it almost sounds like jetting to me, 400es's are notoriously cold blooded, they take about 10 min to warm up and when they do its great, but i had the same issue last year, when your motor is cold it needs more fule to keep it running, thats why you have a choke to lean out the air fuel mixture, if it is already too lean you will notice it boggs on hard accel and slowly starts to increase in power or when you snap the throttle it will cough or stutter out white smoke, i never adjusted my carb and i was stupid not too, i just thought that i had bad rings and bad valve seals or something, if it is fine when you first start it or when the engine is cool and it gets worse as it warms up then i think it is a jetting issue, piston ring failure or valve seals wouldn't go this early, try adjusting your fuel mix screw, when it was originally set up it might have been at a different elevation or it might have been a different season which would cause them to have to lean it out, if that dosnt work then thats one thing you can rule out