The spring is installed, so I pressed on the needle to get it to fully extend. There is metal-on-metal contact with the piston. Notice the slide is fully open and very close to being flush with the venturi. So the button cannot be much of a stop before the full extension or it would raise the slide to where it would not be flush anymore.

With a flat bottom button, you have enough thickness in the plastic to make the button a durable "stop" for the rod but this restricts the piston travel by about 4 1/2 millimeters. The plastic is very thin at the concave peak of the original button and I don't think it would survive getting struck by the metal rod over and over. I do think the end of the rod contacts the plastic, but there isn't any real pressure applied to the button. I have an idea to improve the design and will share this after I make the small part I need.
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cfairweather
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Re: GL1000 Carb Button Design

#6

Post by cfairweather »

Sometimes simple solutions are the best and I think I found one without making a part. Nobody likes the idea of metal on metal contact and I definitely don't like metal hammering plastic, so it occurred to me how to solve this with a simple o-ring.

The goal was to find a way to provide a bit of shock absorption when the piston reaches the top without hitting the plastic button or having metal-to-metal contact with the piston. The solution is to use a tiny o-ring around the rod. The o-ring will stay at the bottom of the rod inside the piston. Using the right size o-ring, the piston will still be flush with the venturi. I found that a 1.2mm X 7.6mm worked fine. Look at the picture of the cap and you will notice the middle part where the rod goes in has a bevel which makes it perfect to mate with an o-ring without cutting it. The other pictures show the slide is flush with the venturi after the installation of the o-ring.

When you let the slide drop into the cap without the o-ring you hear a familiar "ping." After you install the o-ring, you hear a soft thud noise so you know the o-ring is working.
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Last edited by cfairweather on Fri Dec 16, 2022 11:14 pm, edited 1 time in total.
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robin1731
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Re: GL1000 Carb Button Design

#7

Post by robin1731 »

What material is the 0-ring? Will it hold up to exposure to gasoline?
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Re: GL1000 Carb Button Design

#8

Post by cfairweather »

The one in the picture is Buna-N which is commonly used in carbs, but it would probably be better to use Viton or maybe even felt if they make one that small. It needs to stay as soft as possible and must be durable. This is a tiny o-Ring. What do you think? I didn't have a big selection of 1.0 - 1.2 mm o-rings, but I think that a 1.1mm X 9 mm would probably be just right. The 1.2 X 7.6mm works, but it fits over a 11mm rod, so it has to stretch a bit.
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Re: GL1000 Carb Button Design

#9

Post by Sidecar Bob »

I have a couple of carbs from the '77 that got me into 'Wings that the pistons broke away from the slides. I don't know how that happened but I don't believe the pistons should make contact.
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Sidecar Bob
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Re: GL1000 Carb Button Design

#10

Post by Sidecar Bob »

I don't have ready access to the 1000 carbs right now but I believe when I replaces the buttons on mine I checked and the end of the shaft hit the button before the piston touched.

Also note that 1100 carbs don't have the buttons but do have a plastic piece that fits into the top of the shaft to limit the slide's travel, as do all CX/GL500/650 carbs.
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gltriker
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Re: GL1000 Carb Button Design

#11

Post by gltriker »

Great Idea, Cedric! tumb2 tumb2

I try to caution folks about the probability of vacuum piston chamber "button" damage as a result of a serious backfiring event into the throat of a carburetor, forcing its vacuum piston (aka slide) upwards into the "cap", violently.

-January 1st, 2018, I had attempted to start trike's ìce cold engine in my garage and take a little New Year Day ride. Getting frustrated because the engine wasn't "catching" I wound the throttle wide open. There was a loud backfire from inside the air filter housing and at that point I just quit cranking any further, rolled it out of the way and left it dormant for another 3 months.

fast forwarding 3 months
While I was rolling trike away from where parked against the wall, I stepped on something I couldn't identify. hmmm
Eventually trike's engine started but wasn't running correctly. Suddenly my eyes were drawn to this :shock:
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-After spotting the damage on top of #1 carburetor vacuum piston cover, I picked that strange piece of something up and placed it on the running board. Then realizing where it most likely came from, I discovered it would match the void in the top of the vacuum chamber, perfectly. I was successful to reinstall it with Seal-All glue.
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The repair seemed to work well. The engine would idle correctly and pull away from stopped willingly as the throttle was advanced. (Pilot mixture screws set at 3 turns out from seated is very beneficial, too ;) )

Eventually-- maybe 3,000 miles and 2 years later-- trike's engine wasn't running well anymore. I won't go into details...
There had been some backfiring into the throat of the #1 carburetor and, unbeknownst to me the Seal-All glue had yielded and the fractured button was slightly lifted. I picked at the button and it popped off.

