fcr39 stumbling?

[e gracie]

having just spent best part off an hour reading through the search engine still little confused as to what to do....

440ims mx cam fmf.fcr39
jetted as supplied at 180 main 52 pilot.up to about half throttle hard to tell if it is half or there abouts it stumbles big time like running out off fuel .question is do i lift needle. or turn out pilot screw....
bit dum at this point....no smirkin now! ,or is it the other two screws on the air intake manifold?
defo appears to be hunting for fuel me thinx


many thanx in advance for any advise given,,cheers.:rolleyes:

[86atc250r]

I don't know your setup but it's likely that you have just the opposite of the problem you've guessed - that sounds WAY too rich on the main.....

Have you done a plug read?

Too rich on the main will make the quad stumble when under heavy throttle - most prevalent in the mid RPMs.

[e gracie]

havent ridden it yet only run it in garage take plug reading 2 morrow thoughn .interesting comment...180 to big had fcr 37 with 170 main and that was fine?HHHMMMMMMMMM:o

[86atc250r]

Depends on many factors, but most setups fall below the 180 main range....

I ran a 170 for a long time in mine & even tried as much as a 175 - but after taking the time to do some more in-depth testing I found that although the bike seemed to run fine with the larger main, I gained significant performance by dropping down to a 162 & I think I've got more to gain by leaning it further - just haven't had time to test & read plugs...

FWIW, my 400 is built to the point that it's faster than my Cannondale.

[lilpoppy]

Well i just installed my FCR 39 mm carb and i was having the same problem. First of all that does sound too rich. Im running a 165 and it is running perfecr according to the plug test. Second it is totally normal for this carb to stumble when you go from idle to wide open throttle. This was what was confusing me about the carb. Whenever you punch it like that it is letting in too much air and basically making your mixture too lean. It should only do this from idle though. If it is still doing this when you are riding then try jetting down and check the plug. The other thing that helps is adjusting the pilot screw by screwing it out about 2-3 turns to richen it up a bit. Basically I have found that it was impossible to tune this thing without being able to ride around.

[jerry-89-250R]

my fcr doesn't stumble from anywhere, idle to wfo no problem

[lilpoppy]

I called sparks racing and they said that they all did it, even their very own quads. If yours doesn't then I am curious if you adjusted it and what exactily you did to get rid of this problem.

[86atc250r]

I added some spring rate to my accelerator pump to cure the stumble if the throttle is punched off idle....

There are accelerator pump kits available that may solve the problem as well, haven't tried any of them personally though.

[lilpoppy]

I dont know much about carbs at all so what is spring rate, what does it do and where is the accelerator pump located?

[dhines]

Yeah, come on Gabe - hook us up on your little secret for the FCR - could be good FAQ material. :macho

I just went out and bought one myself and I'm trying to get it dialed in. While it runs pretty well, if I'm lugging out of a tight corner in the woods in say 2nd gear (at low RPM's) and then punch it, things just don't happen quickly enough. I have to end up feathering the clutch to get the RPM's up - something that was not really necessary with the stock carb.

I'd like to hear how you have yours set up overall. I've done a good bit of research on this site and others and have gotten a lot of contradictory information on things like how many turns out the fuel jet should be, etc. Let us know what's working for you.

Dennis

[Glamis400ex]

This is just a guess on what Gabe did...I believe he pulled or stretched the spring out just a tad to make it more stiff, thus giving the spring more "load" and giving quicker acceleration, reducing the "lag" or "stumble" of idle.

Just a guess.....I could be completely wrong...:huh

Glamis

[e gracie]

fitted 170 mj not much off a difference i dont fink.will have to wait till 2 morow and ride it as a few say u need to ride it to get a decent plug reading ..any suggestions on what the needle clip position should be or there abouts on this set up b4 i ride it?
and possible pilot jet screw setting 2 out?

IMS 440
FMF POWER CORE 4
PORTED HEAD
MX CAM
AND THE PROBLEM FOR NOW FCR39


also any photos or imfo on what u guys did to raise the tank/ 10mm rubber added to original p[osition at a guess or ??????
many thanx in advance..eg

[jerry-89-250R]

i run my needle in the second position from top, i used a piece of rubber about 1/2" thick, just laid it under the tank too raise it. Jetting is trial and error, it changes from climate and elevation, you need a starting point, riding and plug check is really the only way too find out what works best for you

[lilpoppy]

Why did you guys raise the tank? I didnt touch myne and it seems to be fine. I havnt heard anything about this. This carb is kind of a pain in the butt not coming with any kind of instructions and I can't seem to get any answers from anyone on this site about it. It is a good thing I'm running soo much faster now because this thing has been a big hassle from the start.