Ok here is my scenereo, I did the muffler mod last year, rejetted to a 155 (150 was stock) ran great, then this spring, my bike ran like crap so into the dealer it went, turned out there was a fly in the float bowl and the choke plunger was bent, ran good after that (dealer had to order plunger) went in after the plunger came in and all of a sudden I had a sputtering and backfiring problem after half throttle, back to the dealer, he said I was running lean he figured so he drilled out the 155 (didnt have any bigger ones in stock) to a 157, still sputtering, so I brought the jet in and he drilled it out a bit bigger, WAY worse. So just to humor myself I tried a 152 jet, runs perfect, no backfiring, no sputtering. I dont want to screw up my engine but does this sound right?? Wouldnt you sputter when lean and not rich?? Besides plug color what are the symptoms of a rich running machine and a lean running one?? Thanks for any help. It is a 2003 400 i

 

[img]i/expressions/face-icon-small-smile.gif[/img]hi doober sounds right about what your saying about your jetting but one thing to keep in mind running to rich might cause that as well. engine might be running hot might want it run rich.i got a 400i 2003 as well dont have any problems yet got 1500 miles on never done nothing but change oil i use that synthectic blend seems to work welloh did the trany trick to. doesnt help the gas mileage thoughwith the trany trick but does have more getup and go. mtcat seemed to be running hot last year when i bought it so ive been putting 15-40 oil seems to be running a bit cooler now.

 

Theres a couple of things you can try, while troubleshooting, to figure out if its too rich or too lean.
Try pulling the choke on , while the sputtering happens. If it gets better....youre too lean. If it gets worse, youre too rich.
Pull the air box lid, if it runs better.... youre too rich, if it runs worse youre too lean.
Heres a page with symptoms of lean and rich.
jetting

 

Thanks hondabuster, it sounded rich to me but I havent taken it for a good ride yet.