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fcr 39 troubles

I've been thinking about your situation. I have, and have dealt with, two styles of FCR (the CR special like yours and the MX style). Taken them on and off bunches of times jetting, orings, chased leaks, fuel contamination issues, etc, etc.. They work great once you get them sorted but getting there isn't always easy as your finding out.

I really think, if you can afford it, you should fix and sell the CR special and get the MX version for the bike your working on. With no magic button you need that separate starter circuit. Or a spare leg maybe. That thing is going to drive you nuts at the worst time on the trail. Plus it's just the wrong application. If you can do it eBay that sucker and get the right one. Would be a couple hundred bucks all done I suppose. Only you can make the call I just hate to see guys suffer without at least speaking up.
 
Let me start by saying any advice I have is from personal experience on personal motos. I don't work as a bike mechanic I just work in similar fields. Any tuning I do is with my butt dyno, dig? That said I do like to fiddle with my stuff to get it right and do have a BSME with almost twenty years working on stuff so I'm invested. Also I raced two strokes for a bit and you can't do that reasonably without being able to jet a little (PWK and PWM Keihin carbs). Never could ride that bike as fast as I could make it go. So I'm an overeducated bookworm mechanical hack motorcycle enthusiast. Take what you want and leave the rest...

In your case my experience is saying you want that choke. On the downdraft CR's installed on the M900 (or any 902cc desmodue for that matter) they are finecky on cold days. I can't even imagine if I had to kick the thing. Even with one cylinder. Half the time it would be a total chore. It takes a refined routine of throttle twists, magic button, and sometimes a little babying of the throttle plus a warmup period. I'd have to say 20-40% of the time I have to hit the magic button twice. Without that button life would suck.

On the DRZ with the FCR-MX it's another story. Pull out the choke button starts right up. It has a magic button as well but the success rate is better. Also the thumper is easier to stumble out to a halt when cold. So since I have a choke on that one I'm going to assume that's the ticket.

From an exterior standpoint they are certainly different. Outside of the aforementioned choke the AP is different and less exposed and the float chamber is bigger. Also, as pointed out by another internet source, the needle for the MX has seven positions and for the standard CR there are five. More adjustability must expect us to be changing altitudes (or something) more.

Here are the major differences as listed by Sudco: http://www.sudco.com/keihin.html
I'm assuming those features listed are some pretty desirable stuff on a dirtbike I wouldn't care about on a street bike (CR style).

More stuff on FCR's see post #2 here: http://www.atvriders.com/vbb/showthread.php?367038-School-me-on-FCR-carbs

The FCR-MX 39 on my DRZ came with it installed by PO (horribly jetted of course) and only has a choke. I think ALL the MX series come with a choke. I promptly threw on an aftermarket full system to ensure I maximized my practical experience in getting it jetted properly. If I had to do it over again and bought the one I wanted I would have gone the next step up. Not for the air cut valve the bike shouldn't afterburn if jetted right (only if you need to make emissions guidelines should that happen) but for the hot start. Crashing, falling and the subsequent flooding makes restart a bitch. That hot start valve on the next model sounds like a great idea. Next model up from that includes TPS and ..... who cares? If the ignition system doesn't need it forget about it. I'm not racing it. So the current one I have is order #016-925 but I would have preferred the #016-926 (reading from Sudco page here). I'll try not to crash too much and it won't be an issue.

So then you say why bother with the standard FCR on the inline 4's, big twins and other road machines? Simple: they all have magic buttons and stay on the pavement. Made to go fast around tracks and stuff. More purpose built and less all purpose. The difference is subtle yet pervasive.

All this of course IMHO. Thanks for asking you made me look and I always pick something up when I do that.

My next carburetor adventure is with a handful of Mikuni TM33 pumpers and DR350's. Sadly Keihin does not make a fit for that application. The jet collection continues!
 
The seat is savable.. so after you get it out of there.. replace the needle and the o-ring, not the seat.

Get some fine valve lapping compound and lap that seat with a q-tip, that'll keep it from leaking with the float new needle.

Now that the engine will be running mechanically correct, you have to be able to discern what is a rich misfire and what's a lean misfire.. rich misfires is very common. I like leaner pilot circuits.. Make it lean enough that you have to start the bike with the accelerator pump. Then you have a base line idle.. figure out the main jet first and work your way back.

Everyone messes up and makes the pilot circuit fat and it shouldn't be. It seems to be the thing to do but it's not. Most times needle position is the only thing I'
ll change.
 
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Jim our backgrounds aren't that dissimilar, I justbhaven focused so much on motorcycles in my free time. Thanks for the Synopsys. I am going to east a bit more time on this CR at least getting it back to working condition, and if it hit a wall I'll look at the MX swap. This will predominantly be a street bike, I think, so stalling it at stoplights is a bigger fear than dropping it.

Chad: I smashed the seat getting it out of the carb body( might be straightenable) I do have an old model Parts Carb (slant slide fcr39) with a functioning valve/seat assy. How donibget the seat out without damaging it, so I can just get the needle only?

Ernie: I will see what I can find. Hopefully I never have to pull the seat again, it'll be even harder with thebfatter boring I imagine
 
zoomsplat said:
A true cheap-ass would soak it in a solution of toxic things to free it up.
Click to expand...

I did try this, then ingot impatient and ripped not no it with a pair of pliers.

Corrosion is the answer. The needle body appears to be aluminum. Aliminium does not like prolonged contact with gasohol which happened to this carb. The carb body must be mostly... Zinc?

In any case: needle from parts carb+ existing seat + precision fiddling = 9.01mm float height and free movement.

HF viton o-ring required some forcing to let the seat seat all the way but it should be fine

Nasty cruddy corroded needle on right. New looking old one on left.

FU: corn loby.
 

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Hook up a hose turn the carb over and blow in the hose it should hold as many psi as you can muster. Easy test before you stick it together to check float valve operation.
 
Indeed it is. So easy in fact that it can be done while holding an 8 week old that thinks 5:45 is an acceptable time to be awake and active

Passed the test too.
 
In would like to thank all you bitches, and bitches is the Affectionate, Couldn't Have Done It Without Y'all usage....


Valve adjust. Clean carb. Follow online "how to start KTM" procedure.... Sorta. BAM! Starts first kick cold.

You'd have no idea how elated I am right now.

Time to register, insure, and stop tripping over the aforementioned lathe so I can fit some 17s on it :teeth
 
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