37 mm keihin FCR works great

huscan · Dec 16, 2011

I have a 1995 FE501. When I purchased it, there was an annoying off idle richness, so I took the carb apart and found the needle was very pitted.
I then purchased a 37mm Keihn FCR off a 2002 YZ250F. Jetting specs are here http://www.thumperfaq.com/wrf_vs_yzf.htm
The carb works great, the bike starts easy (once I replaced the stator) has excellent power from low rpm to high rpm.
The only area where it is not perfect is snapping the throttle open just above idle it will die (as if you were to hold the kill button), and if you snap the throttle open at low rpm (about 3000) it will kind of maintain its level of power for maybe 1 second and then accelerate hard. I have adjusted the needle a few times, but end up back to original setting.
Because I own other bikes that require gradual operation of the throttle at low rpm, I am used to it, so I feel the way the carb is stock for a yz250F is fine on a 501 Berg.

RDP501 · Dec 16, 2011

If the bike had the stock carb on it and it were jetted correctly (new needle, correct pilot jet, correct air screw setting) there would be no hesitation when snapping the throttle open from a dead idle. In fact, the 501s with the 38mm flat slide Delorto had quite a bit snappier throttle responce than did the later ones with the 40mm round slide Delorto carbs. And yes, I have done a side by side comparison between the '96 FE501 I once had and a friend's '98 FC501 back in '98 right after he first bought it. If the Keihin was jetted correctly, your bike would not die when rapidly opening the throttle.

Once, soon after I bought my '96 FE501 (new), I put the 38mm Mikuni that belonged on a Husqvarna 500XC 2-stroke I also had on the Husaberg to see if it would work and experienced the same symptoms you are experiencing but never spent the time to get it jetted correctly. Most likely, your main problem with the Keihin is too much slide cut-away and the wrong needle/nozzle combination. If it were me, I'd go back to the Delorto and install a new needle (K35, if I remember right). My '96 FE501 was one of the easiest starting and cleanest running bikes I've owned.

zaga · Dec 17, 2011

Hi

Don't have to talk about clean crab, etc.. you also should check if the body of the carb has grooves made by the wheels of the slide and if the slide has a gap on the body, also if the o-ring from the plate of the slide is ok.

That been said, don't know if the carb is a little small for the 501, but that shoulden't be a problem at low revs.

Also don't know your needle.

You are talking of problems in the region of the Pilot Screw, is it set right ? Also you are talking about snaping the throttle, that also implicate the pump, you might consider to make a travel limiter to the pump.

Most important you should try to determin of your bog, is a rich bog or a lean bog, then you try to correct it.

:cheers:
ZAGA

zaga · Dec 17, 2011

This might help

[attachment=0:2w0ajp91]carb_troub_s_2.jpg[/attachment:2w0ajp91]

ZAGA

Chas · Dec 17, 2011

The 37mm Keihin FCR carb should be a major improvement over the stock carb, although a 39mm unit would be even better. The bogging problem, as already alluded to, is most likely due to an 'overactive' accelerator pump. The stock AP fuel squirt oftentimes is 3 seconds or more. This must be reduced down to 1 second or less.

ned37 · Dec 17, 2011

iirc, it's all in the owner's doc

huscan · Dec 17, 2011

WOW, great replies.

My main reason for posting about the 37mm fcr was because I could not find any info on installing this size carb (that originally came jetted for a 250) to a berg. All the information I could find was on the 39mm or larger. Others may want to look at this size carb, because they are available and cheap. I got the 37mm carb for $70. with maybe 10 hours use, because the previous owner had upgraded to a 39mm for his yz250 years ago and had the 37mm sitting around. While I have not ridden my bike with a 39 or 41 FCR to compare, I can say that this bike does not seem restricted at high rpm at all - It seems to rev "to Infinity and beyond"

I did spend quite a bit of time adjusting the idle mixture screw and ended up around the stock setting.
I am certain that it is lean when snapping the throttle at low rpm.

Was very interested in the comments on the stock Dellorto, because I really like the idea of keeping the bike original, and if the dellorto works that good that is pretty amazing.

zaga · Dec 19, 2011

Hi huscan

The Dellorto its not the FCR, but I've been riding with dellortos all my life with no problem.

I can tell you that a well tuned dellorto its best than a out of tune FCR, thats my opinion.

If you get one dellorto right you wont have that hesitation, offcourse with the FCR right you won't also have that.

Do you have the original carb ?

:cheers:
ZAGA

huscan · Dec 19, 2011

Yes, I still have the original carb, and I was also looking at the one for sale in the classified section.

Taffy · Dec 20, 2011

as ned says, there are good jetting guidelines in the doc and they won't let you down. a 37mm keihin is the equivelant of a 38mm dellorto so you are lacking for nothing IMHO. your ports and valves are tiny so the carb isn't letting you down. I doubt the 39 would make any difference in that case.

knowing the YZ jetting I would say you're just way too rich at present. I do a jetting kit which would do you fine. essentially the same jetting goes across all the engines and all the crabs with just slight adjustments to be made.

I have a jetting kit for sale and you can review it on my web site (address below).

regards

Taffy

zaga · Dec 20, 2011

Yes, to go on with the FCR, you should at least buy the needle from Taff, to be closer to a good setup.

:cheers:
ZAGA

huscan · Mar 18, 2012

I did some reading on the FCR, on this site and other sites. Yesterday I -

1- soldered the leak jet closed
2- drilled and tapped the carb for a 1 inch long m5 bolt and spring in order to limit the accelerator pump squirt. I adjusted this to take away about 1/3 of the travel of the accelerator pump lever.

Hesitation and bog are now gone.
On dry pavement it will wheelie and or spin the tire by snapping the throttle wide open from idle in first gear. Easier to drive now, because the bike always does what you tell it to with the throttle at any rpm, without hesitating or bogging.

Summary: A completely stock 37mm FCR from a 2002 YZ250 installed on my 1995 Husaberg FE501 with only the above changes (all other settings are stock yz250 - main jet, needle clip position, air screw etc.) jetting seems perfect, power is excellent, even at high rpm where it does not feel restricted at all by the smallish carb.

37 mm keihin FCR works great

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