I'd say it's advisable for an H-Series KDX Billetron install to run a 90°, 70°, 45°, or perhaps 40° elbow cable guide for the top of the carb, just due to the KDX chassis design with the gas tank in very close proximity.
They shipped my Billetron Pro Series out with no cable adjuster or cable guide (they sell what they call a slip cable adapter @ $13-15 which is just a threaded adjuster for cables like the KDX/KX that don't already have one), but it appears we will definitely need one of the short radiused metal elbows with threads on the carb side and an adjuster on the top side to receive the cable housing termination fitting, as stacking the slip-in/clip-in stock cable into their slip-fit adapter really makes the cable's carb exit far too tall and rigid.
This may necessitate a KX125/KX250 cable from a PWK36/PWK38 carb application if the stock KDX cable doesn't have enough cable length to add the adjuster, & absolutely will require the longer KX125/250/500 2-stroke cables if you want a quick turn aftermarket or KX throttle (recommended!). I have a Motion Pro quick turn throttle from my older Lectron install. Lectron cables for the older carbs are only made for the quick turn throttles with the pulley requiring longer cable length beyond the housing, same as KX throttles.
I have 2 different 90° cable guides that just arrived tonight. The 45° & 40° versions are a lot more common in the USA though, & they may work better due to being more likely to fit the stock cable length. The Sudco 45° cable adjuster guide elbow may be best, but I also see a lot more of the 40° versions. I'd think 40° would be cutting it close to the tank, but perhaps not.
I'll report back soon.
I'll also be reporting back to Matt @ Lectron to make sure he knows of this.
EDIT - to run Kawasaki KX/KDX cables, it appears that an adapter of this sort would be helpful for cable fitment:
EDIT of my edit - those adapters are a 6mm diameter end (0.236" o.d.), but the Lectron slide seems to need a very slightly smaller end around .228" or .230" according to my machinist's drill bit set as inside bore gauges... That are slightly less than 6 mm and am scratching my head as to what actually fits that hole in the Lectron...
The specific Lectron cables by Barnett are intended for the older Lectrons (SAE thread on top cover???), and the new Billetron carbs advertise that they use the stock OEM cables in most applications. From what I'm gathering I believe you're supposed to use your stock slender Keihin PWK 's vertical cable end in the larger horizontal cylindrical hole as the slide is a universal fit for both styles of cable? No slip-on adapter end required???
I also had found YZ owners questioning this same thing on ThumperTalk.
Here's a YZ125 owner's picture from ThumperTalk (test fit, there's a long return spring included thats not pictured):
On a KDX, shortest length 40° or 45° or up to 90° cable adjuster elbow is definitely almost mandatory as the tank sits so close to the carbs. That's how other members had designed cables for the older Lectron carbs in the earlier posts on this topic, custom cable with a 70° or 90° elbow.
Also the 45° Mikuni/Keihin cable adjuster I bought from Niche Supply on eBay was not made well at all, and the Keihin PWK cable end doesn't quite slide through them due to burrs from the manufacturing process (tubing seam on the inside). Bummer. The 90° short elbow will be it...
If I can get my tire change, wheel bearings, and other work done today before 2PM, I might actually get to putting this on today. If not, maybe tomorrow night. Honda buddy coming over this afternoon with all sorts of mechanical problems on two different XR's, needing help...