400ex Jetting Question

[BUCKEYETIM]

I just installed a WB e-series slip-on (12 discs with spark arrestor removed), K&N air filter, air box lid still on, on my '03 400ex. I followed the directions given to me by dyno jet by installing a DJ146 main with three and 1/2 turns on the fuel screw and the dyno jet needle on groove #3. It seems to be running pretty good , but I am still getting the occasional backfire on deceleration. The guys at Dyno Jet are telling me not to be worried about it as long as it is running ok. What do you guys think? I currently live in GA, so the alt. is somewhere around 300. Any suggestions?

[F-16Guy]

Backfiring on deceleration is usually caused by a lean idle circuit and/or an exhaust leak. Most people on here will tell you to go to a 42 pilot jet, but I think a 40 is plenty for a mostly stock bike ( pipe, filter, jets). I have a 42 pilot in my 416, and I think it's a bit rich, even with my mods. The rest of your jetting sounds pretty good.

[04'400ex'er]

I would take the slip on off and put silicon for a gasket. if it still backfires then go up one size on the pilot and you will we through with the backfireing.

[BUCKEYETIM]

Well, I took it out again today for a short ride. I am not getting any abrupt backfires, but instead just some popping on deceleration at lower RPMs. I called Dyno Jet again and spoke to them hoping to get a different person. The guy I talk to today said that he also had a 400ex and had the same popping while he was jetted correctly. The way he explained it made me feel pretty confident that it was fine, but he told me that I could go out 1/4 to 1/2 turn on my fuel screw if I wanted. Should I even worry about it, or should I just leave it as is? The bike is running pretty damn good.

[cals400ex]

if it is fairly loud, i would just get a larger pilot jet. if not, you should be fine. a way to kind of "check" to see how lean you are is to let the bike idle for a minute or so after it is fully warm in a fully dark place. a shed or garage or at night works good and look at the headers. if they are glowing, your probably not too rich for sure. make srue the bike is idleing and your not giving it gas. the stainless headers tend to glow pretty easy so don't be shocked if they are. i ususally jet my bike so my headers are just to the point where they don't glow.