The choke linkage on a 1975 GL1000 carburetor has a screw on the actuating arm that enables synchronizing the choke action between the left and right carb banks. On later year GL1000 carbs it's absent. If you want to sync the chokes you do so by, consistent with traditional automotive carburetor practice, bending the linkage rod. It's rarely needed however.
Twisted throttle shafts are virtually endemic to GL1000 carburetors. It happens when the machine is resurrected from the dead and someone inadvisably exerts extreme force on the throttle grip to overcome the varnish buildup inside the carbs keeping the throttles from rotating. The thin, delicate brass throttle shafts easily distort.
The earliest 1975 GL1000 carbs also have a jet screwed into a blind hole. The hole doesn't go anywhere. On later 75s and all subsequent years the blind hole is still there but there is no jet screwed into it, or even threads. The hole itself is an anachronism, that is, a leftover from before the carb was repurposed from its original role on a Honda car engine.
GL1000 float pivot pins are pretty unique among Keihin carburetors. They are a smaller diameter than most of the pins on Keihins, they have a taper on one end, and, the carburetor's posts that accept them are made like no other Keihin's posts. Specifically, the two posts on the GL1000 carburetor have differently sized holes, similar to what you'll find on older Mikuni carbs. Therefore, the pin must always be inserted in a certain direction.
The hoses that connect each carburetor to the air chamber (plenum) are not critical and are not sources of performance issues. They do not have vacuum in them. They do not have positive pressure in them. They do not have fuel in them. You could even run the carburetors without them and it wouldn't make the slightest difference.
It's pretty well known by now that on all the four-cylinder Wings, the float bowls are arranged so that the drain screws are close together, that is, biased toward the center of the carburetor set. This allows easy access to the screws by pointing a screwdriver between the intake manifolds.
What seems to not be well understood is that the drain screws require maintenance. The large brass screws seize to the float bowls. They also tear easily if a custom-ground screwdriver is not used on them. And, since debris and rust in the fuel finds its lowest point at the screw, their drilled passages often clog up and thus prevent proper function of draining through the screw.
The vacuum top on these carbs seals to its carb metal to metal. Two things are significant about this. First, the plastic slide ring is not, as many suppose, a gasket or any kind of seal. The seal is purely aluminum top to aluminum carb body, both of which are precisely machined for this purpose. Second, this means any imperfections: dirt, metal burrs, dings--even left over polishing compound--on the vacuum top's machined mounting surface can degrade that vacuum seal, and just as importantly, tilt the top enough to cause binding between top and slide. And if a spec of dirt can have this effect, what do you suppose chrome, paint or powder coating will do? I won't guarantee rebuilds on carbs that have chromed or painted vacuum tops.
Speaking of vacuum tops, some believe they and their slides are matched during manufacture. This is not so. This fallacy probably arose from an assumption, seeing that Honda sold them only as a set. But this was for economy reasons, not for technical reasons. Manufacturers often group more than one part under a single part number. Experience in the industry would have prevented the unfortunate and false assumption.
The factory float valves are still available. They are so much higher quality than anything else out there no one should think twice about preferring them.
The GL1000 carburetor body is designed to strip its float bowl screw threads. Several things make this so. First is the unusually soft aluminum alloy that is likely half zinc. Next is the very small (4mm) diameter screw. But the worst piece of the puzzle is the 1940s design of the float bowl gasket. It has no support. When tightening the screws, the only tightening resistance is in the gasket itself. The metal to metal contact of the later GL1100 float bowl is absent. Thus the screw never fully tightens, and worse, a tightened screw in the beginning soon is no longer tightened after the gasket has shrunken. Worse still, if the factory fiber-reinforced gasket is not used, and an aftermarket plain rubber gasket is employed, there is simply no hope of ever having a liquid-tight float bowl. I repair the threads on every GL1000 carb set I rebuild and use only the original type fiber-reinforced gaskets.
The most important part on GL1000 carburetors is the main jet o-rings. They are available fron no one, not even Honda, and definitely not any aftermarket source who inevitably has the wrong sizes that pinch or tear. There is only one source, a Japanese company who does not like to sell to the public. However, a couple of outfits retail these very parts. One of them is Amazon. It makes a difference.
The second most important GL1000 carburetor part is the idle jet. The design of this carb is such that a much wider range of throttle operation depends on the idle jet than would seem intuitive. Overlooking this part has negative consequences therefore. And wouldn't you know it, it's the hardest to get to, the easiest to lose, and the jet most in need of cleaning or replacement.
- The GL1000 carbs should be installed onto the engine a certain way. That is, with the various attachments connected and tightened in a particular order.