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My son and I have been going back and forth on mufflers for his KTM 250 SX-F and have went though about three until we settled on the FMF 4.1. We have tried the HGS (which was good), a Yoshimura, as well as a Pro Circuit system, but for his riding style and what he was looking for from the engine character, we decided on the FMF 4.1 system. Why? Here are some reasons why the FMF 4.1 system works well for Aden and the KTM 250 SX-F.
First off, the FMF 4.1 system looks beautiful and is easy as pie to put on. Installation literally took 10 minutes and I like that FMF pipe springs last longer than the stock KTM pieces. Aden rides lower in the rpm and only revs his bike in the air (which he thinks helps him but it really doesn’t) so he needs some low end rpm response as well as some low end torque to help get the KTM back into the meat of the power (or “recovery”). You will get more low to mid RPM response as well as more mid range pulling power with the 4.1. Top end is increased slightly plus the over-rev is as good as stock. It is one of the few times that I was impressed with a muffler because it was actually better than stock in almost all areas. THIS IS VERY RARE! Coming out of corners the KTM 250 SX-F now has some more excitement in second gear and pulls harder than the stock system. Third gear roll on is also noticeably more reactive to your throttle hand (increased connection) and can allow for a lazier rider to have more pull in softer loam. We did pull the spark arrestor/insert out with good results and decided to leave it out for maximum throttle response and torque feel. Leaving it in will kill a little low to mid response, yet you still will get better torque feel over stock.
The downside is that you must keep up on your muffler packing (every 10-12 engine hours) because the FMF will blow out and that could damage your expensive titanium can. Just in case you’re numbers guy, did I mention that you will lose almost two pounds with the titanium 4.1 system? If you slap on spring forks on your KTM, you will be gaining almost three pounds. You can get most of that weight gain with the spring fork back down with a 4.1 Titanium FMF Muffler system.
Things to look out for? I can burn a lot of pants on the headpipe of the FMF system. The magabomb portion of the headpipe can burn your pant just above the boot when sticking your leg out in deep ruts. This has happened on only a couple different brands of gear (FXR, FLY) with the FMF headpipe installed. Overall we are happy with this system for the way Aden likes to ride and will be refurbishing his mufflers every 10-12 hours in order to keep the most performance possible. Hopefully FMF will have these back in stock in the near future so be on the lookout over on their website that gets updated weekly on what is in stock. Damn you Covid!
