Q: FIRST AND FOREMOST, IS THE 2018 KTM
125SX BETTER THAN THE 2017 KTM 125SX?
A: Yes. Small changes were made to the orange
tiddler for 2018, although nothing significant. But, in the
world of 125cc two-strokes, the KTM 125SX is in a good
place since their isn’t much worthy competition—save for
its brother, the Husqvarna TC125. What about the YZ125
you ask? The YZ125 is an anachronism that hasn’t been
significantly updated in 12 years. It is outdated and out-powered, not only by the 125SX and TC125, but also by
the TM MX125. As for the KTM 125SX, it was completely
redesigned last year. Our biggest gripe in 2017 was with
the then-new Mikuni TMX carburetor and its horrible jetting. It was a nightmare, but some of it was our fault for
not reading the manual. KTM protocol demands a fuel/oil
ratio of 60:1 rather than the normal 40:1. Once we changed
our mixture, it was better—although it didn’t entirely solve
our 2017 jetting issues.
Q: WHAT CHANGES WERE MADE TO THE
2018 KTM 125SX?
A: The entire KTM SX lineup got an orange frame
and a new look. The rest of the small changes were in the
forks and jetting in 2018.
(1) Fuel mixture. Last year the 125SX called for a lean
60:1 fuel/oil mixture. For 2018 it has been changed back
to the normal standard of 40:1 (and the jetting has been
altered to accept this difference).
( 2) Jetting. With the pre-mix ratio changed for 2018, the
jetting has been changed as well. For 2018, the 125SX’s
Mikuni TMX 38mm carb has a different needle (a 6BFY42-
75 instead of a 6BFY43-73). Although the main jet remained
480, the pilot was upsized from a 25 to a 35.
( 3) Forks. The 125SX got the same update to its WP
AER air forks as the four-stroke lineup. There is a new
piston, air seal and updated valving.
( 4) Head stays. The head stays are now made out of
aluminum instead of steel.
( 5) Frame. The frame is exactly the same as in 2017,
save for the color. It went from black to orange.
( 6) Shock. The valving has been revised to work with
the new fork valving.
Q: DOES THE 2018 KTM 125SX RUN BETTER
THAN THE POORLY JETTED 2017 MODEL?
A: Yes. Thank goodness. It was a nightmare last year
trying to sort out the jetting. This year it is in the ballpark
off the showroom floor. We only went one size richer on
the main jet. We still think the Mikuni TMX 38mm carburetor is inferior to the Keihin PWK carburetor that came
standard on the 2016 125SX. The old Keihin carb was less
temperamental and ran cleaner throughout its range of
power. It should be noted that KTM did not switch from
Keihin to Mikuni due to a performance boost. It doesn’t
take a high IQ to figure that out. Although KTM has never
offered a satisfactory answer as to why they changed, we
assumed it was a production capability issue with Keihin,
a price difference between Keihin and Mikuni or a little of
both. Whatever it may have been, we miss the no-hassle
Keihin (not that we didn’t have hassles over the years
before KTM got the Keihin dialed in).