Installing a used 39mm FCR-MX on a ’03 640 Adventure

Installing a used 39mm FCR-MX on a ’03 640 Adventure... a bedtime story.

I’ll begin by saying right up front… this isn’t a "how to" or a DIY guide. Consider it mild entertainment... proceed at your own peril.

I’ve always been quite happy with my Mikuni BST40. After correcting the jetting, venting and slide issues, it worked fairly well for where I ride and the way I ride.
I don’t motocross my bike… flat track it yes, but I’m not really a big air, berm slammer kinda guy… at least not on a 400 lb. Exxon Valdez of a motorcycle, so I've never had the mysterious BST stumbling/stuttering issues that have been reported by others.

In Western Wooooshington there are logging and fire roads that turn into jeep trials that turn into single track that turn into rabbit trials... that turn into absolutely nothing.

Jeep class trail turning into single track… Godspeed having a gravity issue.



The part where the water is… that’s the middle of the trail.



A trail about to go from wide single track to rabbit class.



By the time you find out that you’re stuck in this nothing, with dense blackberry growth on all sides… to the point where you could just get off, let go of the bike and it would only fall over about 6 inches and stop, it’s too late and turning around isn’t so much an alternative as an imperative.



All these wonderful trails are better approached with something other than the previously mentioned 'EV 400 LB'… like a KTM 200EXC, or a trials bike, or maybe even just your feet.
But… being stubborn, I prefer to throw excessively large motorized objects down a trail rather than small, light practical ones.

Pause for intermission........
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What does this all have to do with a carburetor swap you ask? Simple answer; aside from the trails themselves being devious, insidious and diabolical… numerous logs, branches and boulders conspire with those trails to make forward motion with a 400 lb, CV carb equipped super-tanker less than ah… progressive.

A CV carb, wonderful device that it is at being smooth and economical, sucks at helping one pop a front wheel over those previously mentioned trail conspirators… at least not without some fancy clutch work.
The benefit of a CV carb can also be a hindrance in some circumstances. The lack of an AP makes for less than snappy throttle response, unless you have a good high velocity and low pressure signal in the venturi.

Seems that as I get older, the more permanent the symptoms of my past injuries become.
In other words… my hands don’t work quite as well as they used to, and although I’m a well practiced “clutch rider”, my left hand frequently doesn’t want to get with the program, but instead only becomes a handy place to store a left glove.
The funny part (OK, sad… but still funny) is that my left hand, of the two hands I own, is the better functioning one.

OK… that wasn’t a simple answer, it was a long drawn out Creeperesque answer.
The short form answer is I wanted to try a carb that might offer me a better shot at getting the front wheel up and over “walking speed” obstacles with a minimum of clutch work.
Yes, I’m asking a lot, no, my bike is not geared optimally for tight trials… yes, we are still talking about a 400 lb, 7.25 gallon, Hindenburg of a motorcycle.

So… this is the story of how a 39mm FCR-MX came to find a home on my bike.

While poking around in the KTMTalk LC4 forums one day, I spy a member mentioning that he has a Keihin 39mm FCR-MX available for sale that was removed from a ’03 KTM 525 EXC… no doubt to make room for a 41mm FCR-MX.
I says to myself, “self, this would be a good opportunity to try an FCR out on the 'ol Graf Zeppelin”.
He was asking $175 for it. I bought it… $185 shipped.

While waiting for the carb, I ordered up some ’04 KTM SXC throttle cables, head and airbox boots, head boot clamps (they are narrower and fit the boot mo’ betta), a 525EXC Keihin gasket kit, a James Dean jet kit (for a 525EXC as suggested by James) and a Zip-Ty extended mixture screw.

When I got the carb I put some fuel in the bowl, tested the AP nozzle squirt “three Ds” (duration, direction and distance), then proceeded to take it apart.



