1975 GL1000 755A Carb linkage step-by-step fix

[cfairweather]

1975 GL1000 755A Carburetor Linkage Issues and step-by-step instructions to fix it
This step-by-step instruction document will be in three posts because I am limited to 5 pictures per post.

The original 1975 design uses “ball and cup” joints and the later models use shaft and bushing joints. The “ball” joints introduce wobble and slop which makes it more difficult to keep the 4 carbs in sync. Let me say that while doing this project, I learned there is a lot of precision built into the original design, even though it didn’t work out for Honda.

My goal was to make the 755A linkage as good or better than the later design and I succeeded. In doing this project, I tried many possible solutions. While all of the methods I tried improved the linkage, one method stood out as a clear winner. I am going to skip all of the things I tried and only talk about the solution that worked the best, a tiny modified M3 nylon flange nut. The best time to do this modification is when you rebuild your carbs.

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Let’s first discuss the original parts:
There are two linkage arms, one short and one long. The joints are all the same. The arms attach to steel pins that have a ball in the middle. The arms have a smaller 5.31 (.209 in bit size #4) hole on one side and a larger hole 6.15mm (.242 in bit size C) that fits over the 5.85mm ball. Two cups hold the arm in place and one of these has a spring behind it. You will also notice there are two tiny holes on the linkage and you won’t see these because they are on the underside when installed correctly. These look like drain holes but they are actually for prying out the cup. There is a black spacer washer that is permanently fitted over the ball; however, this washer is often brittle, broken or missing after 50 years. Note: The permanently mounted black washer that fits on the shaft can be replaced with a nylon M4 X 10mm X .5mm washer. You will have to cut the washer and then work it onto the shaft. The slit will not negatively affect the linkage operation as it is simply a spacer and prevents metal-to-metal contact and helps prevent wobble. Here is a source: https://www.ebay.com/itm/176062536916?var=475581960471

There is also a plastic washer and an e-clip to keep the arm in place. The e-clip is hard to find because the groove is about 1.7mm in diameter. The shaft is 4mm in diameter on the side with the washer and 3mm on the other side of the ball where the e-clip goes. The 6mm hole is offset slightly to allow easier installation of the arm.

IMPORTANT UPDATE: 2/19/25
After making this modification on two carbs, I learned you cannot easily get the linkage installed if you use glue on both ends of the linkage. Glue only one end or don't use glue at all. This is because when you slip the linkage onto the shaft, it must go on at an angle and then it becomes straight once the ball is seated into the cup. After you install one end of the linkage with a bushing, you will not be able to get the required angle when you try and install the other end. The bushing prevents the linkage from bending and the required angle cannot be obtained. So, you must insert the bushing on one end AFTER the linkage has been installed on the shaft. So, don't glue the bushing into the linkage or at least don't glue the bushing in on one end. If you decide to glue one bushing in, you must install that end LAST. I have come to the conclusion it is probably best to not use glue at all so both bushings can be easily inserted into the installed linkage, just before you install the e-clip.


Tools and items needed:
1. 4ea M3 nylon flange nuts.
2. A 5mm nut driver or socket. The M3 nylon flange nut will be modified using a modified 5mm nut driver or socket. The modification is simple; just use a #3 drill bit and drill out the end. You also need to make the end flat using a file. This will be used to make the nylon nut round and it will be the perfect size.

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3. A drill and a Dremel with 5mm (est) ball file.

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4. Vise
5. 10mm wrench or a vice for holding the arm when tightening/loosing the 9mm cap ends
6. 9mm socket for removal of the spring and cup cap.
7. Super glue
8. Drill bits: #3, #4, #31
9. A M3 Screw. The head will be removed.
10. Tiny screw driver and dental pick to remove e-clip


Procedure:
1. The first thing you need to do is remove the arms from the carbs. Then use your 9mm socket and remove the caps from each end. Remove and clean the spring/cup caps from each end, then put these aside in a safe place as we will reinstall these later.
2. Remove the head of a M3 screw and then place the cut end into your Dremel. 5 or 6 threads should be sticking out of the end. When you install the nut on the screw about 1mm of the threads will not engage. We will be using the length of the screw to resize the thickness of the nut. So, the nut will be initially stick out about 1mm beyond the end the screw.

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Continued in next post...

[cfairweather]

3. Put the modified 5mm nut driver in a vice. If you have not modified it, do that first.
4. Mount a M3 nylon flange nut on the Dremel, turn it on, and push the nut into your resizing tool. Next, we will use the Dremel as a mini lathe to remove the melted plastic and trim the thickness of the nut.

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5. I recommend a craft knife to get the melted plastic off. Use your knife and “mini lathe” to trim the thickness of the nut down to the length of the screw. The thickness of the bushing will be about 2.7mm.

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It is critical to get all of the melted plastic off so the new bushing is perfectly clean. Finally, insert the nut into the resizing tool one more time and you will have a new bushing. Remove the bushing from the Dremel. You will need 4 of these, so repeat the above steps before going to then next step. Tip: You can use this same idea to make hard to find shoulder bushings.
6. Place one of your linkage arms on a workbench and prepare to install a bushing. The short linkage has one small hole on each side and the long arm has both small holes on the same side. The bushing will be a snug fit.

