[FredFlintstone]

Appreciate a bit of help with a pair of 2017 to 2021 Kawasaki Z650 throttle bodies please. I have identified these as a good retrofit to a Yamaha XS650 project. The first picture is with throttle plates down facing and closed. The throttle cable mechanism and TPS (in second picture) can be clearly identified. But as can be seen, there's another pair of throttle plates visible and they're clearly shown open when the throttles are closed. In the second picture with the throttle bodies flipped there is what I assume is a servo motor of some description and I assume it's purpose is to drive this second set of throttle plates. I don't really understand the purpose of the second set of throttle plates. If I buy these throttle bodies I intend to remove the second set of throttle plates and/or the servo motor. I think that's feasible. Intend to set the engine idle speed on the throttle stop screw. Am I right in my thinking? Thank you.

[FredFlintstone]

OK, having done a bit more research I've found the following explanation of the throttle bodies.

Dual Throttle Valves

Late-model sport bikes often use large-bore throttle bodies to generate high levels of power. However, with large diameter throttles, when a rider suddenly opens the throttle, the unrestricted torque response can be strong. Dual throttle valve technology was designed to tame engine response while contributing to performance.

On models with dual throttle valves, there are two throttle valves per cylinder: in addition to the main valves, which are physically linked to the throttle grip and controlled by the rider, a second set of valves, opened and closed by the ECU, precisely regulates intake airflow to ensure a natural, linear response. With the air passing through the throttle bodies becoming smoother, combustion efficiency is improved and power is increased.

Now I've looked at it a bit deeper, I don't think these are as good an option as I first thought. They're 36mm and I think I really need 32 or 34mm throttle bodies. The much more modern Kawasaki also develops almost 2x the power of the old Yamaha despite them both being 650cc. I'm going to look for another set of throttle bodies and injectors a bit closer to the performance of the XS650.

[PSIG]

I think you're on the right track, and we have discussed secondary throttle blades here before. While you can make anything 'run' and the extra blades complicate things; they help to solve several issues and why they are used on the bigger ITBs. If you are not going to use the secondaries, then ITB bores of more appropriate size (smallest possible allowing full power) is the usual direction for best control and results.

[DStage]

I'll allow myselfe a side comment that this topics reminds me. My friend and I are planning to put Honda CBR ITB to FSO Polonez Truck It runs an OHV 1.6 engine that is a modified and bored out polish version of an old Fiat 1300/1500 engine from early 60s The thing is it's really restricted on the inlet and the original throttle (in this version a mono injector, there are carburetor and MPI versions too all on nearly identical 1.6 OHV) is barely larger than those individual ones on ITB. Yeah.. let's see how it goes

We do plan to port the head at one point and even did some preparation on a spare one by adding material from the back (tig welding aluminium). We also have a custom made camshaft to put in. And the ultimate goal is to add a compressor
(a supercharger for US folks) to make it look like a V8 muscle car/truck shrunk down and the engine cut in half One of those projects just for fun