Apparently, I was absent from school the day they said “do not take a Keihin FCR-MX TOO apart”… because I took it completely apart, including the part that you would call the 'upper float bowl', or 'lower carb body'.
I can call the part what ever I want, because as far as Keihin is concerned it is a persona non grata item on their parts list and not something to be removed.

After looking at the extremely complicated and tiny rubber seals that go between the lower carb body and the main carb body, and a few choice “Oh shit… I hope I can put this back together without it leaking its guts out” internal thoughts, I did in fact put it back together, and to jump ahead momentarily, it did not leak (so far).



Removing the “lower carb body” (that’s the name I’ve decided to call it) does have it’s one advantage in that if you’re not happy with the direction the AP nozzle squirts it’s fuel, you can adjust it at that time.

Bla bla bla… Rebuilt the carb and installed the jets and needle James Dean recommended for a 39 FCR on a 640 ADV at slightly above sea level.

As the PO had already done the 'BK mod', which is to install a screw acting as an adjustable AP travel limiter, I set the AP to provide a 2.5 second squirt. Slightly less than the stock “perfect traction” setting and a good amount more than what you’d use on an off-road only, constant state of wheel spin, bike.


So what was easy and what was less than easy about installing this carb? I’ll make a little list for you to ponder.

A substantial amount of wire harness, sub-harness relocating was required. Not a big deal, but something that does require creative thought as to what should go where.

More creative thought was invested in things like the carb vent hoses. Keihin apparently has issues with being sued, so they have 1,652 vent hoses on the FCR-MX carb.
OK… actually only 5 if you count the float bowl drain, but still that’s at least 2 too many.
I ran a hose across the carb from the left upper vent to the right upper vent… effectively eliminating two hoses that would otherwise need to be routed down behind the engine.
This left me with a total of three hoses to route behind the engine and down.
If I ever crash and the bike ends up upside down and on fire… I might be in deep Kim Chi.



The vacuum hose for the fuel pump.
Believe it or not, I spent more time considering my options on this than any one thing… and I’m still not 100% thrilled about my final decision.
It’s a matter of the hose being so close to the exhaust header and making sure that the hose is not so tightly curved as to possibly kink and kill the pump… that will have 640A owners all in a tizzy.




Mmmmm, what else? Throttle cables were not too big a deal… routing is the key to happiness here. Under the backbone and behind the radiator hose for a bit, then out the right and up. With a steering damper, this may be your bestest and only goodest choice.

A bit to add to the cable part.
FCR-MX carbs... I believe the second generation version and beyond, has a black plastic cover over the throttle bellcrank/AP lever and exposed portion of the throttle cables.
The carb is a snug fit in the frame, so to remove that plastic cover and get at your cable adjustments, you have to loosen the clamps and rotate the carb to the left... but, to do that, you have to remove the TPS sender.
So... it's a good idea to:
1. Put a whitness mark on the TPS and carb body so you don't have to re-set it and...
2. Make sure you have your throttle cables correctly adjusted the first time. You know the routine, check the free play at left and right lock with the tank in place or the cables temporarily zip-tied down to duplicate the tank holding the cables against the frame backbone.



The head to carb intake boot.
First, you can run both the original boots… they don’t fit as well as boots for an FCR, but the do fit and you save about $50.
If you do chose to buy the head to carb intake boot, it is in your best interest to buy the corresponding clamps as well. They are narrower by a few mm and smaller in diameter as well.
The BST boot clamps will fit, but they distort the boot shoulders and you can’t tighten down the carb end one enough to get a good, happy-snuggie fit.

The TPS.
I’m not going to get into discussions about the advantages or disadvantages of a TPSs being connected… or not. There are lots of interwebnet threads on the subject, mostly heated… mostly anecdotal rather than objectively or data analyzed.
Let those who read (and can think for themselves) decide.

