On most 4st. motocross bikes under the air filter is a restrictive backfire screen. By using an air filter system without the backfire screen. When you increase air flow which will increase power. Usually, you can expect 1-2 hp gain with better throttle response.
There is also a second problem the OEM filter is only held down by two clips. The clips do not hold the air filter down very tightly, dirt may get past.
For a note - I have used the No Toil Super flow kit on a few Yamaha over the past few years and always worked great.
There are two companies that make kits to eliminate the Backfire screens TWIN AIR COMPLETE POWERFLOW KIT or NO TOIL SUPER-FLO KIT.
The Twin Air kit uses an aluminum cage to replace the backfire screen with a flame-resistant air filter and lock down screw for the filter.
Thick, flat foam, greaseless sealing ring ensures maximum contact with your air box while acting as a breathable gasket for increased air passage. Smaller than stock to increase air space and circulation around filter
Twin Air Powerflow Kit Air Filter Kit Yamaha YZ450F 14-17 / YZ250F 14-18 - 152219C
Twin Air Powerflow Kit Air Filter Kit 18-22 Yamaha YZ450F / 19-22 Yamaha YZ250F - 152222C
The No-Toil Super-Flo Kit uses a plastic back-fire screen-less cage, a pre-oiled flame-resistant filter, and a new filter lock down screw.
No Toil Super-Flo Yamaha YZ250F 2014-2018 / YZ450F 2014-2017 - SFK18951
No Toil Super-Flo Air Filter Kit - YZ250F 19-21 / YZ450F 18-21 - SFK18053
The best part about installing either the Twin Air or No Toil Kits, you will feel the difference in POWER.