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Thumpertalk Kitted FCR-MX 39 Pilot Circuit Issues - Hanging Idle

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All,

 

I apologize for starting a new thread on this topic, especially as a first post, however I absolutely cannot figure out the issue, and can't find any posts from after 2012 regarding removing the PAJ in the FCR, so I think that perhaps this (or something else on the carb) may have changed.

 

 

Background Bike Info:

DRZ400SM, 3x3, FMF Q4 w/ powerbomb header

Carb is an FCR-MX purchased from the Thumpertalk store in May of 2014.

Jetting:

-155 Main Jet

-EMN needle w/ Clip @ 3

-45 Pilot Jet

-PAJ removed

-R&D Racing Flex Jet Extended Fuel Mix Screw (from TT store)

 

Problem: Idle will hang if throttle lightly applied and released.  Snaping it open/closed returns it to a far lower idle.

 

Notes:

-Carb is brand new and throttle wheel returns fully and squarely to the stop screw every time; no cable issues, sticky slide, etc.

-At a low idle RPM, going below about 1/4 turn on the fuel screw will kill the motor, seems happiest at 1-2 turns out.

-Have tried fuel screw combinations from 1/2 turn open to 3.5 turns open, and the same hanging idle issue persists.  It debatably seems a little better when richer (3+ turns open), although seeing as I have to turn the screw almost all the way in to kill the bike, I feel like my pilot circuit should be jetted roughly correctly.

-Have tried various idle screw settings from barely running to fairly high idle.  Problem persists regardless.  The higher the nominal idle RPM, the higher it hangs when I induce the hang.

-Checked for vacuum leaks.  Everything is nice and tightly installed, no sign of leaks.  Verified w/ carb cleaner.

-Bike runs fantastically at all other conditions, however this issue is very scary on track as it makes the engine braking very unpredictable

 

Any suggestions?  Should I not have removed the 100 PAJ?

 

Huge thanks in advance.

Edited by devgru38

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Try going out 1/4 turn at a time with your flex jet screw and see what that does. 

Thanks, but see 3rd note in original post.  I've already tried the full range of adjustability with not much effect.  Basic pilot circuit tuning method leaves me at around 1.5 to 2 turns out.

 

Do I need the 100 PAJ in?

Edited by devgru38
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Thanks, but see 3rd note in original post.  I've already tried the full range of adjustability with not much effect.  Basic pilot circuit tuning method leaves me at around 1.5 to 2 turns out.

 

Do I need the 100 PAJ in?

 

Sorry, (adhd).  I don't know, someone with more knowledge might chime in later.

Edited by billdirt1

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I know many will start tossing suggestions, and a few knowledgeable folks will make suggestions. Personally, I don't do the crystal ball deal.... I run the bike on a dyno, get an AF curve, and adjust the carb or fix the issue. 

 

You said you have no vac leaks,, ok.. well thats good as that is what your issue sounds like.

​You said your carb set up is as recommended.. well good, as that would have been a suggestion to try.

 

If the csrb is set up perfectly.

Installed correctly 

and you still have issues like hanging idle...

 

either you have a lean condition you're not aware of....or a set up issue you don't recognize 

 

​Either way.... a quick run on a dyno with an accompanying AF chart will get you pointed in the right direction. 

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Right, I installed per those directions to the letter and, 

Oh, ok, well that was not stated in your post, so I started with the simplest and most common solution to.. " I worked on my bike and now it does not run right" 

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I know many will start tossing suggestions, and a few knowledgeable folks will make suggestions. Personally, I don't do the crystal ball deal.... I run the bike on a dyno, get an AF curve, and adjust the carb or fix the issue. 

 

You said you have no vac leaks,, ok.. well thats good as that is what your issue sounds like.

​You said your carb set up is as recommended.. well good, as that would have been a suggestion to try.

 

If the csrb is set up perfectly.

Installed correctly 

and you still have issues like hanging idle...

 

either you have a lean condition you're not aware of....or a set up issue you don't recognize 

 

​Either way.... a quick run on a dyno with an accompanying AF chart will get you pointed in the right direction. 

 

Definitely felt like it would come down to some Dyno time, but figured I'd have one last shot on here to make sure I wasn't missing something obvious.  PAJ should, in fact, be out then on the new FCR-MX39's?

 

Much thanks!

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Oh, ok, well that was not stated in your post, so I started with the simplest and most common solution to.. " I worked on my bike and now it does not run right" 

 

Haha no problem; I'm sure 99% of the posts you deal with are such.  I have done my homework and don't want to waste anyone's time!  Typically the reason I never end up posting on forums, just lurking and offering advice when I can.

