400ex backfiring

[xtullyx16]

Hey all,

I have an 04 400ex, just recently i had the motor rebulit ( 426, hot cam 2, new rings) I jetted it around the end of summer last year ( havent rode it much since). At the time it wasnt jetted properly, ( it was missing at about half throttle position) so I moved the fuel needle up one notch i believe, and it seemed to help... now the bike is running fine ( kinda slugish in 3rd gear) but when i let off the gas the bike backfires when it reaches lower rpm.. i can hve it pinned but it only backfires when it really slows down, and i mean lound popping.. my dad thinks the timing is off by a few degress and it its not the jetting. What do you guys think? any guaranted steps to figure out if its the timing or just jetting issue?

[JOHNDOE83]

pilot jet needs to be larger, try turning out the fuel/ air screw to 3 turns, if that doesnt help, get a bigger pilot jet.

[xtullyx16]

what are your opinions on exhasut gasket are bad or leaking? i called my local atv shop and he said he would think its that.

[xtullyx16]

Originally posted by JOHNDOE83
pilot jet needs to be larger, try turning out the fuel/ air screw to 3 turns, if that doesnt help, get a bigger pilot jet.

Is a new spark plug a good indicator of how the bike is running? i've heard theres to many variables to rely on those.. also when you say 3 turns out you mean 3 complete 360 degree turns from all the way in, right?

[JOHNDOE83]

they will not go bad or leak unless youve takin your headers off, they are crush gaskets, and will sound like ticking if they are bad not a backfire.

In the atv how to's section, I have a plug reading for jetting thread, it explains how and what to do for proper jetting, its based on plug reading.... or the "go big or go home" method.

the thread is also in the 400ex section, theres alot of helpful guys in there as well.

Yeah 3 full turns out, turn the screw in all the way, not to tight, snug.

turn it a full 360 degrees out three times, if it still backfiring, try a bigger pilot jet....like a 42.

this is a common problem when getting pipes and filters.

[wilkin250r]

It's almost impossible for the timing to be off. If the flywheel key is in place and intact, then your timing is spot-on.

In all my electrical experience (which is extensive, I'm an engineer), I've NEVER seen a CDI timing go bad within the CDI itself, it's always outside. If the timing is off, it's because the trigger is off. The trigger in this case is the pulse coil mounted just outside the flywheel, and the key between the flywheel and the crankshaft. I've seen cases of sheared keys, but it almost always causes the bike to run like crap everywhere, not even rideable.

I just don't see it being the timing. Sounds more like a pilot jet/air screw adjustment.

[xtullyx16]

Originally posted by wilkin250r
It's almost impossible for the timing to be off. If the flywheel key is in place and intact, then your timing is spot-on.

In all my electrical experience (which is extensive, I'm an engineer), I've NEVER seen a CDI timing go bad within the CDI itself, it's always outside. If the timing is off, it's because the trigger is off. The trigger in this case is the pulse coil mounted just outside the flywheel, and the key between the flywheel and the crankshaft. I've seen cases of sheared keys, but it almost always causes the bike to run like crap everywhere, not even rideable.

I just don't see it being the timing. Sounds more like a pilot jet/air screw adjustment.

its been so long since i jetted it but i think i have a 40 or 42 in there? anyone know what comes stock? 38?

edit: turned fuel screw out 3 turns, gas came out my overflow tube?! is that normal? tried to start it but it wouldnt start just kept firing, tried it 2 turns out, 1 1/2 turn out and alll the way in, it would not start?! flooded engine?

[xtullyx16]

Update: got it to start with air fuel screw all the way in.. but why when i turn it out does fuel leak out the overflow tube and does it keep pouring out? stuck float? or is my carb just draining?

[wilkin250r]

That seems really odd, I can't see those two being related. Your idle mixture screw should have anything to do with your floats and overflowing.

So let's take things one at a time. If you're overflowing, you have one of three problems. Either you have gunk in your float valve that doesn't allow it to close, your floats are stuck, or your floats are cracked and they aren't "floats' any more.

Stuck floats can often be fixed (temporarily) by banging the side of the float bowl and jarring them. Anything else requires a carb rebuild.

[xtullyx16]

Originally posted by wilkin250r
That seems really odd, I can't see those two being related. Your idle mixture screw should have anything to do with your floats and overflowing.

