One other thing i've run across with these symtoms was exccessive carbon deopits in the combustion chamber causing the rings to stick and white smoke on take off , you could try adding some Sea Foam with your fuel it cleans the entire fuel system ; as well as cleaning exccessive carbon deposits in the combustion chamber i've seen it work on small engine's from as little as lawn and garden equiptment and as big as automobile engine's !

You can get it at all Auto Parts Stores and most Walmart super centers for around $7.00 plus tax , it works on both 2 and four stroke engines ; gasoline and deisel fuel this is the best fuel additive i've found and it's been on the market with a satifactory money back guarranty since 1942 !

Good luck with your 400ex , williebee......

 

Usually when your having white smoke it means you either have oil passing by the piston which means it needs bored out so you probably should pull of the top end and mic it all