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FCR MX 39 Leak jet

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I have heard some talk about this mod. I have a FCR MX 39 for my DRZ400SM. Can someone please explain what this mod is about? Can I safely do it myself (weekend mechanic), or is there a kit available for this?

I tried to search this, but did not come up with any solid results.

Thanks!

Edited by ratu

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wow, I thought lot of ppl knew about it, am I posting it in the wrong section?

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I just did the machine work to install one about a week ago

I would not suggest it for someone with just a drill press.

In essence, it's drilling a hole in the float bowl in a pretty specific spot. Miss it and the AP will leak.

Then tap the hole so the new jet sits flush

it's a 4mm-0.7 thread

If you drill it from the AP side, and try using a drill it will walk off location.

I use an endmill to do it accurately from the AP side so I can hole it

Then I tap it by hand from the inside.

If you're carefull and have some skills you can do it yourself.

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PBMW did a good job on my project bike, which is owned and raced by someone else on these forums. It is definitely a precision milling operation. Keihin made a starter mark on the inside of the float bowl, but you take your own chances by using it. I plan to make a jig to drill from the inside, but not on someone else's bike.

The key is that the hole must exit through the same oriface as the start of the AP circuit, as sealed by an o-ring on the AP bowl.

I found reasonable success with a #50 leak jet and stock AP spring; however, I'd like to try something slightly larger, due to environmental variations.

Edit: Sorry. I should have explained its purpose. By adding a leak jet to the carb, it allows the accelerator pump circuit to work as it was designed. A leak jet can successfully remove the bog from an FCR, without over-pressurizing the AP -- the premise behind o-ring, safety wire, and heavy spring modifications. As an added bonus, fuel economy should go up, as excess fuel is recycled into the float bowl, as opposed to being needlessly burned.

Edited by lynk

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you can buy a float bowl thats already leak jet capable from several sources.thats your best bet for a weekender.

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you can buy a float bowl thats already leak jet capable from several sources.thats your best bet for a weekender.

Sorry, but where can i find that fload bowl from my FCR?

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I would highly recommend having someone like PBMW drill this out for you.....

I will say that with all that has been done to my Carb including this Leak Jet... that it is so very close 👍 Just a few more tweaks and it will be like night and day from the jetting and settings that were in it say 3 weeks ago🤣.

The throttle is very very responsive ........front wheel comes right up with a twist of the throttle in second gear 🙂

And like Eddie has said......JD has some and didn't know about the Honda :applause:

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couple options-

oem float bowl,likely honda being the cheapest. pre 07 fcr 450 should work.

From any other model would it work ? From ktm bikes '07 and after ?

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From any other model would it work ? From ktm bikes '07 and after ?

The carb I have uses a leak jet. It's off of an '06 Yamaha YFZ 450 quad.

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The carb I have uses a leak jet. It's off of an '06 Yamaha YFZ 450 quad.

Will this float bowl fit to a 1st generation FcrMX ? I have a FcrMX39 with a removable air bell from a Yamaha yz 426 .

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so, if fcrmx has an LJ - it doesn't need o-ring mod or stiffier ap spring?

i've found three honda's float bowls, according to RonAyers fiche.

they differ by leak jet size?

btw, what about ajustable leak jet kits?

they worth it?

Will this float bowl fit to a 1st generation FcrMX ? I have a FcrMX39 with a removable air bell from a Yamaha yz 426 .

it seems that yamaha do not sells them as parts

http://fiche.ronayers.com/Index.cfm/Module/Main/TypeID/3/Type/All-Terrain_Vehicle_(ATV)/MakeID/4/Make/Yamaha/YearID/47/Year/2006/ModelID/2147/Model/YFZ450/GroupID/335075/Group/CARBURETOR_

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so, if fcrmx has an LJ - it doesn't need o-ring mod or stiffier ap spring?

no,the mods still apply.

the leak jet is is just another avenue of tuning.

with my set ups i find that no leak is optimum.

adjustable leak jets work fine.

if you want to experiment with leak jet tuning a leak jet bowl or the alj will get you where you want to go.

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no,the mods still apply.

the leak jet is is just another avenue of tuning.

with my set ups i find that no leak is optimum.

adjustable leak jets work fine.

if you want to experiment with leak jet tuning a leak jet bowl or the alj will get you where you want to go.

What he said. I've seen a bunch of guys say that they used a carb like mine, and they block off the leak jet because it wasn't needed. (this is for the DRZ)

On my personal carb, I used the jet that was in the bowl, and tweaked the AP timing screw until I got rid of the bog.

Oh... and the bowl off my carb should fit any of the MX versions. (not the slant style)

Like Eddie said... it's one more thing to tune, but on the street, I don't know if it will gain you anything other than the weight reduction of your wallet.

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if you want to reduce the volume of the squirt,adding a leak jet will do that.

the o-ring,spring or wire increases squirt pressure and forces the entire volume to be injected as early int he stroke as possible.

the o-ring is less agressive than solid wire with the spring in between.

this is regardless of leak jet.

leak jet just controls the total volume being injected.

the 07 and newer crf fcr carbs have the pump linkage revised to do exactly what wiring does.

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no,the mods still apply.

the leak jet is is just another avenue of tuning.

with my set ups i find that no leak is optimum.

adjustable leak jets work fine.

if you want to experiment with leak jet tuning a leak jet bowl or the alj will get you where you want to go.

Spot on

The leak jet is just another way of getting to where you want to be.

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It looks to me (weekend mechanic with DRZ400SM) that installing a leak jet on my FCR MX from TT will just open yet another door for tuning.

I do not ride trails, mostly stay on paved roads or fire roads…so I am not putting around in low rpm or low speed anyway. So that means I do not need to get the leak jet installed?

However, I did notice that my bike stalled couple of times at low speed, so would installing the leak jet resolve this problem, or something else is going on?

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no idea.that really depends on the rest of your set up.

bike stalls on quick throttle opening? if not,than you dont have an accelerator pump issue and adding a leak jet wont fix that.

by your pm you sent me it sounds like you are running no pilot air jet with a needle other than EMN. if its a needle with a leaner straight diameter,than you will need to run the pilot air jet.

jetting is like a puzzle. pieces from 2 different puzzles dont work together.

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Ok, here is my set-up

2008 DRZ400SM

I am taking sea-level setting – as I am in Vancouver, WA.

3X3 air box mod

Full MRD SSW exhaust system

FRX MX 39 kit from TT – just did a bolt on with all the jettings the FCR came with

- 200 Main Air Jet

- 160 Main Jet

- 45 Pilot Jet

- EMR Needle

Per instructions, I did REMOVE the 100 Pliot Air Jet.

However, I did NOT install the 017-262 EMN needle which came with the kit (missed that somehow following the FAQ FCR install inst)

The bike runs little sluggish, and has stalled on me couple of times, not on WOT, but just slow gradual throttle.

I plan to

-- get the extended fuel screw (2 turns? – after you tight it, then open 2 turns?)

-- install the 017-262 EMN needle which came with the kit (install at clip 3?)

-- do the o-ring mod )seems pretty straight forward with the carb on the bike)

Do I need to do anything else? The carb is currently on the bike, for the above three items that I need to do, do I need to pull the carb out again?

Will appreciate your valuable input, I am willing to listen.

Thanks!

I can post up some pictures of me bike if you want to see....

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