Popping on Decel, Leaking Pipe

[Mitragorz]

I've got a lot of popping on deceleration, and searched the boards and learned that it could be caused by either a leaking header or not having the mixture screw out enough. I wrapped my headers not too long ago with heat wrap, so my first assumption was that my headers weren't in correctly. I took them off, put them back in, and had no indication of any leaks. So, I decided to try the mixture screw solution. As I'm trying to find the screw (I have no idea where it is, I've never done any carb work or jetting...) I had an idea. I'll stuff a rag in the exhaust to double-check my headers. Obviously if the exhaust is blocked, if there's a leak, I'll be able to feel the air. Well, the good news is that the headers aren't leaking, but some of the rivets themselves around the pipe are leaking...

So, could that be what's causing the popping, or is it more likely the mixture screw? I'd really prefer not to replace the pipe, since when I put everything back together after the wrap, I used a spare exhaust clamp I had laying around to secure the pipe to the headers... and subsequently pinched the two together in a fashion that won't allow them to be easily seperated :mad: (It's a Curtis full ehaust, so the header and the slip-on would both have to be replaced...)

Would adjusting the mixture screw be a solution even though the exhaust does leak from the pipe?

Also, where is the mixture screw? If I need to take the carb off to get to it, how do I do that?

Thanks.

[UNBROKEN]

the rivets will not make enogh diffrence to backfire, sounds lilke ure a little lean on the idel circut u can try backing the mixture screw out alittle and if that doest help u may need to go up on the pilot jet, have u cheacked for any carb leaks as well ?

[Mitragorz]

I don't feel / hear / see anything leaking from the carb.

How do I get to the mixture screw? Like I said, I'm not familiar with the carb.

[Mitragorz]

Ok, so it doesn't seem to be the mixture screw... I backed it out 6, yes SIX full turns from snug, and it still backfires.

I don't know what the guy who owned it before me did to it (other than beat the crap out of the poor engine) jet-wise, but I assume improper jetting could also be a probability? Maybe?

What about gas? It's 11:1, and only has 87 in it from last September or so (I Stabil-ed it before I left for the past 8 months on business), and I only learned after the fact that it should be run at a higher octane. However, I don't know if too low an octane will actually cause it to backfire. I've yet to put fresh gas in it. What do you think?

[krt400ex]

the octane rating shouldnt cause backfiring. you do need to run premo in it though for sure.

if you backed the screw out six full turns, and it made no difference, then you need to up your pilot. go to a K42 pilot and try 2.5 turns out on the screw and see what happens...but im not sure that you are thinking of the right screw because it is not supposed to be screwed in snug at all...

check the jetting. i dont like other brand jets than stock, so the #'s i give you will be for the keihin brand jets. start with a 158 main and a 42 pilot, fuel scre at 2.5 turns OUT. if you install an adjustable needle, try it at the second or third clip from the top and see how it feels

the needle conrtols anything past 1/4 throttle, but not past 3/4 throttle. the main jet is what controls the last 1.4 throttle, while the pilot circuit (pilot jet and fuel screw) control the first 1/4 of the throttle movement.

in other words, the pilot circuit control the bottom, the needle adjustment the middle, and the main jet controls the fuel delivery to the top end. good luck.

if it is popping on decel, the most likely problem is a lean pilot cicuit.

[Mitragorz]

Originally posted by krt400ex
im not sure that you are thinking of the right screw because it is not supposed to be screwed in snug at all...

I've got the right one, I just snugged it up to use as a reference for turns out. I started with 2 and worked my way up to 6 seeing no change.

I'm not sure what # jets are in there now, is there any way to tell just by looking? Like I said, I don't know what carb work the previous owner did, and this is my first time digging into the carb.

I assume this is where I'll do the jetting?
http://img409.imageshack.us/img409/2120/p1020531sm2.jpg (http://imageshack.us)

I opened up that black plastic cover on the side of the carb, but didn't see anything adjustable. I was going to open up that bottom part, but one of the screws was stripped... If that's the spot, then I guess I'll have to get that one out somehow. I'll pick up a new set when I get the jets.



Any special tools needed for this?


Thanks for the help so far.

[Mitragorz]

Ok, well I got that stripped screw out and got into that bottom piece there. Had to hacksaw across it and use a flathead instead of a phillips, so it's essentially useless now.

Anyway, I looked at the needles in there, and it looks like I've already got in a 158 main (I assume that's the part sticking out with the "158" on the side), but there's a 38 pilot. I'll pick up a new pilot and that set of screws right now.


As far as an adjustable needle, I'm not sure if I have one or not. Where would I look / how can I tell? Anyone have a pic to help me out?

This carb is a whole new can of worms to me.

[Mitragorz]

*sigh*


I went and put in the 42 pilot, and once again, went up to 6 turns out, and she's still popping...

Do I need ANOTHER size up on the pilot, or could I just be looking in the wrong place... I'm just about at my wit's end.

actually i think sparks recomends a 45pilot for their exhaust.

[leasureryan]

I am at a 168 main and 45 pilot....Kehin ofcourse. Screw at 4 turns out. All I have is an HMF full system and EHS airbox lid with uni filter......I still have a ton of popping, and backfiring. I think I need to jump up to either a 48 or 50 pilot, and honestly, I think I can go bigger on the main as well. Also running a 110/93 octane 50/50 mix. Go bigger until you feel power loss, or see a fouled/wet plug. Then back it back down 1 knotch

[krt400ex]

this is rather interesting. what elevation you at? my quad with the twin air filter, yoshi pipe, and no airbox lid is good on a 42 pilot, 155 main, and 2 3/4 turns out...

