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Written By: Gardner Tarlow
125s, To Inject Or Not To Inject?
When Kris offered to let me test the 2023 KTM 125 SX FI machine, I was more than stoked to get the opportunity to compare KTM’s new 125 FI to some other well carbureted 125s. With that being said, I would have been even more stoked to try a 150 FI bike but unfortunately for the vet riders or bigger guys, the 150 SX FI is not part of the 2023 lineup and currently there is not an aftermarket KTM 150 kit for this bike. Hopefully in the near future KTM will provide a 150 kit for the 125 FI bike but until then, be happy that our Austrian brethren have continued to push the limits of two stroke design and technology and just keep on smiling.
Great bikes come and go but nothing beats the sound of a well jetted 125, two stroke. The crisp sound of a 125 always gets my attention and brings me back to a time from my past. At 185 pounds, I’m obviously not the intended market for a 125, but with that being said, be it moto or enduro, I am a huge fan of small-bore, full-size bikes. My current favorite track bike in my garage is my 2022 KTM 125 that Andrew Langston rebuilt from the bottom end up (thanks KTM) to a 150 and its jetted absolutely spot on. My 150 does a great job of pulling my extra weight around without having a temperamental, modified 125 motor. With the 2023 KTM FI bike ideally being tuned spot on I wondered how it would compare to a well jetted, stock 150 bike.
The first thing you notice looking at the 2023 is the new plastics, graphics and new orange flatter seat. The new front fender has fins on the backside of the fender designed to divert dirt and mud away from the radiator intake. The airbox has been completely redesigned to redirect and increase airflow to improve engine performance. An electric start device now replaces the missing kick starter and the electric starter is located under the motor in front of the clutch cover. The engine start and stop buttons are combined on a single unit on the throttle side as well as a map switch with both lean and rich mapping located on the clutch side. The pipe appears a bit smaller and more compact as well as the aluminum silencer is now black coated, shorter, and more compact. The Chassis is an all-new chromoly steel frame and the subframe and swingarm are all new design. The rear shock has been updated to lessen squatting on acceleration, which in turn helps overall handling and cornering. There is more contact area with the frame for grip as well as the foot pegs are significantly larger. These changes were created to provide a more stable chassis compared to previous generations.
A well jetted 125 is a blast to ride and the only thing better than a well jetted 125 is one that never needs to be jetted in the first place. The 2023 EFI 125 solves that issue (in theory) and in its stock form, is about as spot on as I think you can get. Pump gas at 40:1 was used even though KTM recommends a different mixing ratio. This particular bike here on the west coast seemed fairly crisp from the moment it is started, but I do notice it being a little rich if I get lazy and try to ride a gear high. At no time did I notice a hesitation, bog or blubber while riding on the track when riding at my normal pace. The power is very linear with a good amount of bottom end, as well as a mid to top end that signs off really late for a 125 two stroke. I have heard of other testers complaining of detonation, however, I did not notice any detonation with this particular bike. This bike really likes to be rung out and performs best when in the upper half of the powerband as you would expect with a 125. I did notice at times the mid-range required more clutching to coach the bike back into its peak powerband as compared to some of the other carbureted 125s I have ridden. I was fortunate to be able to compare the 2021 TC125 and SX125, 2022 YZ 125 as well as my newly converted 2022 SX125 to a 150. I liked all the bikes that I rode and used for comparison purposes. I think riding these other bikes allowed me the ability to appreciate what the 23 KTM 125 SX FI bike is all about. Some of the other bikes power bands are more forgiving and easier to ride when mistakes are made but the KTM 125 SX was in no way ever at an appreciable disadvantage from a power point of view. Ideally, I’d like a YZ125 bottom end and a KTM top end but that’s not currently available from any stock bike set up. Comparing the FI 125 SX to the well jetted 150 SX, I didn’t think was a fair motor comparison as the 150 makes 3 plus more horsepower stock. In comparing my 2022 chassis to the 2023 chassis there were appreciable differences in the feeling of the two bikes and I do believe that the newer chassis seems to feel calmer and more stable than the previous chassis. My ability to ride long ruts really well is not part of my arsenal of skills, that’s not to say I can’t ride them but I am not the guy at the track that others watch to perfect their skills. I do feel like this 2023 chassis being calmer, allowed me to set up better entering ruts and allowed me to remain comfortable within the rut through area 2 and 3 more consistently. This to me was a real positive for the 125 as my own 2022 150 chassis set up was not as consistent with this feeling.
I’ve put almost 20 hours on this stock bike and feel very comfortable about where it could be improved if at all. With the exception of maybe the YZ 125, most 125s require spot-on shifting when cornering and really expose rider errors when made. This EFI bike was no different, however, with a bit of clutch slippage, recovery was manageable and similar to the other Austrian 125s ridden. My issue was with the mid-range and my need to abuse the clutch to keep the bike in the power band was consistent at numerous tracks. This probably has more to do with my size versus any motor issues but it was an issue that was exposed none the less that was not appreciated on the carbureted 125s.
Being an all new FI bike there are limits (at this time) that allow for modifications to the stock format. The inability to adjust the electrical power valve for me took away the ability to modify the power band as I was able to do with previous mechanical power valve bikes. The ECU on this bike is locked (unless Twisted Development or other similar companies) and at this time there is no aftermarket ECU’s or an ability to remap this ECU. I believe once the ECU issue is addressed this will allow for a lot of changes to the personality of the motor. Currently the bike does have two motor map switches; white map is standard and green map is rich, which is designed for sandy tracks and conditions. When trying the richer setting I didn’t notice any significant difference in the power character of the motor other than it seemed rich in the bottom half and had an occasional lag sensation on track. This green map may be good for very sandy tracks but otherwise was not of any use for myself here on the west coast.
In hopes of bringing a bit more punch to the overall power of the EFI bike we tested both the FMF Factory Fatty pipe and Pro Circuit Works pipe/shorty silencer. At the time of testing, I was only provided the FMF pipe without the matching FMF silencer, for this reason the stock silencer was used. This FMF pipe seemed to provide overall better power throughout the lower and mid powerband. The pipe alone improved the sense of needing to abuse the clutch noted before in mid-range as well as helped with small corner mistakes where the stock set up suffered slightly. I believe the stock pipe felt like it had slightly better top end and over rev compared to the FMF but not enough for me to trade away the bottom end and mid range benefits that the FMF pipe provided. The FMF pipe was definitely an improvement over stock and I am uncertain as to what difference the FMF silencer would make.
Next was the Pro Circuit pipe/shorty silencer. Installation of the PC pipe and silencer was a bit of a pain compared to the FMF. Despite installation frustrations, once installed and after a few motos it was obvious this set up was better than both the stock and FMF set up. The bottom end coming out of corners was snappy and more responsive and pulled hard through mid and top end. The over rev with this pipe and silencer was stronger than both the stock set up and the FMF Factory pipe alone. I am uncertain what difference the FMF silencer would make over the stock silencer set up but with the configuration I had, the PC set up was preferred. This improved performance with the PC works pipe, shorty silencer as well as pump gas was impressive and put this bike in closer contention with my 2022 KTM 150 in stock form. With more performance modifications and an adjustable ECU becoming available in the near future, it won’t be long before the carbureted versions become a bit obsolete for all but the die-hard carb tuners. Stay tuned for more tweaks and mods to come.
As far as reliability and maintenance goes, I have had zero issues with the 125SX after almost 20 hours of ride time. I have been literally changing transmission oil and air filters only. I