http://www.xs400.com/forum/showthread.php?t=6479
Hi guys, I found this one, key words was bog down. I believe something that we don't know is 'when did the bike ran good last?' Were you on it yesterday? It ran great? or you ran it last month, and didn't put any fuel stabilizer in the tank? (Seaform, stabil) Seaform was stating that it never got off the ground as a "fuel stabilizer" which is what it was created for. be interesting to hear if an appropriate amount of seafoam would fix your issues? ((X amount times the amount of fuel in tank) 3 gallon tank) just my thoughts meandering. otherwise, double check that previous thread, I am sure it.
here is another one.
http://www.xs400.com/forum/showthread.php?t=10051
and a another one, This one atleast the thread starter came back and admitted what happen, or how it got fixed. Drewpy thought some crap got up into the carbs,and I know I did have a bug in the diapham area. I found it prior to getting it on the road, so I dont know what symptoms it would have gave me. It was some sort of beattle. Okay, well there is three spots, based on the bog down keywords. Okay this seems to be indepth,
http://www.xs400.com/forum/showthread.php?t=6112
You know I had used the term Italian tune up, thinking of just getting it on the road, and WOT, wide open thottle. Early in the season, I don't remember what caused the issues, I think it was still chilly, the bike sat for a week, to 10 days, and then my wife drove it, it sucked, then I drove it to work, it sucked all the way to the interstate. then I hit it hard, there was some sort of garbage that worked it way out, cause unlike that previous guy in one of the threads stating he is back to 50 mpg, I was back to my normal 40 mpg. Early in the season, I was using the non-corn sryup gas. now I dont' worry about it unless I need to leave it alone for more than a few days.
Good luck guys.