Notice the witness mark where the end of the vacuum piston shaft contacts the underside of the button.
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When the vacuum piston cover's button is no longer sealing that opening, the vacuum piston shaft will tĥen protrude further than usual into view until the vacuum piston rim bottoms inside the vacuum piston cover. Fortunately the vacuum piston wasn't damaged.
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-
There's an example of broken vacuum piston (slide) in this link. Post#1
https://ngwclub.com/forum/viewtopic.php ... es#p772162
Last edited by gltriker on Fri Dec 23, 2022 4:05 am, edited 3 times in total.
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cfairweather
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Re: GL1000 Carb Button Design

#12

Post by cfairweather »

Thank you PidJones, RedGLBx, Robin, Bob and Cliff for your valuable input. I believe Bob is right about the shaft touching the button before the metal touches, but at that point the rod is almost at the end of travel, so maybe it softens the metal to metal contact a bit. The fractured pistons may be a combination of a backfire event and/or metal fatigue. I have seen a couple of these fractured pistons too. Cliff- You did a great job explaining your experience and your pictures helped too.

Here are my conclusions from what we know now:
1. Honda went to metal tops and plastic shaft tips to solve the issues we are discussing.
2. The piston needs a cushion at the end of travel, but not on the fragile thin plastic at the top of the button.
3. The logical place to cushion the piston is at the bottom of the shaft. This is a better place than the button because the load is spread across a much greater area than the button. The tip of the shaft is beveled, which further reduces the area where it contacts the plastic. The o-ring around the shaft method will spread the load onto the beveled seat.
4. The o-ring needs to be durable because fuel can enter the pocket part of the piston where the o-ring is located. Robin had this concern, and it is valid. We need a material that can survive long term exposure to fuel. We all know these o-rings get brittle after exposure to fuel over time. Maybe a ring made out of durable plastic or TPU would be better? I am leaning toward TPU. I use this stuff a lot and it might be a perfect fit for this application.
5. The exact size of the o-ring is critical. It needs to fit into the beveled edge of the seat so the piston stops just before the metal-to-metal contact can occur. The piston must be flush with the venturi too. My testing led me to the 1.2 X 7.6mm ring but there may be a better size. I believe a 1.1mm X 9 or 10mm would work slightly better, but this is one of those trial and error things. I need to order a few close to this size to determine the best one.

Again, thank you for your thoughts.
Last edited by cfairweather on Tue Dec 20, 2022 9:10 am, edited 1 time in total.
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Re: GL1000 Carb Button Design

#13

Post by robin1731 »

"The piston must be flush with the venturi too."

I wouldn't worry too much about that. The slides spend very little time open all the way and I doubt you would see any performance difference if the slide was down into the venturi a mm or two.
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1985 Honda Elite
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and a rotation of various purchases
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cfairweather
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Re: GL1000 Carb Button Design

#14

Post by cfairweather »

Robin- I agree it won't affect the performance much. I have the flat bottom buttons in my 1975 GL1000 and I took it up to 100 with no problems. The flat bottom buttons keep the slides in the venturi about 3-4 millimeters. It will definitely affect performance, but most people would not be able to tell the difference. Still, I would like to make it as good as possible.
Last edited by cfairweather on Tue Dec 20, 2022 9:07 am, edited 1 time in total.
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Re: GL1000 Carb Button Design

#15

Post by Sidecar Bob »

Cliff: I think we have a chicken s egg situation. Could it be that the bad running was caused by the cap leaking where it was cracked, rather than the backfire cracking the cap?
When I noticed that one of the caps on Mr.H was cracked I epoxied washers around them and coins on top, expecting that to prevent further deterioration. One morning several years later it started easily and ran well when I left home but about half way to work it started to struggle and when I got there I found the coin and the top of the button missing. There was no backfire or other unusual event when that happened; I believe vibration and exposure to the elements caused the epoxy to eventually let go and the failure of the original plastic was probably due to exposure to sunlight over many years.

Re the broken slides, they were intact when I got the bike and I don't recall a backfire but that was 30 years ago and I didn't keep records then so it is possible. What I remember is that it was running poorly so I opened the carbs for cleaning and found the slides broken. It was long before the Internet and I had no money so I tied them back together with wire to get it running until I could do something more permanent and ran it like that for about a year.
Mr. Honda ('83 GL1100/Dnepr) summer How a motorcycle evolves thread
The Famous Eccles ('84 CX650EI/VeloUral) winter Never Ending Build (CX500forum)
Click: Colour schematics for all GL1000 & GL1100 and GL1200 standard models plus instructions on how to download the full size version
"A guy with two sidecars can't be all bad." - Cookie
Another guy with two sidecars..... Hmmmm... must be something to that....
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