Need jetting help for YZ426F!

CJG · Nov 24, 2001

I just put an FMF Power Core IV Q silencer on my 426, and I'm having a problem with the engine bogging down at less than 1/4 throttle. I changed to a 168 main(stock162) like FMF recommends but the poor low RPM performance persists. I also tried both a 38 and a 45 pilot(stock 42), changed the clip to the 5th notch on the needle, and tried adjusting the fuel screw with every combo and I just can't get the low-end fixed. Ive tried nearly every combination of the above to no avail. Does anyone have any experience with this bike/silencer combo? What jetting worked for you? Any gear heads out there have any ideas?

Thanks alot!

Boozer · Nov 25, 2001

have you done the BK mod? in standard form, the accellerator pump pushes a stream of fuel that lasts anywhere up to 5 seconds. Through research, teams have found that 5 seconds is way too long, and that a stream of about 0.5 seconds works best. Go to www.thumpertalk.com and look at the YZ400/426 forum. If you can't find much there, look at the WR400/426 section and conduct a search. You will find out everything about this mod there, including photos of how to do it. Ive done it to two WR's (250 and 426), and the 426 benifited the most from it. The low end bog has been eliminated. If you have trouble finding the info, let me know. (i bet you've already done this and im sounding like an idiot!)
Oh, and your accellerator pump timing may be out of adjustment also.

CJG · Nov 25, 2001

Thanks for the tip Boozer! Actually, I had never heard of the BK mod, but I'm definitely going to look in to it.

Thanks again!

Scott Senatore · Nov 26, 2001

Hi CJG how do you like the quiet core so far, is the reduction in noise considerable.

CJG · Nov 26, 2001

Hi Rick#3,
I like the Q alot, now that I finally figured the jetting out. I've noticed a fairly big reduction in noise, and I'd say that FMF's -6db claim is probably pretty accurate.

As far as performance goes, I can't tell a huge difference from stock. Except that it seems to pull just a little better on the low end, and. of course, it's quieter.

moto614 · Nov 26, 2001

hi rick#3,
did you do the bk mod?, and if so what jetting did you end up with?
i've got an fmf q s/a on the way and would like to know

thanks
jeff

moto614 · Nov 26, 2001

sorry i ment "cjg" it's late i need sleep

jeff

IBWFO · Nov 26, 2001

I'm not sure you should need a jetting change if you do the "BK" mod. (I didn't)
The "BK" mos is named after Brian Kenny, who is Timmy Ferry's mechanic.
I have the Yoshimura TI pipe and header on my bike and after doing every combination like you I ended staying with the stock settings and just changing the clip position. I have done the "BK" mod but I didn't have a bog like some so it wasn't a drastic improvement. The Thumpertalk.com site is well worth your time and I'm sure there are many that have the same pipe/silencer as you that could help you.
Regards

moto614 · Nov 26, 2001

hi ibwfo,
im on thumper talk all the time tried the "search" to no avail about the fmf q silencer,at least the info i was looking for. my bike runs great the way it is(no bog) so iam reluctant to change any thing(jetting sucks) but i would like my bike quieter and i don't really need any more power,i mostly ride tight single track woods.from what i read if you do the the bk mod without the jetting your only doing half of it.i don't know maybe i want the cake and the ice cream, quiet and fast??

Scott Senatore · Nov 27, 2001

IBWFO, what is the BK mode you are referring to.

IBWFO · Nov 27, 2001

It is where you time the "squirt" of gas into the carb. On the 426 the carb is a ''pumper" carb, which means when you blip the throttle it squirts fuel into the carb, wheather it is started or not. The "BK" mod makes sure the timing of the squirt is where it should be. On bikes that have the low-end bog it is an unbelievable modification!
Try this site.
Regards,

http://mx393.tripod.com/carbinfo.html

SFO · Nov 27, 2001

A ranger told me that the only 426's to pass the sound test at the last enduro at midlecreek were running the FMF "Q" pipe.
I didn't do the AP mod but did end up going down to a 155 main with a ekp needle in the middle.
My bike was very "boggy" on the bottom with too big a main jet. Just dropping one size made a big difference.

CJG · Nov 27, 2001

moto614,
I did the BK mod and it made a big difference in low-end performance for me. I'm still experimenting with jetting, but right now I'm running a stock pilot(42), stock clip position(4th), and a 175 main. Everything is working pretty good right now, so I doubt I'll move very far from my current settings, if at all.

moto614 · Nov 27, 2001

thanks for the input cjg, the check is in the mail:)

Need jetting help for YZ426F!

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