Hey "uchi" There is a mod called an advance timeing key mod. Im no pro but were discussing this same topic on another site.I am starting to mod my 400ex as well and asked just about the same question on another site im a member of and thats what i was told can be one upgrade amongst others. There is actually a mod im doing to mine its a 3 stage I mod ill get the link and post it here in a lil bit.

 

i was told that the 426 was the best all round ( between 416 & 440 ) & i took mine tto the shop & had them to order me a 426 kit & bore the jug , stupid me i didnt check the size untill i was together and it was the 416 & i cant tell a big differance , it gave me a little more on the bottom but i also went from 14 tooth front sprocket to a 12

 

http://www.partsforatvs.com/de...725&cn=M3Q+400EX+POWER Here is that kit i am getting ready to put in mine. I dont know what the rules are here about posting other sites as far as other forums but if you would like to know where my post is posted that is about the same question message me and ill give you the site info. It has some of the same but some different suggestions.