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Definitely felt like it would come down to some Dyno time, but figured I'd have one last shot on here to make sure I wasn't missing something obvious.  PAJ should, in fact, be out then on the new FCR-MX39's?

 

Much thanks!

Far as I know.. I have not purchased an FCR from TT nor installed one from them in a dozen months... but I highly doubt anything has changed in what they are sourcing and providing. 

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Far as I know.. I have not purchased an FCR from TT nor installed one from them in a dozen months... but I highly doubt anything has changed in what they are sourcing and providing. 

 

Gotcha.  Yea, I just feel like since this is indicative of a lean condition, and that air restriction is out, it may be liable.  I am only managing to kill the motor with the fuel screw all the way in though, so...

 

I'd rip the carb out and throw the PAJ back in to test right away if I weren't nursing an arm injury that makes it a challenge.

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If you still have the return cable on the throttle try turning the throttle away from you when you get a hanging idle. If the idle calms down then you know that the cables are getting stuck.

 

It seems to be a common problem that if the cables are not just right, they will stick. 

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All,
 
I apologize for starting a new thread on this topic, especially as a first post, however I absolutely cannot figure out the issue, and can't find any posts from after 2012 regarding removing the PAJ in the FCR, so I think that perhaps this (or something else on the carb) may have changed.
 
 
Background Bike Info:
DRZ400SM, 3x3, FMF Q4 w/ powerbomb header
Carb is an FCR-MX purchased from the Thumpertalk store in May of 2014.
Jetting:
-155 Main Jet
-EMN needle w/ Clip @ 3
-45 Pilot Jet
-PAJ removed
-R&D Racing Flex Jet Extended Fuel Mix Screw (from TT store)
 
Problem: Idle will hang if throttle lightly applied and released.  Snaping it open/closed returns it to a far lower idle.
 
Notes:
-Carb is brand new and throttle wheel returns fully and squarely to the stop screw every time; no cable issues, sticky slide, etc.
-At a low idle RPM, going below about 1/4 turn on the fuel screw will kill the motor, seems happiest at 1-2 turns out.
-Have tried fuel screw combinations from 1/2 turn open to 3.5 turns open, and the same hanging idle issue persists.  It debatably seems a little better when richer (3+ turns open), although seeing as I have to turn the screw almost all the way in to kill the bike, I feel like my pilot circuit should be jetted roughly correctly.
-Have tried various idle screw settings from barely running to fairly high idle.  Problem persists regardless.  The higher the nominal idle RPM, the higher it hangs when I induce the hang.
-Checked for vacuum leaks.  Everything is nice and tightly installed, no sign of leaks.  Verified w/ carb cleaner.
-Bike runs fantastically at all other conditions, however this issue is very scary on track as it makes the engine braking very unpredictable
 
Any suggestions?  Should I not have removed the 100 PAJ?
 
Huge thanks in advance.

 

My 434 did that at sea level until I installed a 48 pilot jet, It needs a 50 or 52 for winter use at sea level. I dont have the xact number of my winter pilot jet at the moment.

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My 434 did that at sea level until I installed a 48 pilot jet, It needs a 50 or 52 for winter use at sea level. I dont have the xact number of my winter pilot jet at the moment.

 

This is what I was afraid of.  Two quesitons:

1. So you're running no PAJ still?  Wouldn't it be more logical to run a 42 pilot jet with the 100 PAJ instead of the 48 pilot w/ no PAJ?

2. I'm only barely able to kill the motor by turning the fuel screw all the way in, which should mean that I don't need to go up a jet, right?  Guess maybe that method for setting pilot mixture is faulty?

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If your getting a reaction to turning the fuel screw the pilot jet is the correct size.....The fuel screw should be very close to 2 turns when done adjusting

 

 

As per you opening post....Problem: Idle will hang if throttle lightly applied and released. Snapping it open/closed returns it to a far lower idle.

 

It sounds like you have a restriction in the throttle cables or throttle wheel or not enough throttle freeplay ...

 

Dyno testing the bike is not going to sort out this issue....There is no way your going to be lean with an EMN needle on clip 3

Edited by Craigo 485sm

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If your getting a reaction to turning the fuel screw the pilot jet is the correct size.....The fuel screw should be very close to 2 turns when done adjusting

 

 

As per you opening post....Problem: Idle will hang if throttle lightly applied and released. Snapping it open/closed returns it to a far lower idle.

