37 mm keihin FCR works great

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.

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

This might help

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

ZAGA

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.

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

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

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

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