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........
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
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.
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.