Here's a picture showing the plenum in situ, you can see the critical point where the frame underside just skims the cyl3 inlet - I used 3 of the runners from the K1100 plenum and shortened them to keep the overall runner length the same as OE. The second picture isn't too clear, but you can just about make out the runners inside the new plenum.
The plenum is made up of 10mm nylon sheet (sides & base) and skinned in 16g aluminium, sealed up with tigerbond - at least the MKII post-backfire version is. The bore in the sides for the AFM has an o-ring seal, as have the 3 elbows that slip over the smoothed out tops of the throttle bodies. The half round relief in the front is to clear the radiator hose. The plenum base fastens down to a convenient M8 hole in the top of the engine block.
Because I have zero knowledge of plenum design (you guesssed, right?) I tried not to steer too far away from the OE layout and dimensions. My concern at the moment is that the bellmouths of the runners are maybe a little too close to the RH side wall of the plenum. I remember reading somewhere a long time ago that the distance should be at least the diameter of the runner, and this layout is a little bit less then that.
When I first got it running it was fine at tickover and low/medium revs, but it wouldn't rev past about 4k. If you held the throttle open it fluctuated unevenly between 2-4k, revving and dying and revving again. I found that if I held the AFM vane open with my finger (technical huh!) it would make it rev higher, but clearly not fuelling right.
Straight away I was ready to blame my plenum design - it was the only thing that I had altered from OE. I played around with it, which resulted in the Bikini Atoll of backfires.
That was all back a few weeks ago (around the time I joined the forum) and the bike was running with a lashed up loom. I was going round in circles a bit and so I pulled stumps for a little while and settled on getting a proper loom built and not rushing to press the starter button anymore.
I'm still finishing off the wiring, getting the tacho to work, but the weird thing is that it's running better and better. It could be a blocked injector - I haven't got round to checking them yet, it could be that the airbox is now properly sealed (most likely candidate, but the problem is still slightly there) or it could be a problem with my wiring that made the alternator rise to 15-16 volts as it revved. The last one sounds unlikely to me, but as soon as I sorted the charging system out and the running voltage got back down to 14.8V, it seemed to rev out a lot better...?
Maybe, it's a combination of things. I've found that it takes a completely different approach to problem solve a previously-working thing (what did I alter/change to make it run bad?) than it does to troubleshoot a new build (check everything from A to Z, systematically)
No clever wisecracks from me eh? Must be morning!