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Yes, we know it is tough to get a full muffler system at this point in time as COVID has screwed a lot of inventory up. However, you can find slip on mufflers as FMF has been busy getting product back in stock! We recently tried a dual injector kit on the Yamaha YZ250F and needed a muffler to try so before we got the dual injector kit installed we took the FMF slip on and the stock system out for a day of comparison. Here is our findings.
Although the stock exhaust is VERY hard to beat in terms of performance, it can get a little raspy once the hours start adding up. The FMF 4.1 system slip on gives the bike a throaty tune and a factory look, but with a more tunable engine character. We have found that running the system without the spark arrestor insert takes away some back pressure, which causes the YZ250F to lose some of that bottom end power we come to love out of this bike. If you want to retain some of that back pressure/low end RPM response cu the spark arrestor out and install the insert.
The 4.1 adds mid range power as well with a little more top end pulling power. We prefer the system with the insert installed because it helps keep most of the stock bottom end, keeps the YZ250F quiet, and can even make it legal to ride on the trails (when spark arrestor screen is left in). You will lose use over a pound of weight when installing the FMF system (over the stock muffler) so that is another added bonus. The FMF system is made well but just be sure to keep up on the muffler packing (every 10 hours) and the canister will stay together longer. The longer you prolong re-packing the closer you get to burning up the can and possibly blowing out the rivets/etc. Oh and I almost forgot to mention that installing the FMF slip on is painless and takes five minutes of your day.
Overall, I am happy with this FMF 4.1 slip on as it improved mid range/top end and made the YZ250F sound much better and didn’t sacrifice as much bottom end as I thought!
Best Yamaha Power Tuner Map With FMF System: Keefer Free Feeling Map (On My Website)
