The carbs are out. The only way they ever get to flow some gas on this bike again is if I admit defeat with this whole project.

With the carbs out, I got my first chance to see how throttle bodies I got on ebay would fit. They are from a Kawasaki GPz1100 B2 from 1982 (imagine, 10 years older than the NC30 and with electronic fuel injection) and I knew from this page that they had a bore of 34mm, roughly similar to the NC30 carbs.

That size is somewhat surprising given that they came off of a bike with almost three times the displacement, but I think they used throttle bodies that were comparatively undersized by today’s standards at the time. Since they don’t have any vacuum slides or anything in them, I suspect they had to keep the bore down to get the response to small throttle openings to be reasonable. As a comparison, modern 600cc sport bikes use throttle bodies with bores in the 38-40mm range, but they are complicated beasts with staged injectors and they also usually have vacuum slides to prevent the low-throttle response from being too abrupt.

The GPz1100 throttle body on the left vs the NC30 carb on the right. The bore and general size are pretty close.

As you can see in the picture, the general size of the throttle body is pretty close to that of the carbs, although the throttle body of course is much slimmer due to the lack of float bowl and slide diaphragm. It’s quite a bit shorter, so this is where some sort of adapter will have to be manufactured to fit it to the airbox.

I was pleased to discover that the throttle bodies actually fit directly in the stock rubber mounts. They’re a tad too large, but that can easily be fixed. The only problem immediately noticed is that there’s not enough space for the throttle position sensor between cylinder 4 and the right side of the frame. (For reference, the center distance between the two intakes is 127.5mm.)

Amazingly, they even fit directly in the stock rubber boots! Some minor adjustment will have to be made, because they have a raised ridge around the end that aren’t on the carbs and make them slightly large, but that should be an easy fix. The real tasks will be to make the throttle linkage work between the front and rear pair and to mount them to the airbox.

This is the airbox bottom that the throttle bodies will need to mount to. The carbs are hard-mounted to this bracket.

The airbox bottom has a rubber trumpet for each carb, the base of which also functions as a gasket to seal the connection to the carburetor, which has a machined groove to mate with a raised ridge on the rubber trumpet.

This is where the carb mounts to the airbox. The groove is where a ridge on the rubber trumpet makes a seal. A similar adapter plate will need to be manufactured for each of the throttle bodies.

This pattern is what I have to match up, a groove and two bolt holes with guide pins. It will take some moderate precision manufacturing to get this to work. As you can see in the top picture, the airbox side of the throttle body is a flat cylinder. They must have originally used a rubber mount also on the airbox side. It will probably be easiest to use the same pattern, to make an adapter plate that screws into the airbox and has a similar diameter as the throttle body such that a rubber connecting pipe can be mounted between the two. This will also make the alignment slightly less critical than if I try to hard-mount the throttle bodies to the adapter plates.