Yamaha have released details of their new motocross models in the U.S.

While colours and other specifications may vary slightly from the Australian models on their release, there are a few noteworthy changes that we will see for 2012.

After both the YZ450F and YZ250F received complete rebuilds from the ground up for 2010, it is not suprising that Yamaha have only made small adustments to their models for 2012, refining two already proven machines.

The 450 has received some engine finetuning, with the aim being to improve midrange while smoothing out the bottem end hit.

The suspension has also receieved some attention, with the 48mm KYB Speed Sensitive System (SSS) forks being revalved to increase low-to-mid compression damping.

On the rear, the KYB 50mm rear shock has revised rebound damping, a new rebound adjuster oil passage and improved valving with less low-speed rebound and more high-speed compression.

The biggest news for the 250F for many will be the news that Yamaha have decided to stick with a carburettor for 2012. While many will be disappointed, Yamaha have made upgrades to their bulletproof 250cc engine that will mate with the carburettor to ensure it won’t fall behind the pack.

For 2012, the 250F gets a 2mm large carb bore from 37mm to 39mm for more top end power, a point of criticism from some on the 2011 model. There is also a new air filter cage that flows more air to go along with the revised engine mapping for more mid and top end grunt.

The engine also receives a piston that is 8.7 percent lighter, along with a revised crankshaft balance and counter-balancer to reduce vibration. The connecting rod and big end bearing get a new heat treating process for more strength and durability.

A more thorough run down on the new Yamaha motocross range will follow upon Australian release.