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Discussion Starter #1
I have a 400 ex carb on my ltz 462.... it pops and backfires just a bit when you get on it and it starts to decompress.... maybe jets are off..... but my problem is idle... I have turned the idle screw to stock 400ex settings, 2 and a 1/4, ive done it all the way out pretty much.... doesn’t make a difference. Only thing that does is the air fuel screw, and it has to be pretty much bottomed out (bottomed our it idles way way way too high but doesn’t choke out) if you turn it down even slightly it doesn’t register until you go quite a bit to where correct idle is close (or so I think) not like 1200 rpm as it normally is.... more like 600-1000..... but sometimes when u give it gas or start too it’ll just die.

I’m thinking it needs a bigger pilot jet. Not sure. Only thing that does any difference is the air fuel screw and it only runs decently when it’s bottomed out and idling high...or what I think is high....

Needle on 5th clip down, I believe... was on 4th previously and done same thing.... and 3rd (stock when I got it) same thing..... idle screw does nothing, unless it’s all the way in (off) then it won’t get gas but the air fuel screw is only thing that does absolutely anything...

I have a fcr and 400ex boot but I’m not sure how much I want to do for the fcr upgrade... I’ll have to move my airbox to get it all to fit, get a new throttle cable and etc... plus fcr needs needle, and jets... I’ve read a whole lot but I’d hate to have to go through all this again on this cause such a tight fit
 

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Take it apart and clean the hell out of it, particularly the jets and idle passages. What jets do you have in it? Zero everything out and start over (needle 3rd clip, fuel screw 2 turns out, set idle if it runs).

I don’t recall ever having a needle on any setting except 3rd or 4th clip.


Are you sure the idle screw is actually moving the adjustment or just spinning? I had a remote idle screw that would just twist and not make any adjustment other than barely closed or barely open. It was weird, so I just replaced it.
 

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I think you should bite the bullet and do the FCR. That said, the 462 will be happier with a bigger pilot go up one size and try it. Your throttle cable might be tight, causing the idle to hang up will it idle down with the cable off?
 

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Discussion Starter #4 (Edited)
Take it apart and clean the hell out of it, particularly the jets and idle passages. What jets do you have in it? Zero everything out and start over (needle 3rd clip, fuel screw 2 turns out, set idle if it runs).

I don’t recall ever having a needle on any setting except 3rd or 4th clip.


Are you sure the idle screw is actually moving the adjustment or just spinning? I had a remote idle screw that would just twist and not make any adjustment other than barely closed or barely open. It was weird, so I just replaced it.


I’ve cleaned it 4 times already, and cleaned it again when I replaced the air fuel mix screw on the carb and adjusted my spring for a easier throttle... I’m not sure ab the jets but I believe the pilot is a 48 maybe? (I think it’s a 42 tbh, 38 is stock I believe, so either 38 or 42) If that sounds right... I’ll check here later... needle clip made no difference when I moved it further down... Only time the idle screw actually does anything is when it’s all the way in and shuts fuel off


I think you should bite the bullet and do the FCR. That said, the 462 will be happier with a bigger pilot go up one size and try it. Your throttle cable might be tight, causing the idle to hang up will it idle down with the cable off?
I haven’t even thought about that, but it’s got some slight slack and the carb bottoms out so I don’t think that’s the problem though... it’ll idle fine with the fuel air screw turned in to where it actually engages the carb (like you’re lightly pushing on the throttle).... idle screw does nothing, it’s only the air/fuel screw...

When you adjust the air screw all the way out (not all the way just back it out to where it’s off of the throttle tab) it shuts off, no gas.... but only time it will idle and not WooterWooterWooterWooter off is when that’s turned in and holding the throttle open a little, otherwise it shuts off from not enough gas....


Only thing I can gues is either pilot is too big (I doubt) and not getting enough air (hence the throttle having to be adjusted to where it gets more air)
Or the pilot jet is way to small and that’s causing my idle problem
 

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The stock Z carb is a 36mm bore. Everything I’m seeing says the early stock 400ex had a 35.5mm, then changed to a 38mm with an accelerator pump in 2004. Factory jets for the 400ex are 38 pilot and 148 main. No idea what you have in there. If you can get to the air-fuel mixture screw (cap has been removed), then they are probably different. Considering your engine bore size, you’ll probably need bigger on both.
 

