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Since the delivery of our KTM 125 SX, my son Aden has been riding it non stop. He also has been barking in my ear about getting a pipe and silencer so it “sounds cool”. While I am usually reluctant to add parts to my kid’s bike for the sake of just doing it, I thought this could be a good way to test a pipe and silencer to see if it actually helps the Austrian two stroke power plant. After I discussed “the conditions” to my kid, I called upon Lil D and the boys at FMF to send me a Factory Fatty pipe and Shorty silencer to try.
What were Aden’s “conditions”? The conditions were that he had to install the FMF products and help me write this test. I took it upon myself to ride the 2020 KTM 125 SX in stock form as well as with the FMF Factory Fatty/Shorty combo at couple tracks. The installation was fairly painless for my 13 year old. He learned how to use a spring puller for the first time as well as how to push the rubber grommets through the silencer mounting points without stabbing himself with a flat blade screwdriver. Once he got the FMF pipe/silencer installed he of course had to start his bike and see how it sounded and I will have to admit, it sounds pretty bad ass. The pitch of the KTM 125 SX with the FMF pipe/silencer is more of a crisp/barking sound rather than a richer/raspy sound that the stock pipe/silencer comes with. Ok, now that we smoked out the garage and pissed mom off, we loaded up the bike and went riding!
I went back and forth with the stock set up as well as the FMF a couple times at a few different tracks and I will say that the FMF Factory Fatty pipe/Shorty silencer combo did help throttle response as well as low end grunt. Coming out of corners (with the FMF Fatty/Shorty) the KTM 125 SX has a little more acceleration pull on corner exits and could be shifted a little earlier without falling off. The mid range pull was also longer and overall crispness of the mid range seemed improved. The FMF Fatty/Shorty seemed to lean out the bottom to mid range (in a good way) and made the KTM feel more playful around the track. RPM response was improved throughout the power and the throttle felt more connected to the rear wheel.
The top end pulling power/over-rev was slightly shorter than the stock set up so I wanted to play with jetting to see if I could get some of that back. I went back and forth on a few different main jets as well as needle jets, but came back to this set up below:
Needle position: 3rd position from top
Jet needle: 6BFY43‑71
Main Jet: 520 (500 Stock)
Idling Jet: 35
Choke nozzle: 80
Needle Jet: S‑7
Idle Air Adjusting Screw: 1.5 turns out/open
With this jetting the KTM 125 SX pulled longer up top and gave Aden/I more over-rev in 2nd-3rd gears. To me these two gears are the most important for Aden as he learns how to improve his corner speed as well as technique. Sometimes Aden will leave it in second or third gear too long (instead of shifting) and having this jetting allows the KTM 125 SX to pull a little longer. This jetting was good for sea level up to 3500ft. Anything higher than 3500 feet elevation and I would go back up to the 2nd needle position.
Overall, there is some performance gains to be had with the FMF Factory Fatty/Shorty Silencer. Aden has put over 14 hours on the system and the silencer still sounds good and not blown out. He did manage to put a small ding in the bottom of his pipe and that made him mad, but he’s ok with it after he took some WD-40 and Scotch Brite to the thing! Proud dad moment! The Fatty is $249.99 and the Shorty costs $134.99. A lot les expensive than a four-stroke system!
Below is Aden’s opinion of the FMF and stock pipe/silencer. I wanted to let him have free range on what to talk about, so he can try to get better at communicating his thoughts into a testing format. During the course of this test I never shared my thoughts about the FMF Fatty/Shorty for confidential purposes. His opinion is unedited, but I did clean up a few grammatical errors for the sake of reading. After reading his opinion, I couldn’t help but write down my thoughts about his opinion. It seemed too good not to share with you all. Enjoy!
Aden Keefer Opinion:
The differences between the stock pipe vs the FMF Fatty/Shorty pipe is that the FMF pipe hits stronger coming out of corners, but on the other hand the stock pipe is a little bit more linear coming out of corners. The stock pipe takes a while for the power to kick in which was ok on hard pack desert tracks that I rode. I did like the FMF pipe more because it was aggressive and you can put the bike anywhere knowing that it’s gonna have the little extra power. The one reason I do like the stock pipe is that since it’s more of a longer silencer I can control it better when I get tired because it’s not so aggressive. Don’t get me wrong though! I think the FMF pipe and silencer is one of the best pipes out there. Not does it only make your KTM 125 SX faster, it makes it look super cool and it sounds way better than the stock pipe. I also noticed when we put VP C12 in the stock pipe/silencer it made it run aggressive, but when we also tried out VP C12 with the FMF pipe/silencer it ran like a race bike. It made it so fast that I was pulling all the 450’s going up Mt.St.Helens at Glen Helen. I also think cleaning the FMF pipe/silencer is rewarding. My dad loves to vacuum because he said he can see his progress with his work. I like cleaning my FMF pipe and silencer because I can see my progress as I scrub it with an SOS pad. So to wrap this up, I think that the FMF pipe and silencer is the best pipe out there for any 125’s out there.
Dad’s Thoughts About Aden’s Opinion:
-I appreciate some of these testing comments and at least it shows that he is listening to me a little. So you’re saying there’s a chance! Hallelujah!
-I like that he could feel that the FMF Fatty/Shorty had more bottom end. That shows me that any kid could install this product and feel the difference.
-It’s funny that he recognizes how the KTM works on different terrain and can appreciate a smoother power at times. I am glad he’s not a balls out rider! Safety first!
-Pulling 450’s up MT. Saint Helens? Eaaaaaaasy there tiger!
-Of course when I was his age I also thought something that was aftermarket was way cooler! Why? Because it wasn’t stock! Duh! After reading his opinion it seems that he’s into the styling of the “Raw Fatty look”. Me too, Aden, me too!
-Here we go with the C12 talk again! That is all this kid was talking about for a few weeks! “Dad, let’s get some C-12”! “Dad, let’s get some C-12”! I broke down and got some. The C12 did make the KTM’s pulling power stronger, but also leaned it out enough to warrant the jetting change. However, the jetting change that worked with the VP C-12 also worked with pump gas too. Don’t be scared to run that jetting with either fuel. Aden wanted it because of the smell! Mom’s still mad because her kitchen smells of C12 and Maxima Formula K2. Sorry, babe! Not really, because our house smells like a race shop now! 🙂
-Aden coming in hot with the vacuum reference! Wow! My secret is out now. Great!
-Good job Aden. We have some work to do, but good job son! Love, Dad!