This does not seem right?

[T.Hopp]

I had a stock 400 w/twin air filter, 6 -1" filter vents in the air box lid. Jetting was stock at 148 (keihn). The bike ran fine. I just installed saturday the Sparks Key along w/HMF slip-on. I figured a rejet was in order. Before switching jets I let the motor warm-up just to see what it would do. It ran good. Throttle response was excelent. I didn't ride it as to keep in good standings with neighbors. To get ahead of the game for when I got to my riding spot, I installed a 155 main. I picked a 155 because search results w/similar set-ups led me to it. The 155 ran worse. It stumbled when pushing the throttle. This result made me switch back to the 148 and see how it reacts while riding. What a difference! I just don't understand why the stock jet runs so well with filter, vented airbox lid, grounded headpipe,and Slip-on? Any thoughts to this?

[Cool Honda Man]

I have been told that if you put a slip on exhaust and an air filter that no re-jetting is needed. Before changing jets again I would run the atv on WOT and see if the bike is running lean.

[frosti108]

yea you better do some spark plug checking... ;)

[K_Fulk]

There jetted rich from the factory. You should be fine with a 148.

[Toadz400]

Weird...I'm at 155 with a Big Gun slip-on.

I wouldn't think that they jet them THAT much more richer to not have to rejet when adding a pipe and vents in your lid?

[K_Fulk]

I ran a 155 on my 440 with porting cam full pipe and no lid. And by looking at my pistons it was still a tad rich.

Plug checks arent that accurate on 400exs. You need access to a exhaust sniffer to get it real accurate.

[frosti108]

whats this exhaust sniffer you speak of? would it by any chance be similar to the thing shops do emmissons tests with? :confused:

[K_Fulk]

Yeah, most places that have a dyno have them to tune.

[frosti108]

o cool. thanks

[stevo3176]

That sounds strange I have a kn in the air box top on no holes and a hmf slip on as well. I put a 155 in too and the thing rips. What is you elevation? Are you using the correct jets? One more I made this mistake once was the choke off?