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A long, but good read from another forum...

First off, there's 2 basic fuel related problems. You either have a rich mixture, or a lean mixture.

A rich mixture is caused by too much fuel compared to the amount of air being used during combustion. Rich conditions can be detected by the engine spitting and sputtering, blurbling, or acting like a rev limiter, rapidly losing and regaining power. In severely rich conditions, you may be seeing black smoke coming from the exhaust. The black smoke you see is actually raw fuel that is not being burnt and is being wasted. By looking at the spark plug, a rich condition can be detected by a black, sooty plug.

A lean mixture is caused by too little fuel compared to the amount of air being used during combustion. Lean conditions can be detected by the engine losing power, yet retaining it's engine speed. For instance, the engine sounds to be accelerating to higher RPMs, yet feels as if it has no power. By looking at the spark plug, a lean condition can be detected by a white, blistered plug.


Secondly, there are 3 basic carburetor circuits: Pilot Circuit, Mid-range Circuit, and Main Circuit. These 3 carburetor circuits can be troubleshooted by knowing the throttle opening they control.

The Pilot circuit is responsible for throttle openings from Idle (0 throttle) - around 1/4 throttle. This circuit consists of pilot air jet(s), the pilot fuel jet(s), a pilot screw (either fuel or air screw), and pilot ports inside the carburetor throat (a.k.a. Venturi).

There are 2 types of pilot screws: a fuel screw and an air screw.

The fuel screw is located on the engine side of the throttle slide in the carb, and controls the amount of fuel that is drawn into the Venturi by the pilot ports. By turning the fuel screw out, you are allowing more fuel to pass the screw, effectively richening the mixture. By turning the screw in, you are restricting fuel, effectively leaning the mixture. Another way to determine whether it is an air or fuel screw is that a fuel screw has a rubber o-ring to keep air from entering the pilot circuit around the screw.

The air screw is located on the air-box side of the throttle slide in the carb, and controls the amount of air that is drawn into the Venturi by the pilot ports. By turning the air screw out, you are allowing more air to pass the screw, effectively leaning the mixture. By turning the air screw in, you are restricting air, effectively en-richening the mixture.

The air jets are hardly ever changed, so we won't go over that. The pilot fuel jet(s) can be changed to bigger (richer) or smaller (leaner), depending upon your problem. A good rule of thumb to use is that if you have to adjust the pilot screw more than two turns either way if it's stock setting, then you need to accommodate by changing the pilot air or pilot fuel jets accordingly.

Remember, the Pilot Circuit is only effective from 0 throttle to around 1/4 throttle. It still functions during the rest of the throttle positions, but it's effect is minimal, and goes un-noticed.

The Mid-range circuit is responsible for throttle openings from 1/4 throttle - 3/4 throttle.

This circuit is controlled by 2 things: the Jet Needle, and Needle Jet (a.k.a. the Main Jet Holder).

The Jet Needle, or needle as many call it, is attached to the throttle slide, and drops into the Needle Jet. All needles are tapered. Either the Jet Needle is adjustable or it is not. If there are more than 1 grooves for the needle clip to sit in, then it is adjustable. By raising the clip on the needle, you are allowing the needle to sit deeper into the needle jet, which restricts fuel, effectively leaning the mixture. By lowering the clip on the needle, you are raising the needle out of the needle jet, which allows more fuel to pass, effectively enriching the mixture.

When the slide raises, it raises the needle out of the needle jet, allowing fuel to pass by the needle and into the Venturi. This is where needle taper comes into play. Unless you are extremely fine tuning the carb, you don't need to worry about taper. You change which part of the taper is in the needle jet by the position of the clip.

Remember, the Mid-range circuit is only effective from 1/4 throttle - 3/4 throttle. None of the other circuits have a drastic effect on this circuit, so if your problem is in the mid-range circuit, then it can't be the main jet or the pilot jet.

The Main circuit is responsible for throttle openings from 3/4 throttle - Wide Open Throttle (you'll see me refer to this at WOT later on).

This circuit is controlled by 2 things: the Main Jet, and the main air jet. The Main Jet is the #1 thing that people change in a carburetor when it comes to tuning them. This is often a big mistake, as it only controls 3/4 - WOT, and NOTHING ELSE. Remember that. A larger main jet will allow more fuel to pass through it, effectively enriching the mixture. A smaller main jet will restrict fuel, effective leaning the mixture. With the main air jet, it allows air to premix with fuel as it goes up into the Venturi.

