1990 yamaha xt 600 problem..white smoke??

[Jaguars7129]

Hello, I notice that whenever i start up my yamaha xt 600, it runs ok for about 15 seconds and then white smoke starts to pour out of the exhaust and continue for about 3 minutes, and then stops and runs fine..The bike is air cooled so it isnt anitifreeze burning and i really want to know what the problem could be. The bike has 17000 miles on it and any help would be greatly appreciated..thanks

 

[helio lucas]

on my xt 600 when i had the loud, straigth silencer, it seems that the packing absorves moisture or something when the bike colds down... and then if i start from cold it seems to do the same thing, after ~1 minute it goes away...
then i have built some internals to quiet the exhaust and never done it again. i have never worry about it, but it seems that the packing i have used (rock wool from house building) doesn´t help.

and yes, it was water since it collect some bubbles on the hand if i put it on the exit of the silencer...