01 yzf426 w/ fmf Q jetting or tuning help?

I have a 2001 426 and just put on a FMF Q silencer with a FMF power bomb header and now have a real bad bog and rich burn at 1/4 to 1/2 throttle. I was running the same header with a white brothers e-series with 18 tuning discs and had no problems. I ride in central California around 1,000 ft elevation and 50 to 70 degrees temps. I tried to adjust the air screw in some which helped but not enough. I pulled my jets thinking they were larger than stock. (2nd owner) but I have 162 main and 42 pilot. I have not checked the neddle yet. The new pipe instruction did not say to change any jetting. I know that I have alot more backpressure now than before. Does anyone have any advise for me to hopefully eliminate some of my guesswork?

The Q likes the stock jetting 42-165 #5 on the needle even my 470 likes it works for me :cry:

I run the same setup on my 01 (Q and powerbomb), but I have no problems with stock jetting. 162 main, 42 pilot, #4 needle clip and about 1 turn out on the fuel screw. Keep in mind it's a fuel not an air screw, so out is richer.

If your sure it's too rich, try a 40 pilot or drop the needle a notch and see what happens.

I put in a e-mail to FMF for they're opinion but I think you have the right idea. I think I may need to drop my needle down one and try the stock jets again. The bike was a little rich even with the white brothers but I had way less backpressure. I already tried to turn the fuel screw in on my last ride but I still had the bog and sputter going up the hills on the trail. It was like riding a 2 stroke 125 and I was missing the torque and the ability to lug the bike. I went home very early that day and have not experimented anymore yet. Thanks for the advise.


I run a 40 pilot, 162 main, clip in the second position from the top, and a ZipTy mixture screw.

Most of my riding is in the 2000 to 5500ft elevations. Back when the "Q" hit the streets I spent a day at 4000ft dialing in the jetting for this pipe. I tried many different variations and found this setup to work the best.

The ZipTy mixture screw makes tuning the "bottom-end bog" out much easier. I often have to make a simple little adjustment on this screw for different conditions and elevation changes, and it's so much easier with Ty's screw that it makes riding the bike much easier too.

I ride the tight and nasty stuff and have been known to be a "revver" type of rider. I use the bike through all ends of the RPM range and do not use flywheel weights on my ole '02 YZ426.

Can you discribe the zipty fuel screw a little more and where would I be able to order one? Thanks for your advice.


Thanks for the tip. I have been looking for something like this. I just ordered the Zipty mixture screw and one of there magnetic drain plugs. I hear these are not a bad thing to use for metal particles in the oil. Do you think that I should change to the 40 pilot and move the clip on the needle 2 notches from stock even if I am riding under 2000 feet elevation most of the time? FMF e-mailed me back today and told me to install a 168 main jet from 162 stock and turn my fuel screw out 2 and a 1/2 turns. I could not believe this considering that the bike is already running too rich now. I think they are a little off on this application with the Q. Hopefully your trial and error will save me all that extra time. Thanks for the advice.


I have an '02 426 and I'm running the Powerbomb and Ti4 system. WHen I called FMF, they told me the same thing: 168 main, 2 1/2 turns on fuel screw, everything else stock (42 pilot, 4th clip position). The bike was way too rich, hard to start, wet plug, ran like crap. After posting on the jetting forum, the feedback was to go back to stock settings: 162 main, 1 1/2~1 3/4 turns, etc. I'm going to try it out this weekend and post my results. Oh yea, I ride So Cal tracks and dez, 500' - 3000' elev.

I would try the 40 pilot, then the needle after I found out what the pilot does. The adjustable fuel screw is too convenient. Does it pop on decel. What are you using for an indicator of rich? Plugs?

My bike has a rich plug and alot of black on the back of the new Q pipe. It also smokes a little if you rev the throttle in nuetral. The bike would stumble and bog when you tried to accelerate up hills in the 1/4 to 1/2 throttle range and had no low end power at all. Then it seems like after the revs are up enough it takes off like mad and can start using the fuel. I tried to turn in the fuel screw in and it seemed to help some but not enough. No pop or backfire on decel. I am going to try the 40 pilot because the bike has always seemed a little on the rich side during quick twists of the throttle. Then I may try to drop the needle if that does not work. I think the needle clip is in the stock #4 position.


Thanks for all your help. As soon as I get some time I am going to try the 40 pilot and then drop the needle only if I still need to. I will post what my best results were in case someoone else buys this pipe.

P.S. Does anymone know where to get a good deal on a cam kit with the Decompressor for the 00-02 426?


I would clean the carb while you have it off. The airbleeds in the carb throat clog easily. Also, check the needle jet for mung. It can build up over time. If the holes get plugged, it will run rich. Finally, the '01 needs the BK mod to eliminate the stumble when you whack the throttle open.

What is the BK mod. I have heard guys mention this before?

You can also buy a 03' YZ450 optional AP diaghram instead of doing the BK mod. Only do it if your bike stumbles off idle though, not all bikes need it. Mine did and the 03 diaphram fixed it.

http://www.thumperfaq.com/ap_mods.htm Read this :cry:

The BK mod seams a little complicated. Was the accelerator pump change very difficult? What is the P-38 kit?


The AP swap only requires the carb to be rotated slightly and three allen head screws removed. Its a breeze........

Not sure about the P-38, I think its like the Powernow which is an insert that goes into the carb boot and supposedly smooths out the airflow (less turbulence, more air = smoother power and better throttle response)

Hows the riding up there in Paso? I've got some friends in Grover Beach who keep trying to get me up there with my bike.


Yes we have some great riding in our are such as Black Mountain (Pozo), Balinger, and my favorite Clear Creek which is near King City. There is Pismo Dunes if you like the sand. What is the AP mod and where would I purchase this? Is that the 450 diaphram? Have you changed your jetting at all?


You can get it at the dealer, its the 'optional AP diaghram' for a 2003+ YZF450. The dealer will see it in his microfilm thingee.

I did change my jetting to the 2001 needle (you already have that) and a differnet pilot and main, but I cant remember which off the top of my head. I think a 158 and 40, but I could be thinking about another bike (I get them confused). I'll look when I get back home from Xmas holiday.

Search the jetting forum for posts by me cause I asked a whole bunch of stuff when I was trying to get mine dialed in and JD gave me lots of good info.

Have you done the GBmod as well?

Your jetting may be spot on, the 426's are known to have a bog at bottom.

Here is a link to the GBmod explaining what it is and steps to get it done


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