[Mitragorz]

I'm at about 15'

[krt400ex]

Originally posted by Mitragorz
I'm at about 15'

yea, try the 45 pilot and see what happens

[krt400ex]

wow, what am i thinking? i just saw that you had a 440 kit on there...you need a 45 pilot...lol

[Mitragorz]

Thanks for the help, I hope this fixes it. I'll stop in today to get the 45, and a new mixture screw as well... During my trials yesterday, mine decided to jump ship at some point between my garage and the end of my street... The little bugger is impossible to find. Hopefully they have one in stock, I don't feel like waiting around!

[Mitragorz]

I put the 45 in, it's out 4 1/2 turns right now, and it's still popping, though it doesn't seem as bad... I'm guessing I should go up another size with the pilot, what do you think?

[leasureryan]

Originally posted by krt400ex
this is rather interesting. what elevation you at? my quad with the twin air filter, yoshi pipe, and no airbox lid is good on a 42 pilot, 155 main, and 2 3/4 turns out...


Who were you refering to? Me? I am at 1050ft above sea level

[leasureryan]

45 pilot on a 440 big bore kit? I would say more like a 50 or 52.

Someone please explain this to me here. elevation has THAT MUCH impact on jetting? I see guys in PA, wich the elevation there isn't much differant than here, and they seem to run so lean of jets in thier quads. Why is this? Why is my internaly stock 400 wanting so much fuel? I have a 45 pilot now, screw out 4 turns, and a 168 main, needle 3rd clip ....and this thing is stilling running lean. Still have tons of backfire on decel. I just don't see how everyone elses jetting is so differant than mine, and it's stock, with full HMF pipe and ehs lid, and uni filter.

Anyone else at 1050ft elevation? Or close? central Ohio? What are your jetting spec? Anyone else have an answer?

[DementeD]

u ever think that maybe urs is the only one whos off?

the only way to know for sure is have it hooked up to a wideband to get ur a/f reading..

[leasureryan]

Originally posted by DementeD
u ever think that maybe urs is the only one whos off?

the only way to know for sure is have it hooked up to a wideband to get ur a/f reading..

why would it be so far off man? Tomarrow....I am going to toss in a 175 main, couple stages bigger, see how it reacts. Also going to toss in a 50 pilot, and turn my srew in to 1 1/2 again, and tune from there. May also raise my needle. I am pretty good at jetting, I just don't understand why my 400ex is the only one wantingto guzzle the fuel like this. It acts like it is bored, camed and port/polished....lol. We will see in the next couple of days after some more tuning. Like I said, runns super strong now, but still got the popping on decel, which means lean, so she must want more fuel

[DementeD]

i dont know what mine is jetted at really but it just seems odd urs needs so much fuel..

just curious is it completely stock internally..and are u positive of this?

also what fuel are u running what oct.

[leasureryan]

yes.....all stock internaly.....only thing this guy before me had ever done was adjust the valves once a year , the 5 times a year he rode it. 1 owner, quad is mint, guy really knew what he was doing, got it from a member here on exriders.com.

I am running a 110/93 octane 50/50% mix.

[supernatural]

i have an 04 with a hmf slip on and it seems to be jetted right, plug good color and no power loss anywhere. when i asked this question before a guy who races quads told me all pipes will backfire on decel, but you only notice it more with an aftermarket pipe because the air flows better and they are louder than stock. i've had it pipped for almost 2 years now and nothing seems to be wrong yet. it only seems to pop on heavy deceleration though. this is only what i was told but figured i'd pass it on

[krt400ex]

Originally posted by leasureryan
Who were you refering to? Me? I am at 1050ft above sea level


no, the person having the problems...lol

i never jetted for a 440 kit b4

[krt400ex]

Originally posted by leasureryan
yes.....all stock internaly.....only thing this guy before me had ever done was adjust the valves once a year , the 5 times a year he rode it. 1 owner, quad is mint, guy really knew what he was doing, got it from a member here on exriders.com.

I am running a 110/93 octane 50/50% mix.

you dont need to run race gas if the engine is stock, and that would probably have alot to do with your jetting issues. alot of times race gas causes you to need to adjust jetting...sometimes radically

[Mitragorz]

Just out of curiosity... I seem to be going up quite a bit on the pilot. What indication would I get if I needed to up the main?

[Mitragorz]

Alright guys, I think I've finally got it. I've got a 52 in, 1 1/2 turns out, and it's not backfiring at all. Occassionally I'll get a few pops on the last bit of decel., almost to a stop, but I think that's fine.

I had it running well with the 50 in, but it was about 4 turns out, so I upped it a little bc I don't need ANOTHER mixture screw falling out.

Thanks a lot for the help guys, I appreciate it. If it wasn't for the forum, I'd probably have wound up taking it into Honda and spending a few hundred $$$ for them to fix it.

Thanks again.

[krt400ex]

Originally posted by Mitragorz
Alright guys, I think I've finally got it. I've got a 52 in, 1 1/2 turns out, and it's not backfiring at all. Occassionally I'll get a few pops on the last bit of decel., almost to a stop, but I think that's fine.

I had it running well with the 50 in, but it was about 4 turns out, so I upped it a little bc I don't need ANOTHER mixture screw falling out.

Thanks a lot for the help guys, I appreciate it. If it wasn't for the forum, I'd probably have wound up taking it into Honda and spending a few hundred $$$ for them to fix it.

Thanks again.


have you noticed a difference in power since you did that? i would think your bottom end should be much better

[Mitragorz]

It does have more low-end. I can't wait to actually take it out and open it up.