The Main Jet only functions at 100% when the slide is open and the jet needle is pulled completely out of the needle jet. At this time, the only thing restricting fuel flow into the Venturi is the size of the Main Jet.


Now for tuning.

If you read above, you should know the difference in feel of rich and lean mixtures. By knowing at what throttle opening the problem is occurring at, you can figure out what circuit the problem is occurring at.

If it's the pilot circuit, there are 3 basic way to tune the circuit. You can adjust the pilot screw, change the pilot air jet, or change the pilot jet.

Adjusting the pilot screw is simple. With the engine running at idle, warmed up to normal operating temps, turn the screw in until it starts to idle rough, then turn the screw out until it starts to idle rough, then turn the screw so it's between those two extremes. To check the position of the screw, you can count the number of turns as you turn the screw in until it seats SOFTLY with the carb body. Reason I capitalized SOFTLY is that the screws (especially the fuel screws) are easily damaged if over tightened. So screw them in until they SOFTLY seat the carb body. Compare your counted number of turns to soft seat and compare it to stock settings (stock settings are determined by counting turns until soft seat before you do any adjustments whatsoever). Again, if you had to turn the screw more than 2 turns either way, you need to change pilot jets (air or fuel) accordingly.

In the mid-range circuit, there are 2 basic ways to tune the circuit. You can adjust the jet needle, or change the needle jet. Raising the clip will lower the needle, leaning the mid-range. Lowering the clip will raise the needle, enriching the mid-range. You can also change the needle jet, but only if your jet needle adjustments make no difference in the way the mid-range circuit operated. If you are running lean on the mid-range, and you've raised the needle as far as it will go and it doesn't get any better, then you should go up in the needle jet size. Many carb manufactures don't have different sized needle jets, so the aftermarket may offer them, or they may not.

In the main circuit, there are 2 basic ways to tune the circuit. You can change the main jet, or change the main air jet. Changing to a larger main jet will effectively enriches the circuit. Changing to a smaller main jet will effectively lean the circuit. You can determine which you need to do by first determining whether you are rich or lean. Changing main air jets, again, is for very fine tuning. Once you have the main circuit functioning properly, you shouldn't have to worry about the main air jet, because the air for the circuit is mostly provided by the air passing through the Venturi. On many carbs, the main air jet is not changeable. They may be pressed in.


So there you have it. I basically touched base with carburetor internals and how to adjust them to tune the carb. Every brand carburetor has different ways of accomplishing the same main goal of every carburetor. That goal is to precisely and efficiently mix air and fuel in the right ratios for efficient engine operation. This efficient operation comes from complete combustion, which cannot occur if you are too rich. Whether Mikuni, Keihin, or whatever, they all do the same thing, just in different ways. Hopefully this will help some of you to understand the functions of the carburetors internals.

If you have any questions, feel free to ask. Hope this helps you guys!

Lastly, you all need to know...

***This is only a reference guide. This is not to be used as a manual for any specific carburetor, as every carb is different. This is only a guide to be used to base your carb tuning off of. In no way am I responsible for the adjustments, or their results, you make on your own machine.***
 

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Discussion Starter #7
Put a 52 pilot in carb and my backfiring and fire spitting has stopped, but it has a slight studder in low throttle (pilot)

turns out my 400ex carb is newer, and has the accelerator pump in it... but ALSO has a middle piece in between the hex jet and the main jet... (hex jet-main jet with holes, middle spacer jet?, then the main jet "cap") went to honda and got the 52 pilot, they never seen the middle piece nor knew where whoever got one from....

the "spacer" is hex shaped also, fits inside the main hex jet, then the actual main jet (cap) screws inside the spacer.... or can run as just main hex jet and main jet (cap)…… if this is making any sense.. ill try and post pictures here soon when I go to install spacer and see if that works

so I took it out and this has been started with just the hex and the main jet (cap) and 52 pilot... im going to try it with the spacer here in a few, and if all fails ill get a 50 pilot tomorrow and run with and without spacer to see if it has any major affect... seems as though the problem is my pilot is a tad too big now...

