Last winter, I installed the Athena 440 Big Bore Kit on my 2004 kfx 400. The fit and finish were great, but the instructions were vague and poorly translated so I recommend a tear down manual. I installed a stage 1 intake and exhaust cam at the same time and the mods seem to work well together. As far as jetting goes, I went up one main jet size and raised the needle one notch. I had already fattened the jetting from stock when I upgraded the exhaust. Cold starts are good with a little choke and hot starts are good as well with a blip of the throttle. Fuel economy remains fantastic as I was recently able to drive 80km on varying terrain and elevations before finding my reserve! A bonus of the big bore kit is you still run high octane pump gas as the compression is unchanged from stock. Ok, Ok, you’re wondering, What about power? Power is great as I basically gained a gear across the board. Anything I needed 1st or 2nd to climb/chew through, I can now do in 2nd or 3rd. The low end is a little fussier as I lost some lug but the midrange is great and the top end is strong. This is the first season since 2004 that I have not wanted any more power! Would I recommend the Athena 440 kit? You bet! Price is good, power is great, and reliability has been great after one season of trail pounding. Have fun!