400ex popping on deceleration, backfire

[unitednations]

I have an 03 400ex that started to pop like crazy on deceleration. Any ideas what this could be? It ran good for like 2 years, and just started to do it. It does it if i let off the gas around half throttle or more. its a constant popping out of the exhaust. I tried adjusting the air fuel screw 1 turn each way, not sure if it changed any. still popped.


HFM pipe
Jetted 40 and 162? i cant remember exactly
air/fuel screw 2-1/2 turns out.
KN air filter
Hot-cam
11:1 compression

Any ideas would be very helpful! thanks!

[05440EX]

Popping on decel is usually a result of being rich, or unburned fuel loading up in the exhaust. Popping on acceleration is a lean condition.

[unitednations]

could it be a leak in the headers or in the pipe as well?
I dont know how the carb got more rich. I didn't touch anything. it just started doing it.

Isnt there a air cut off in the carb or something? do these go bad?

anyone know of a way to find a leak in the exhaust? a redneck way haha.

[JOHNDOE83]

No! poping and backfiring on decel means to lean.....

www.jetsrus.com has the info.....

[honda400ex2003]

turn your f/a screw out a 1/4 turn at a time until it goes away.if you are more than 3.5 turns out you need a bigger pilot. it is a lean condition also. steve

[rooster300ex]

if it just started to do it i bet its a exhuast leak. check your header bolts. Samething happend to me. Quad ran fine and then it started to do this. After the header cooled off i wiggled it and sure enough it was loose.

[rmcfadden68]

i had the same problem with my 05 and it turned out to be loose header bolts

[unitednations]

yeah i dont think its a jetting problem. it didnt re jet it self. i did just replace the float in the carb tho. maybe i screwed something up in there. idk.. but ill redo the headers. they seem tight but it could leak. No way to really tell. cant put your hand there long to test for air haha

[05440EX]

If you turn the A/F screw, don't you want to turn it clockwise (in) to richen it up? Its an air screw and by turning it in you decrease air and increase fuel.

[JOHNDOE83]

use a process of elimination....

if its not the headers, if your plug is good, if your gas is good, its your carb....

something inside like a jet could be cloged not letting enough gas in, clean your carb and if it still pops and backfires....id say get a bigger jet.

[troutman561]

Check your exhaust system! I had the same problem when running the 400ex I am bringing back to life and in the process of cleaning it I took the heat shield off the header the and pipe was cracked in 2 places I never would have seen if the heat shield stayed on... I would eliminate exhaust system problems before going in and messing with fuel/air mixture and such..

[unitednations]

so turning the air fuel screw in richens or leans the engine? now im confused lol

[05440EX]

Turning the screw IN will close off the air and in turn RICHEN the mixture, turning it out will add air therefore leaning the mixture.

[honda400ex2003]

got it mixed up it is a fuel screw not an air screw. out richens it, in leans it. i would say that it has a leak somewhere if you did not do any mods to induce this problem. check the air boot, exhaust, bolts for all of it, and clamps to make sure they are tight. take some carb cleaner or something and spray it around the carb and such to see if the idle increases or not. you should be able to find the leak. steve

[unitednations]

wow one guy is saying in richens it and one is saying out does :confused: ahhhhhh

but i think i may here a leak? i revved it up alittle and it sounds kind of like air rushing somewhere. like a turbo? idk maybe im hearing things now that i have a problem with it :rolleyes:.. ill prolly just go reseat the headers, (bought 6 of them crush gaskets, i tend to mess up alot ) and if that doesnt work ill screw with the carb. and change the plug and what not.

[rooster300ex]

honda400ex2003 is correct. You turn the fuel/air screw OUT to richen it up, and IN to lean it out.

[honda400ex2003]

thanks rooster. If i am at least 90% of what i am putting down it doesnt go. lol. steve

[rmcfadden68]

out richens it. in leans it. ive been thu this same problem twice and both times it was exhaust issues. once it was loose head pipe bolts the other time the hose clamp between the head pipe and pipe rusted and broke. in both cases air gets in the exhaust and ignites exhaust gases. may even throw flames out the pipe. if u did no mods than go over exhaust to find the problem.

[unitednations]

well i cant tell if my headers are leaking, but my stupid hmf pipe is separating at the rivets. air leaks out of that. maybe thats it. what should i use to plug that up. damn pipe is only 2 years old.

[rooster300ex]

Originally posted by honda400ex2003
thanks rooster. If i am at least 90% of what i am putting down it doesnt go. lol. steve
lol