AM I ON THE RIGHT TRACK? just to be sure im not working myself around the main problem..
went from 48 pilot to a 52.. will try 50 as soon as I can get it.. I have no clue what the main is, no numbers on it... but no problems from 1/2-3/4 to WOT... runs like scalded dog... just hard to even idle or idle long, or just slowly cruise

(with 48 cruising) it would kinda sputter and act starved until more throttle was given
 

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Sounds like it’s getting better. The 50 might be right.
 

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Discussion Starter #9

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Discussion Starter #10
So found out the main jet is a 165... stock pilot was a 48....

Above is the spacer I was talking about, even Honda guys had no idea what it was really or what it would do... when I get the 50 pilot I’ll just run it both ways.... see which is best

Think I should up the main as well? And what would you think to? With or without the “spacer”?

When I give it throttle it squirts a good stream ab 2-3’ easy.... maybe ab the size of a spaghetti noodle, before it’s cooked.... (couldn’t think of anything else to describe atm)
Not really much “sluggishness” on the WOT or 1/2-3/4, maybe it could use a little more fuel?? Maybe a little more would be too much? I have no clue really, so accepting all ideas...

Thanks in advance
 

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That's really not an ideal carb for a 462. There's reasons everyone switches to FCR's, but If you really want to use that type of carb, I'd get one for a TRX450R. I think those were a 42mm version of that weird thing. They're cheap, and that's a lot more appropriate for the build. The jetting would be pretty close to the same as it would for the TRX with the same type of mods. It would be a little less of a guessing game.

As far as this carb goes, you have to remember than you have it on a much more aggressive engine than a 400ex. The bigger the engine is, in relationship to it's carb,the less accelerator pump shot it needs. It's moving a lot more air through the carb with a lot more signal to the needle and main jet. The accelerator pump is just there for the brief moment that the throttle opens, and no extra fuel has made it through yet. With more signal, that moment gets shorter. Your description of the accelerator pump makes it sound like it's too much.

Backfiring on deceleration is always either a lean condition on the pilot and/or fuel screw setting, or an exhaust leak. The carb indicated that it wanted a bigger pilot, if you were 3 turns out on the fuel screw, and still not rich. One size would probably do it. If you get too rich, it will stumble at real light throttle.

That jet extension thingy is more or less converting the jet type. Those carbs are supposed to have a Keihn Hex jet in them. It really doesn't matter, as long as you get the right size jet in it, which will be trial and error. I'm wondering if that was part of a dynojet kit.
 

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That thing has to be sitting at the very bottom of the carb bowl, picking up every piece of dirt there is.
 

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Discussion Starter #13 (Edited)
You’re right, I’m just using this ex carb as a substitute right now until I can get my fcr and all my parts together for it... after my clutch of course... any idea all the jets for the fcr (39 I believe) for a 462? I mean for a rough start....or any idea a good online credible place to get a jet kit for fcr...... mine didn’t come with any, or even the needle so I’m starting from nothing on the fcr... got it for free from my buddies yfz450 (05 or 07 I believe) so can’t complain

As for the ex... 50 pilot helped a lot, still can’t really get idle correct but I’m just gonna leave it and run it till the fcr upgrade...

As for the middle piece you’re right, it’s a extension convertor... I bought a 170 main today, ended up being too rich or well I thought so, it fits in the bottom hex (first one)... was acting like it was kinda wide open with just ab 1/4 or so throttle and kinda ran ruggedish..... (170) it’s too big for the second, the adapter..
the middle piece (adapter) is for the 165 jet I had in it, it only fits those size jets (smaller thread diameter)

I still have a little flame coming out just here and there, once in a while, but no where near as bad...

Appreciate all the help!
 

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Pretty sure my 462 was a 48 pilot, 170 main, 3rd clip on needle, 2 turns out at sea level with an FMF full system, custom intake with K&N, stock carb boot, no lid, new style hot cams, and stock valve size. That should get you close.
 

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That sounds similar to mine. Pilots seem to land anywhere from 42-50, I like the NCVQ needle 3rd slot, and the main in my 462 is like a 190 main, but the carb is a 42.5 and it has Web cams, porting, and +1 valves which effect the main.
 

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Discussion Starter #16
What about the leak jet? Or whatever in the bottom of the bowl? And the “starter jet” in the carb? The other beside the main and pilot...?