So let's take things one at a time. If you're overflowing, you have one of three problems. Either you have gunk in your float valve that doesn't allow it to close, your floats are stuck, or your floats are cracked and they aren't "floats' any more.

Stuck floats can often be fixed (temporarily) by banging the side of the float bowl and jarring them. Anything else requires a carb rebuild.

so should i just take a small hammer and knock on the bottom of my carb a couple of times? does it help that my overflow tube doesnt drain any other time?

[JOHNDOE83]

I would have to agree with wilkin, 90% of the time theres something stuck in the float valve.

You have to remove the float needle and ceet to clean it, I use a air compressor to clean my carbs, it blows everything out and ive had better luck cleaning them with compressors then soaking them.

I had the same problem once, there was a millimeter size chunk of dirt holding the ceet from working properly.

You can beat it and usually its a temp fix and it just lets you know to clean the float valve.

[xtullyx16]

How do i remove the float needle? is it pretty simple if i get to my floats?

[JOHNDOE83]

Yes it very simple, once the float bowl is off, where the float connects to the carb is a tiny pin holding it in place.

I use a paper clip or a safety pin or whatever you can use to push it out then the float and needle will come out.

Be EXTREMELY careful not to bend or ruin or lose any pieces.

Good luck and let us know the outcome?

[wilkin250r]

I would highly recommend, if you're ever going to do any type of work on your quad, to get yourself a repair manual. It will go step-by-step, really simple, with pictures.

Taking apart a carb can be scary the first time you do it, but trust me, it's really simple.

The only tricky things are:

Carburetors have a few small parts, and they do NOT like dirt. The best is to put a clean towel down on a workbench and put all your parts on that.

There's something odd about Keihin carbs. If the carb has never been apart before, the float bowl screws are on there REALLY tight. But at 25 years old, chances are it's been apart.

A can of aeresol carburetor cleaner is great stuff, and it's cheap. Don't be afraid to use the whole can, spray every passage both directions.

[xtullyx16]

Thanks wilkin and Johndoe,

I tried to mess with it last night but i was unsuccesful, i tried to take the easy and loosen the octagon shaped screw on the bottom of the carb but I could only reach the jet, not sure if it was the main or pilot/slow jet..

I wil look into getting a manual, any reputable dealers or should ebay have it? or my local stealership?

edit:

Sweet! found a free pdf service manual in 400ex section.. Also have you guys heard of ccp stabilizers?

[wilkin250r]

Originally posted by xtullyx16
i tried to take the easy and loosen the octagon shaped screw on the bottom of the carb but I could only reach the jet, not sure if it was the main or pilot/slow jet..

That's the main jet, so when it comes to jetting your quad, now you know the easy access for that. Any other jetting (pilot and needle clip) requires the carb to be taken off and taken apart.

[xtullyx16]

edit: got the info i needed on pilot needle, cept i dont know if my needle not having o ring and washer will affect much if anything.. not sure?!

Also i cracked my one of four valve caps while adjusting my valves :macho :devil: oops!

[xtullyx16]

put the gas tank back on after cleaning and inspecting the float.. im guessing the float level clearance is off because when i turned my gas on it took maybe 5 sec and the gas came dripping out continuously ... im prolly going to take my carb to my local atv guy.. hes always working on 400ex's

[JOHNDOE83]

Did you try to get the float out?

Cause the needle and ceet only come out when the float is removed?

Thats were the dirt and grime gets stuck.

[xtullyx16]

Guys.. I feel ashamed and slightly slow.. I took my carb to my local atv mechanic and he looked at it on the spot.. he hooked up a hand compressor and hooked it up to my vacuum.. he applied like 10psi and my floats held fine.. so he was looking at it, and he started talking about this drain bowl screw at the bottom, He was like ya know if you loosen this the fuel will drain out of your carb.. so I was like i dont think ive touched it...:( heres the sad part.. I came to realize after he checked it, and it was loose, that I had loosed my drain bowl screw thinking it was the airfuel screw.. soo my floats were prolly never bad.. im just kinda dumb..:mad: :ermm:

[JOHNDOE83]

LOL...thats funny.

sometimes, my quad wont start, and im like WTF, and cant figure it out, then I realize, the on/off switch is off, or the gas is turned off....lol.