 

It sounds like you have a restriction in the throttle cables or throttle wheel or not enough throttle freeplay ...

 

Dyno testing the bike is not going to sort out this issue....There is no way your going to be lean with an EMN needle on clip 3

Craig well done..

 

I so often assume folks have done the obvious checks before jumping to a jetting problem....

I assumed he had removed the plastic cover, checked for throttle wheel interference with the cover or frame, removed both cables from the carb  started motor and confirmed or denied if the issues was still present.  it never occurred to me, someone with  a hanging idle linked to throttle input would not start with the mechanical items that control throttle input. 

 

Though a dyno session would still sort it out..... by showing it's not a jetting issue  :p

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Craig well done..

 

I so often assume folks have done the obvious checks before jumping to a jetting problem....

I assumed he had removed the plastic cover, checked for throttle wheel interference with the cover or frame, removed both cables from the carb  started motor and confirmed or denied if the issues was still present.  it never occurred to me, someone with  a hanging idle linked to throttle input would not start with the mechanical items that control throttle input. 

 

Though a dyno session would still sort it out..... by showing it's not a jetting issue  :p

 

 

Notes:

-Carb is brand new and throttle wheel returns fully and squarely to the stop screw every time; no cable issues, sticky slide, etc.

 

 

I'm not here to waste your time, guys.  See my first bullet point under "Notes" in my original post; I've run the motor with cables loose, actuating the wheel by hand.  Tried to get a 0.001" feeler gauge between the wheel and stop to make sure it's returning.  It's definitely returning full and square to the stop.

 

I'll have to live with it until I can throw dyno wideband in the tailpipe and see what's up.  I was mainly posting to ensure that the PAJ removal was still a thing, as the last post that discussed whether it should be removed was from 2012, and I had no idea if the carbs had gone through design changes since then.

 

Thanks again

Edited by devgru38

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Edit: Sorry for the double post.

 

Ah crap, this issue just got a whole lot worse.  On the way in to work this morning, with the fuel screw out about 2 turns, the bike's idle would drop super low, and die.  After pulling over, upping the idle speed significantly, and continueing my ride it did exactly the same thing.  I had to crank the idle speed screw significantly to get it back to where it was yet again, and set it to a very fast idle just to get me the rest of the way to work.  By the time I got there a few miles later, it was barely idling again...  Turned the screw in another few rotations to get it back up to a normal idle before having to head into work.

 

I had to turn the thing in at least 4-6 full rotations in the course of my ride and i'm still at a lower idle speed than when I started (after warming the bike up fully for 2 minutes on choke in my garage).  And yes, I visually verified throttle wheel return to stop every time I turned the idle up.

 

Any ideas would be greatly appreciated.

Edited by devgru38

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Edit: Sorry for the double post.

 

Ah crap, this issue just got a whole lot worse.  On the way in to work this morning, with the fuel screw out about 2 turns, the bike's idle would drop super low, and die.  After pulling over, upping the idle speed significantly, and continueing my ride it did exactly the same thing.  I had to crank the idle speed screw significantly to get it back to where it was yet again, and set it to a very fast idle just to get me the rest of the way to work.  By the time I got there a few miles later, it was barely idling again...  Turned the screw in another few rotations to get it back up to a normal idle before having to head into work.

 

I had to turn the thing in at least 4-6 full rotations in the course of my ride and i'm still at a lower idle speed than when I started (after warming the bike up fully for 2 minutes on choke in my garage).  And yes, I visually verified throttle wheel return to stop every time I turned the idle up.

 

Any ideas would be greatly appreciated.

Air leak

Main jet not screwed in tight or at all 

Damaged fuel screw, or bore in carb body

carb not set up as you think it is

any number of other reasons....

 

 With what you have posted.... I would start the daig from the beginning,, assume NOTHING... 

New parts come mismanufactured, broken, incorrectly installed in an assembly.

User make mistakes and do not see them for what they are, because they KNOW they did it right.

 

None of that is flaming.. just experience and observations from a lot of years fixing other folks "fix's"

 

Pull the carb.. verify the air boots integrity and installed correctly.

Manifold O ring, jet sizes, locations in the carb, and installed correctly..

Passages clear

Fuel screw undamaged and no damage in the body.. all parts present and in the correct order

needle jet in good condition, needle undamaged and the size you think it is.

 

Basically inspect every part of the carb as if you think it is wrong, worn out, broken or missing. 

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