The EXC TPS plug doesn’t match the 640A TPS plug, but the wires do… as does the TPS function and range of operation.
All I did was to remove both incompatible male and female sockets and replace them with a 3 pin, waterproof Deutch socket set.
It’s then just a matter of setting the TPS position according to millivolt readings thru a range of throttle operation.



The enrichner assembly.
You can use the Mikuni enrichner in the Keihin carb with one modification. The Mikuni enrichner piston has a fuel 'needle' on it that must be removed to function as a Keihin enrichner piston.
Simply snip off the needle and carefully file or very carefully grind down the needle so that it is flush with the surrounding piston.
I did it with a fine file under a large magnifying glass.

Not much else to say (hard to believe I know) except installing a take-off 39mm FCR-MX on a 640 ADV is not that big a deal, and should be even easier on a less complex LC4 such as an ‘E’ or an ’03 SXC.

So far, I’ve only gone on a few street test rides and I can say that the FCR is neither better nor worse than the BST… just different in the way it responds. I can say that James Dean's recommended kit and starting point for jetting is pretty damn close.
I’ll have a better impression when I can go for a dirt ride and get the final idle mixture and AP duration sorted out.

That’s all folks… if you’ve taken the time to read this whole thing, I hope you got a chuckle or two out of it.

C

Creeper eagerly awaiting a solid ride report on the new carb as I have one on my 525 mxc and the idea has crossed my mind on the swap,but really I'm pretty happy with the bst,but sure can't wait for your opinion after some more miles. Good writeup Creeper keep us posted!

00 300 exc
03 525 mxc
06 640 adv

Ta Chris.

It's good enough to be a "how to" or a DIY guide for me too ! Just need the $$$ to get on for moi. (but I am going to give the BST one more float adjustment before deciding to ditch it, or keep it and save the $$$ for some other bike stuff)

It's more of a "shit I encountered in my travels" thing than a blow by blow how to. That would be hard to do anyway because of the number of variables involved in a take-off carb.

I really didn't have any issues with the BST other than the walking speeds "snap"... or lack of it.
I mean, if you pull in the clutch a bit and wing the throttle a bit, then partially release the clutch enough to get a bit of air (but not so much you shoot off a cliff ), you can clear logs and stuff... but that's a lot of work for my left hand, even with a hydro clutch.

See, I'm the oposite, using the clutch is no worries, but the BST is what annoys me. I just had my suspension re sprung and re valved so that should reduce the bog a bit (hopefully), and lowering the float level lower will hopefully eliminate it.

I will have my souped up BST running within a week or so. Today I just began work on making the LC4 rideable again. That ids2 is some piece of work, you know. That is the hardest thing in the universe to repack. I felt like filling it with FFF and sealing the ends just to get some satisfaction.

I scoped out a location for the O2 sensor on the right side of the midpipe. I should be installing that tomorrow.

BST40 mods:

* grose-jet instead of useless float needle valve system
* dial-a-jet installed onto airbox boot
* bore divider on intake side
* loaded's fuel mixture screw

I wanted a full set of needles to work with but the ones I ordered don't fit correctly, so I only have OEM to work with. Hopefully that will get me somewhat close to correct AFR. That's the most important part, and the reason I'm going back to the BST. I want to dial it in to a reasonable mixture with the jet4power meter. Honestly I have no idea what the stock settings will look like on the meter.

I think when I was jetting by seat-of-the-pants I was going around in circles. I couldn't have been anywhere near the ideal mix. I know on my two stroke it was all over the map until I dialed it in with the help of the meter. This should be a lot easier, without having to worry about premix fouling or low exhaust temps screwing with my readings. The downside is that I have no needles to swap on the BST so I'm scratching my head as to how tightly it can be tuned.

One thing I did before was I installed an intake bore divider on the BST. I think it ties in with something Creeper mentioned above about intake velocity. It added quite a bit of off-idle torque, in fact it was the one time I actually had it running like a champ, but then it broke and I didn't feel like fixing it so I went off to the pumper carbs. I got better at machining the divider by slotting the bore and using thicker material, so that's what I'm doing to the BST. I need to get a baseline AFR reading first though.