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7. Using a #31 drill bit, drill out the bushing. Make sure you drill a straight hole. A drill press will help, but is not needed.
8. Now we need to clean up nylon on the inside of the joint. I recommend first using a #4 bit and you can do this with or without a drill. Just carve out the excess nylon so it won’t interfere with the spring and cup. You need to be careful not to touch metal when using the #4 drill bit. We only want to remove excess nylon, not metal. Then insert the round Dremel ball file (about 5mm) to form a smooth, tiny cup on the nylon bushing. Finally, you need to run the #31 bit through the hole for one last time. Make sure the entire shaft of the bit goes into the bushing to ensure a perfect size. Don’t forget to clean out the joints before you reassemble. I also recommend a small dab of grease for the cups. Tip: When installing the arm, use a dental pick through the bushing to get the cup out of the way.

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[cfairweather]

Continued from previous post...

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Install the clip without a washer because the bushing flange is basically a washer. There will be no room for a washer.

[Fred Camper]

Nice thread with lots of documentation. Looks to be a good addition to shop talk if others agree.

[CYBORG]

Nice thread with lots of documentation. Looks to be a good addition to shop talk if others agree.

I agree.

[cfairweather]

Thank you. I have decided to share an alternate option to this fix for those that don't mind drilling a larger hole in the linkage arms. This option is actually easier and uses an aluminum bushing; however, it does require you to drill out the small hole to a sightly larger diameter.

[cfairweather]

I decided I would provide an alternative option for those that don't mind drilling out the small holes on the linkage arms. If you do this, it will allow you to insert an aluminum bushing and this is probably the most durable option. It is really simple, just use a #3 drill bit or a drill bit that barely allows you to insert in a 3.1mm X 6mm X 2mm aluminum bushing. You may need to create a cup with the Dremel tool as previously discussed. The E-Clip is hard to find. Tip: The same size clip is used for the needle jet on a SOHC CB750. Here is a link to buy them: https://www.dimecitycycles.com/keihin-n ... idual.html

With the aluminum bushing option, you will need to use a M3 X 8mm (or 7mm or 9mm) washer (spacer) before installing the clip and the thickness will be between .50mm and 1mm. They typically come in two thicknesses, .05 mm and .08mm. Use the thickest spacer washer(s) to ensure the linkage has a sung fit to prevent any wobble.

Insert the bushings after the linkage has been installed and then install the e-clips.


Here are some pictures:

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After you do this linkage modification, you will need to bench sync the carbs and then do a final sync once the carbs are back on the bike.

[cfairweather]

I decided to put together a kit that contains the parts you will need for this task. Here is a link and the writeup on how to install the kit: https://www.ebay.com/itm/276948312813

This upgrade kit is for the 1975 GL1000 755A carburetor and it is designed to improve the linkage operation. A drill bit is included and will be needed to do the upgrade.

The ball & cup linkage design was only used on the 1975 GL1000 755A carbs. The design introduces wobble, slop and makes it difficult to synchronize the four carburetors. My solution contains bushings that will eliminate both the wobble and slop. You will need to slightly enlarge the smaller holes on the linkage using the included bit so the bushings can be inserted. The bushings are inserted AFTER the linkage has been installed on the shafts. You will need to remove the carbs for this upgrade.

Items included in the kit:

4ea Aluminum bushings to keep the linkage aligned
4ea TPU spacer shims
4ea Stainless washers that are the correct thickness
4ea E-clips that are hard to find and can be expensive when you do find them
1ea High quality cobalt drill bit

To use the bushings, you must slightly enlarge the smaller holes on the linkage arms using the included bit. On the longer shaft, the two smaller holes are on the same side. On the shorter arm, the smaller holes are on opposite sides. You will notice that the larger holes on the linkage arms are slightly bigger than the ball on the shaft. The other side is a smaller hole, and these are the ones you need to make slightly bigger to allow the bushing to be inserted. After you correctly make these slightly bigger, the bushing will fit, but the hole will still be slightly too small to fit over the ball. Make sure you drill a straight hole. Do not try and drill the hole at an angle so it aligns with the offset larger hole on the other side. That larger hole is offset to allow you to install the linkage arm. You can drill a straight, perpendicular hole without a drill press if you are careful and have good skills. It is best to remove the nut plug on the ends of the linkage and then remove the springs/cup mechanism before drilling. Put the linkage arms in a vice before you drill to keep them steady.

Clean them up after you finish drilling and you are ready to install everything. You might use a tiny amount of grease on the joints before reinstalling the caps. The original design has a black rubber washer on the 4mm part of the shaft that serves as a shim between the linkage arm and the shaft mount. These old shim washers are either very brittle or they have already broken off. They are rarely in good condition, so I included new replacements that I printed on a 3D printer. Simply remove the old ones and push the replacements on over the ball of the shaft. After you install the 4 shims, you can install the linkage arms. There are two tiny holes on one side of the linkage arm. These go on the underside when you mount the arms. After you have the linkage arms installed, you can insert the bushings. Then you install the stainless washers and finally, the e-clip. You will need to bench sync and then do a final sync when the carbs are back on the bike.

The clips are easy to lose, so be careful. Put it in the groove and while holding it in place with your finger, then use a screwdriver to press on the edge of it and it will clip into place. Make sure the linkage works smoothly without binding.

The buyer accepts all responsibility for everything associated with the items in the kit. This includes, but is not limited to the installation process, any possible problems related to the items, any possible hazardous materials used to make the items and any binding of the linkage. The pictures that show linkage parts are provided so the buyer can see the bushing installed. The linkage parts are not included.