I am about to buy a raptor and this summer I am going to put jets on it. I have heard that jetted quads don't run as well in the winter. Is this true? If it is, why? And how do you get them to run better?

 

Changing the jets doesn't in and of itself make the quad more sensitive to weather changes. The basic principles at work are that warm air is less dense (less oxygen) and cold air is more dense (more oxygen), hence if the quad runs perfect in 80 degree summer days, it will run lean on 20 degree winter days and vice versa. If anything, when a motor is jetted correctly for weather, it makes the most power in cold weather because the air is denser. The same principle applies to elevation also. If you modify the airbox and put a pipe on, then you rejet to compensate for the increased airflow. If you rejet for 70 degree weather, it should work for a range of 50-90 degrees aproximately. When it gets cold, then you would need to rejet. This is not unique to the Raptor, all internal combustion motors need to be jetted based on weather conditions. If you look at race quads, bikes, or cars, the mechanics pay attention to temperature, elevation, humidity, barometric pressure, etc... and make changes for a little as a 10 degree temperature swing. This has to do with the compression ratio, cam specs, and timing settings. From the factory, the Raptor is in a very mild state of tune, making weather and elevation changes less important than on a highly modified machine.