Any idea where I could find jets, needle, float valve piece... I guess that’s it... online?

And is the yzf & yfz the same carb? Like use the same things.. can parts interchange?. found a kit but it for yzf, not the yfz (quad) the one I have (yfz)
 

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Most carb parts aren’t very interchangeable. Carbs from quads and bikes are usually different to some degree. For example, the thumb throttle can house the return spring for the throttle, but the bike has the return spring on the carb. One throttle cable on the quad instead of 2 in a push-pull setup on a bike.
I don’t usually mess with the air and leak jets. Rarely a need to unless you’re doing some serious tuning.
 

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What about the leak jet? Or whatever in the bottom of the bowl? And the “starter jet” in the carb? The other beside the main and pilot...?

Any idea where I could find jets, needle, float valve piece... I guess that’s it... online?

And is the yzf & yfz the same carb? Like use the same things.. can parts interchange?. found a kit but it for yzf, not the yfz (quad) the one I have (yfz)

Actually, yes

The carb kit for a 05ish YZ450F dirt bike will have the NCVQ needle, if you don't already, and a much smaller leak jet. The pilot and main might be in the ballpark, might not but that's minor. They usually come with a new float needle, and a new bowl o-ring too, which is nice.

Leak jet is what it sounds like. It is a controlled leak to the accelerator pump. The YFZ came with a 70, if I remember right, which is huge, and why they stall so bad stock. (It's not the light flywheel like everyone says) But too lean to run properly is just how the EPA likes it.

What your engine actually wants for a leak jet depends on how aggressive it is, and how big it is compared to the carb that is on it. 400's and 434's tend to be lower like 35-40. They need more pump shot. Bigger engines like a 462 may be happy with a 45-50. Like I said, it depends on the build.

The start jet, I seldom ever mess with. It is for the choke, and to be honest, when I tune a FCR to where I'm happy with it, you can walk up to it, smack the throttle half way open once, then hit the button and it's idling. Winter time is the only time I need the choke.


As for where to get jets, any catalog has Keihn jets in it. I have a big assortment at the shop, and most shops that tune anything should. You could try them. I would buy the pilot you think you will need, and one on either side of it.

As for the main, on a FCR, They are a pretty small change from one jet size to the next. Increments of 5 will be as close as you will be able to judge without a wideband. I'd start off changing by increments of 10 to find the ballpark, then move by 5's to find the happy place. For example, my 462 will run just fine with a 165 in it. It's lean, but it runs ok, besides missing a some power. 190-195 is where it runs best. By 210, its got a bit of a studder in a couple areas of the rpm range because it's rich.

One thing I learned when I got the dyno, and the wideband, that nobody ever told me, is that the air fuel ratio at wide open, swings all over the place with a carburetor. Especially these, where the carb feeds one cylinder. The best jet size is a compromise keeping it from getting too rich at the rich points, and too lean at the lean points.
 

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That's really not an ideal carb for a 462. There's reasons everyone switches to FCR's, but If you really want to use that type of carb, I'd get one for a TRX450R. I think those were a 42mm version of that weird thing. They're cheap, and that's a lot more appropriate for the build. The jetting would be pretty close to the same as it would for the TRX with the same type of mods. It would be a little less of a guessing game.

As far as this carb goes, you have to remember than you have it on a much more aggressive engine than a 400ex. The bigger the engine is, in relationship to it's carb,the less accelerator pump shot it needs. It's moving a lot more air through the carb with a lot more signal to the needle and main jet. The accelerator pump is just there for the brief moment that the throttle opens, and no extra fuel has made it through yet. With more signal, that moment gets shorter. Your description of the accelerator pump makes it sound like it's too much.

Backfiring on deceleration is always either a lean condition on the pilot and/or fuel screw setting, or an exhaust leak. The carb indicated that it wanted a bigger pilot, if you were 3 turns out on the fuel screw, and still not rich. One size would probably do it. If you get too rich, it will stumble at real light throttle.

That jet extension thingy is more or less converting the jet type. Those carbs are supposed to have a Keihn Hex jet in them. It really doesn't matter, as long as you get the right size jet in it, which will be trial and error. I'm wondering if that was part of a dynojet kit.
Ive heard thats what the fmf carb kits use.
 
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