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XR650L FRC tuning Help (After Lots of Googling)

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Hi Guys,

I have done my reading and feel like I understand corroborator theory, but am weak when it comes to practical application having worked almost exclusively on bikes with FI systems. I am taking most of my information from this page, as it seems fairly comprehensive.




I will keep it brief since there is allot of information:



·         OE Airbox w/ K&N Filter

·         41mm Keihin FCR MX Flat Slide Pumper Carb, (Sudco kit for this model, with pods and exhaust)

·         Full FMF exhaust


·         Weak power, less than jetted (poorly) OE Carb, (will not power whillie)

·         Idle RPM does not change while adjusting IMS (although changes can be felt while riding)

·         1/2 "X 2 " hole in air box side cover (courtesy of previous owner)

·         At extremely low RPM, bike jitters and bucks much more than with OE carb (On/Off throttle response)


·         40 Degrees, Roughly Sea-level

·         IMS adjusted for good throttle response (Runs weaker if I turn it in, stumbles if I turn it up)

·         Does not lean/bot under snaps to WOT

·         Runs best at ¾ throttle

·         Little or no popping chopping WOT to closed throttle


From what I have read, all this suggests that the IMS is correctly adjusted and that the bike simply requires a larger main jet. However, the fact that the idle did not change while the IMS was being adjusted freaks me out and makes me concerned either the pilot jet or slow air jet are incorrect. 


To be clear, to adjust the slow air jet, I unscrew it? Does this jet function only when the choke is engaged?


Thanks for the Help Guys!


*Edited for Clarity 

** Please excuse the title typo, it's FCR



Tuning FCR.gif

Edited by Mayt13

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I'm not exactly sure what all you are asking but opening (turning out) an adjustable slow air jet leans out mix.  The adjustable idle air jet functions the same as the IAC in the fuel injection system except that your screwdriver takes the place of the stepper motor.  All of these screws are just valves letting more "stuff" in as the passage is opened (screw taper is moved away from the seat).  Not too complicated really.  


With both an adjustable slow air jet and an idle mixture screw you have two variables affecting the low speed mix.  The tricky part is that the idle mixture screw may add either air or fuel depending on your carburetor model.  This can get very confusing very quickly if you try to make adjustments to either and aren't completely sure how your carburetor is plumbed.  


The bottom line is it takes a minimal amount of fuel to run your engine so, knowing that you have a stumble down low, it may be that you need more fuel from the idle mixture screw (if it does indeed add fuel) and not more air from the idle air jet.  Adjustable air jets are great for really dialing a bike in but I'd suggest setting it 2 turns out from lightly seated and then leaving it alone and trying minor adjustments to the fuel only (again once you've determined that your idle mixture screw adds fuel).  


Now, because of the relationship of the adjustment screw taper to the thread pitch these adjustments are not linear. Meaning one turn from zero to 1 out doesn't have the same effect as one turn from 1 to 2 out.  However, if it seems that minor adjustments to any screw make major changes in the mix the it is likely that the tapered end has been damaged causing the once smooth taper to now be stepped or bent.  These screws can also leak air around the threads which can also cause heartache.  Make sure the spring and seal (if that is part of the design) are present and correctly installed.  Both of these problems happen quite often.


Oh, and dump that K&N, especially if you ride offroad.  All the stuff you feel hitting you while riding is also grinding away inside your engine while using it.  This is especially true with a modified air box.

Edited by hollerhead

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Can you temporarily disable the accelerator pump so that its effect isn't masking your feel of the different carb circuits?

If your pilot jet is too big or small you'll have frustration trying to get the idle mixture right.

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What jets are you running now?

the slow AIR jet actually affects the whole throttle range right up to almost full throttle, that chart is not accurate,I am running a wide band O2 and can see and feel what is going on, the slow AIR jet actually makes a very big difference,

what slot is the needle on? there are 7 slots on that carb needle.

If this is the Slant FCR from Sudco, mine came jetted WAY too rich on top,out of the box it had a 170 in it, I'm now running a 145 but this is with stock air box/snorkel removed, Uni foam filter, stock exhaust.

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This was Sudco's response to my inquiry: My text is normal their's is in bold.


What really freaks me out is RPMs do not change while adjusting the IMS! That means you are too rich or too lean on the slow jet, does turning it out make things better or worse? Does it run better cold? Better warm? With the choke on? 
What are the stock Needle/Jet sizes on the 41mm Kenhin FCR MX Pumper 650L kit? Stock jetting in that carburetor would be 170 main, 50 slow, EMR needle in the 4th groove, 200 main air, 1.75 turns out on the slow air screw, 1.5 turns out on fuel screw. 
And what does the next size up cost? Slow jets and main jets sell for $4.67 each.
Is there a tuner's kit with many different Needles/Jets? We do not, we just sell them individually.
Edited by Mayt13

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I cant seem to stop thinking in terms of FI or CV carbs! I have a hard time warping my head around the idea that for a given altitude/ambient temperature, the soul independent variable is throttle position. i.e. these carbs must be tuned from the bottom up regardless of how they run across the rev range. 


At this point, I think you guys are right on track. I purchased a larger pilot and a larger main jet (because I suspect it will be lean, but this is just a guess). I'm going to play with the IAC and see if I can get enough air flowing that turning up the IMS does anything.


My reasoning: IAC screwed in=> less air => Air/fuel=x for good combustion => tuned IMS to x => but fuel charge limited by IAC at x => tuning out IAC => more air => turning out IMS=> more fuel => more charge => more power. 


IDK if that reasonable at all, but that's where my brain goes.


:confused: I don't get it does the idle not responding to the IMS mean its wayy to lean or way too rich, or either! Could screwing in/out the IAC help? Should I just throw in the larger pilot? 

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A pilot jet size too far off in either direction will make the adjustment screws ineffective.

If you get too frustrated, consider buying a wide band O2 sensor / gauge. That will give you a definitive, quantified measurement in real time for any throttle position.

I have no experience with the Sudco FCR, but many people have reported they come with very rich jetting. So maybe start by moving toward lean.

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Its a wonder that jetting works at all.  Way too rich.  A 50 Pilot Jet (slow) is about 3 sizes too rich and probably why you aren't seeing any RPM changes as you mentioned


Seeing you have drilled out your airbox


Try this


42 Pilot Jet (slow)

NCVT 3 clip from the top.



Calibrate your pilot circuit first using Eddie's method. Be sure to do this with the engine oil hot.  Five minutes of idling doesn't cut it. The OCEM(x) needles will work but I don't like them because they have a slight lean spot from 1/8-3/8 throttle.  The NCV(x) have a dual taper that fixes this.


Be aware that because you have a sudco FCR MX carb you will have to mod your AP by doing the o-ring mod to the linkage and also you will have to mod or change your diaphragm.  Refer to this page (near the middle of the webpage) about these AP mods.

Edited by D0T-C0M

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