[xtullyx16]

so john how the hell do u adjust ur a/f screw from without taking off the carb? or rotating it, but still the heat shield and gas tank need removed? a special tool llike a 90 degree philip screw driver?

[JOHNDOE83]

some people JB weld a bigger screw to it, some use the bent screwdriver, I just pull the carb.

The bent screwdriver only works if you can see exactly how many turns your doing, the JBweld or spot weld works but its hard to do it right, besides your only gonna adjust it once and leave it.

I ususally just turn it, If you just loosen up the clamp, take off the fuel line, you could just pull the carb off and turn it upside down and change it.

All you remove is the line, the clamp, your seat, then pull the carb, I make sure my clamp holding on the carb is accesible from the side of the atv without removing any parts.

[xtullyx16]

I put all the plastics back on and started up my 400ex and right away i could tell it was idling very wierd and smoked looked to becoming from head pipe and where slip on and head pipe connect. This was pure white smoke..

The past week ive only done these things to it:

Took apart carb and cleaned it and took apart float assembly

Put new crushable gaskets in exhaust manifold

adjusted my throttle cable

Im thinking from the looks of it my head pipe isnt fully flush and my motor isnt sealed which is causing a sever lean condition..

also note when i turned my quad on to idle and warm up.. the head pipe by the engine turned bright red..

Please help!

[JOHNDOE83]

your motor is running severly lean if the pipe is glowing, you need to up jet right away.

[odog]

i had my carb rejetted about 3 times but it didnt fix my popping it was the exhaust gaskets i needed new ones because when i bought my headers i didnt put new gaskets on them so if jetting dont work you might wont to look into that.

[xtullyx16]

JohnDoe - I havent touched the jets so its sucking in air somewhere right? the only thing i can think of from what happened is A) when i started it up it over revved for a second then started sputtering like the air filter was off or something.. so maybe clutch cable is to tight and lifting slide and since my flanges are bent to hell from b4 and my gaskets were put in by myself i think its pulling in air around the flanges....

[JOHNDOE83]

If it over rev'd with no explaniation "Like the carb WAS working properly" and "The throttle wasnt stuck" and you cant figure out why, then my next guess would be a air leak.

[xtullyx16]

Well ny flanges are bent to hell and I'm assuming its those and my gaskets might not be all the way in.. I have a full hmf.. do you think they would replace the bent flanges?

[xtullyx16]

Got the airleak fixed.. i think it was a mixture of the flanges and carb boot to engine... Now i cant get it to idle and it doesnt wanna start hardly.. Ik the air fuel screw is turned 3.5 turns out and my idle screw is probably outta whack because ive turned it each and everyway for the past hour.. gettin pissed off:mad:

[zach0199]

It was reccomended to me to only go out 3.25 turns on the a/f screw and if its still not good enough go up another on the pilot jet.

[xtullyx16]

I'm not touching the af screw unless I'm advised to do it.. there's to many variable to judge.. I want this thing to fire up more than once every 15 tries... then I can fix the idle setting.. Johndoe need your help bud ha

[MO-EX-RIDER]

what size jets are you running and what type of air filter set up? if its hitting every 15 tries, i would look at the valves again or timing

[xtullyx16]

i think slow jet is 38.. i havent touched timing.. It ran started more often when i had the air leak around my header.. makes sense because my local honda mechanic did turn my air fuel screw out to 3.5 and it wasnt out that much b4. that richened startup so Im guessing when i fixed the air leak, i richened it way up and im flooding the motor.. but I need to know how to reset my idle screw, it doesnt seat so im not sure where its at!

[JOHNDOE83]

Popping on decell is a lean condition of the pilot jet like I said before, if you look in the carb and still have a 38 pilot jet, you are way to lean for a 426, you will need a 40 or 42 pilot jet for sure.

You said earlier that you went up on the needle, the higher up the clip is on the needle the leaner it will make it, the lower the clip, the more rich you make it.

[xtullyx16]

I havent touched any of the jets.. the af screw was adjusted and this quad was running fine before i took the carb apart and the valves were adjusted. The only problem was a pop on decel.. i think just adjusting my af screw to 2.5 will help with the startup.. then i can figure out the idle speed..