Incorrect fuel curve. How do you change it?

[Kansas T]

I got my 400ex dynoed the other day and the fuel curve is way off. At low rpm the a/f ratio is 13 to 1. In the mid range the a/f ratio goes to 13.5 to 1 and then at higher rpm the a/f ratio keeps dropping until it gets to 10.5 to 1. I have a K&N filter, dynojet kit, open air box, HMF slip-on, and an XR400 cam. The dyno opperator commented that most dynojet fuel curves look like that. Will switching to keihein jets and removing the dynojet needle level off the fuel curve or is there another way to correct it?

[wheeltrax]

going up higher on your main jet will help.

[Kansas T]

Wont that also richen it up even more at higher rpm's?

[nosliw]

Originally posted by wheeltrax
going up higher on your main jet will help.


ok......

he's at 10.5:1 in the upper rpms. meaning 10.5 units of AIR to every 1 unit of FUEL. the lower the AIR number means the more FUEL will be in there relatively.. why would he want to go up on the main jet?

[wheeltrax]

ah yes.. sorry, my mistaking. thanks for pointing that out lol. I know ideal all around ratio is supposed to be 14.7:1 then leaning the main jet would perhaps help you a bit.

thanks nosliw for pointing out my mistake lol

[nosliw]

Originally posted by wheeltrax


thanks nosliw for pointing out my mistake lol

hey that's what im here for :D jk!!lol

but anyways kansas t,

lower the main jet, maybe up the needle clip on your jet needle, and screw in your fuel screw a tad. all those things will lean out the mixture in their own throttle positions.

[Kansas T]

I think ideal a/f ratio for power and long life should be around 12.5 or 13 to 1. 14.7 is the correct ratio to keep catalytic converters working on cars. I really dont think changing the main jet one way or the other is going to flaten out the fuel curve. I'm used to working with fuel injected vehicles and I can simply alter the fuel maps on them via a laptop computer. I just dont know how to alter the carb to do what I want.

[Mobile Dyno]

your pilot looks good and your needle to. What size main do you have? If your at 10 than you may need to go 5 dynojet sizes smaller and that should get you in the range. Does the bike spit and sputter topped out?

[Kansas T]

I currently have a dynojet 170 main in it. It doesnt ever spit, sputter or even run bad, but it doesnt really pull in fifth like I think it should. It just gets so rich it stops pulling. Wont going smaller on the main jet also make the midrange rpm leaner than it already is? All the numbers I posted are from a full throttle run, no part throttle.

[Mobile Dyno]

Didn't the Dyno operator test the pilot and the needle? Going smaller will lean the needle a little bit. It will lean more around 5-6k rpm but it won't hurt anything. I say go to a 160 dynojet main and that should get you were you want to be. let me know if you try it.

[Kansas T]

No, nothing was tested. I dont think they have had the dyno very long and its purpose seems to be more bragging rights than tuning. I would have prefered alittle tuning time to make adjustments and try some things, but I couldnt do it there. I'll probably dyno again soon if I get some changes made and find a different dyno shop.

[Chino886]

Hey Kansas T,

Curtis Sparks recommends a 155/45 combo on the 400 EX, check out their website.

[wilkin250r]

Originally posted by Kansas T
Wont going smaller on the main jet also make the midrange rpm leaner than it already is? All the numbers I posted are from a full throttle run, no part throttle.

Yes, smaller on the main jet will also make the mid-RPM WOT leaner, but that really doesn't matter much. When you're actually riding, how much time do you really spend WOT at 4000RPM? Usually, when you're full throttle, you increase RPMs rapidly, so a lean condition in the mid RPMs really won't have much affect.

I wouldn't put much concern into your main-jet effect on low and mid-range RPM. They don't really matter, because you never spend any time in that RPM range at WOT.

If I were you, I would concentrate your WOT jetting for the higher RPMs, and leave the low and mid-RPMs for other throttle positions.

[Kansas T]

Ok, thanks for the suggestions. I guess I'll drop the jetting down alittle bit and see what happens.