400EX Backfireing

[HondaRacing23]

I have a 2000 400ex with a 426 EX kit in it. I have a hot cam stage 1 with a K&N air filter. I have myu needle 2.5 3.0 turns out and a 42 pilot with a 155 main. it doesnt matter what i do the bike is always backfireing. WHY? i have dont eveything to this bike and it is always running crappy and i cant find out why? thanks

[nosliw]

maybe your headpipe isnt bolted all the way down to the head. if it is then when is it back firing? wot 3/4 1/2 throttle?

[HondaRacing23]

when i have it pined then let off. or when im going through a little trail and come to a straight strech and get on it then when i let off to turn it is like "pop pop pop pop" it runs fine all except the backfireing. Also I have the HMF full system and I hate the headpipe because it has two sections where it bolts onto the head. It does leak I know that and i have did everything possible to get it not to and it still does. if you or anyone have had this same problem let meknow what you did. im at the point now to sell it on ebay and get the new HMF headpipe. thanks guys for all your help...

[pead_24]

go up on your main jet sounds to me your running lean?

[boricua]

if it pops when you let off the throttle, usually completely off or close to it, its more then likely the leaks in your exhaust. air gets sucked in the pipe causing a lean condition and it pops out the back....too lean a jet may also play a role, but i would start with the leaky pipe first....

[enduro400rider]

my 400ex does the same thing i have adjusted the jetting atleast 10 times and it doesnt help. the only thing i could figure out is it only started doing it when i removed the spark arrestor does yours have one installed in your pipe if not it might be the same reason mine does it. i dont know why the spark arrestor would affect it like that but it does on mine. when i pull in the clutch it stops backfiring and it only backfires when i let off the throttle during the first 15 20 min of riding and then it doesnt do it any more and thats after warming up for about 5 min so the only thing i can think off it the spark arrestor hope this helps

[HondaRacing23]

mine dont have a spark arester on it. you are telling me to put it back on? it dont might since but ill take your word for it and try it cant hurt i guess. thanks for the advice. i will also go up to a 160 and see what that does and change my plug to a 9Z instead of the 8Z thanks

[EPDP99]

dont put the spark arrestor on it if it didnt come with it. I would get some new crush gaskets for the head pipe and replace those first.I had a similar problem with mine and those took care of the exhaust leak. Its like $5 try.

[HondaRacing23]

i tried the crush rings and that didnt work. my pipe came with a spark arrester but it didnt come installed on it. it just came with it.

[boricua]

i thought the spark arrestor just stopped the sparks from leaving the pipe.......is there more to it than that?

[HondaRacing23]

i dont know that post is making me wonder....:confused: :confused: :confused: :) :)

[EPDP99]

the spark arrestor stops the sparks but it will restrict the flow and hinder performance.

[cals400ex]

the hmf pipe that my buddy has does leak too. here is what i would do:

158-160 keihin mainjet
needle clip in the 4th position
3 turns out from lightly seated on the fuel screw with the 42 pilot jet in place.

also, make sure your air boots holding the carb on and where it screws to the head is snug.

[EPDP99]

cals400ex is the jetting master.....take that for gold!!!

[HondaRacing23]

ill give that a shot and see how it does

[enduro400rider]

with the spark arrestor installed it would slightly lessen the back flow ( back flow makes the poping sound) not much but it was enough to make a difference on my 400 but thats with a different pipe then yours i have a yoshi but maby it will do somthing for yours i m not saying its a guarenteed fix but its worth a try

[Jonesy]

id say u are running a little lean. i have a similar setup 426, hot cam stage 1, HMF slip on. I run a 165 mian jet and 3.5 turns out on the air/fuel screw. Mine backfires for about the first 20 minutes like someone else said but stops eventually. i know mine is because the exhuast is leaking around the head, but after it complely warms up it seals better i guess.

[HondaRacing23]

mine is leaking a little around the head too. the old HMF header pipes SUCK if you ask me. they wont do anything about it either. they wont me to spend more of ym hard earned $$$$ to get a new one just like everyone else. oh well thats how this world makes money i guess. im going to try to put a 160 in it and run 3 turns out and go from there. ill post some results. im going changing the plug too.