I actually didn't have a good experience with dial-a-jet before when I used it. I ran it for a while on a two stroke. I became convinced that a two stroke doesn't care at all about fuel atomization at the carburetor, so the dial-a-jet didn't offer anything. On the BST, my belief is that it will provide enough supplemental atomized fuel to cover the transition periods of the slide. So in effect, it should behave similarly to an accel pump.. as a quicker acting siphon than the main jet.

I think my ignition is kinda flakey too. I need to go through my electricals and make sure things are proper.

BST40 mods:

* grose-jet instead of useless float needle valve system
* dial-a-jet installed onto airbox boot
* bore divider on intake side
* loaded's fuel mixture screw

Sounds like quite a project.

But Man, I won't be doing all that to the BST, I just want it to stop boging when hitting obtacles and jumping.

Who's needle table was this, cugino pegaso's?

Attached a jpeg of the table for ya potato

Sweet pics creeper. The wires were the biggest challange when I test fit my FCR. I had no exhaust problems at all. I wonder why some guys do and some don't? I am still running my BST for now. But after my MX trip I will be going FCR. I just don't want to change anything that could effect range or reliability right before a big trip.

As far as BST bogging goes I have completely fixed mine. It is 100% float issues. It is really really easy to set the float wrong on the BST. My bike would sputter and almost turn off when I got into rocks and steep hills. The 2 combined where a clutch grabbing throttle milking fest that only led to gettng tired and pissed off. I can pound through miles off ass deep sand whoops, jump as high as I dare, and do rocky steep climbs with ease now with my BST. The only issue I have at all with the BST now is the fact that it makes my 55HP bike a total dog. It should wheelie at will with only throttle. I shouldn't need to have special wheelie powers to do a power wheelie on the road. For some guys it makes the bike easier to ride. But like Creeper said it can be trouble when you need to pop the front wheel over something.

Another good read, thank you Creeper!

The Southern California Home for the Millivoltly Challenged (SCHMC)
was hoping some knower of things TPS could explain further please?

Thank you, Nick-Founding Member SCHMC

Clickie the pickie............

Groovy, thanks. Sorry should have been more specific, so the same values apply regardless of carb type?

Congrads, I just did mine not too long ago, yet went with the 41mm..
One suggestion an old time told me, route those breather hoses up and out of the way.
It should allow you to cross deeper water without stalling.

Yes that's what I ordered from. I don't understand how those 6DH* are even considered. They are so different from the OEM and FactoryPro.

There appear to be several OEM (various Japanese bikes) needles for the BST40, but who knows what they measure. Would be nice to find a series somewhere.

As the intent of a TPS is to provide input for a 2nd dimension to the ignition map rather than any "information" for the carb... yes, the ignition input settings would be the same for any TPS equipped carb.

Basic ignition systems are based on RPM; as RPM increases, timing advance does as well... until full advance is reached.
With a second dimension of throttle position relative to RPM, a second map layer can retard the timing if throttle position doesn't match the RPM parameter, or leave it at the higher advance curve if it does.
Even smarter ignition systems use the spark plug resistance as a reference of cylinder pressure... a 3rd dimension to retard timing under excessive loads.

C

Interesting... if you want to discuss the mechanics of these adjustments to timing and their utility I am all If this is something that I can investigate on my own dime, then throw a dog a bone (link to further reading?).

Several brands of motorcycles use spark resistance as a third igniton reference... the only one I'm aware of that uses it on a carbureted bike is H-D.

Again I thank you sir.

A few too many "free patents online" sites and a NASA pdf about the electrical properties of aircraft skins and...

Jacobs ignition system described

Optimal combustion efficiency - more bang for your buck - having your cake and eating it too - OK, why aren't we all doin' it?