Makes betterpower, use the JD jetting kit, changing the crankshaft mainbearings to one piece C4 rated bearings adds more power and more revs.
MODS- HGS exhaust, VHM head and the VHM Piston work well. Vortex ignition with special maps.
This new lighter / slimmer bike is very good. The engine is all new, the lighter engine likes to be in the higher RPM. Piston squish clearance is a little too wide for racing. Early problems of crankcases cracking around the main bearing should almost be gone now. Checking-adjusting the crankshaft end float is wise when the engine is apart, some are too tight. The powervalve design does require more power now to drive it, and also can cause the power to drop off, and the rider may feel this. We have been using hard coating (nitriding) to have make the levers smooth.
Regular cleaning and greasing of the powervalve flap and rollers is required or they will stick in one position with some loss of power (best to do this before every race meeting!) KTM have made the powervalve very easy to get to, the linkages can be modified for better performance. Stock jetting is almost perfect. Using the older 2012- 2016 ktm sx125 Keihin carby on the 2017 produces more power. The 2017 mikuni carb is jetted too rich. Heavier riders may need to adjust gearing and do a few mods to make sure the engine stays in the higher RPM, as the revs drops the engine slows quickly.
Best mods- HGS or VH special exhausts, VHM cylinder head and the Vortex ignition with the HGS special mapping works well, change the carby and intake boot on the 2017 best for racing. Carb boring works well. Using the VHM or the Vertex racing piston ( single ring ) adds more power. The VHM high inertia crankshaft is available now. The 2017 WP air forks work well. Items to watch- gearbox is very lightweight, dont overload it, like a broken chain jamming it , gears will break. Only comes with the 2 ring enduro type piston. need to change carby if racing. Suffers detonation easily, powervalve can jam up after 10-20 hours.
The KTM / Husky is very fast after some simple mods are made (great bike). Easy to get parts in Australia.