Problem....missing, popping under hard throttle, jetting guru's help ASAP please!!

Hey guys,

Well here goes first problem I have had with my 01' 426. I have done the BK Mod, checked the valves, did the blue wire mod 2 rides ago. Also installed a new UNI air filter, visi fuel filter, etc.

Well I noticed after riding with the new air filter, fuel filter, oh and a new spark plug and the blue wire mod that it was not as crisp as it had been after the BK mod. But it still was OK for a ride about 30 laps on my track in about 80 degrees one afternoon about two weeks ago. So I got it out yesterday and it was popping and acted like it was starving for fuel or flooding out under hard throttle. No decel pop though. It usually has a little decel pop while it is warming up though and I didn't notice that yesterday. I have stock jetting. I tried turning out the air screw on bottom of the carb from 2 turns to 2.5 turns. Did not affect it at all. I live in VA and am pretty close to sea level. Riding conditions will be hot I imagine pretty much from now on varying from 80 to 100 degrees.

What do you think jetting wise. Really don't want to change jetting because it has always run so good with the factory jets in all differences in weather. Even after I did the BK Mod it still ran great with the factory jets. Do you think it might be cuz of the blue wire mod, Fuel filter restricting the fuel, New air filter too much of too little air. I am stumped and before I start tearing the carb apart thought I would get your opinions. Tried putting blue wire back in and made no difference. Someone with similiar riding conditions what jets are you running with factory exhaust and BK Mod. Any help will be greatly appreciated. Let me know!! Thanks, Frank :)

Good luck. Check out my bad stator question from today. I also have the bk and blue wire mods. I went up and down the scale with jetting and adjustments. Nothing worked. Then tested a carb from another bike and that didn't work either! Check out my post. Does your bike run as bad as mine sounds?

I dont think this is jetting,,,,,

it sounds more like a mechanical issue, i.e. electronics, stator or timing is off.

Maybe I missed it but have you checked the timing as perscribed in the manual ?

You said you checked the Valves, Did this mean you removed the Cam Chain ?

Also if its flooding as you say, tear the Carb apart and start cleaning and measuring.

Measure float hieght and so on. Make sure the carb is spotless

Well 193 that doesn't make me feel to good. I don't think it is an electrical problem on my bike though cuz it stills rips even under the popping and missing. I was still overshooting jumps etc. I think I need to change the carb around a little main, pilot needle etc. But just hoping somebody with same bike and same exhaust bk mod etc. can tell me what jetting they are running. I can always set everything back the way it was but that usually doesn't work. You know why do we mess with ours bikes when they are running fine? LOL I mean as much power as a 426 has why would we want more. LOL

Everyone always wants more power. I ride a ZX9R Ninja on the Street and it will run 170+ mph and I still want it to go faster!! Motoman where u at I bet u can help?? Come on guys, help us out!! Thanks in advance! Frank :)


Hopefully it isn't electrical. I didn't think mine was either. My bike still has lots of power, I just don't trust it to even keep running through each little test ride. What a pain. I've spent so many hours on this thing. I was way faster when my bike ran! LOL>


Check out my post on the wr board under running rough at 1/2 throttle. Mine is not the original but a reply. I would love to get some confirmation on what symptoms you are having. It may allow me to confirm or deny some suspicions.



I believe the "air screw" that you adjusted is commonly known as the pilot screw? The pilot screw is found on the bottom of the carb. F.Y.I. this screw adjusts the amount of fuel in the pilot system not the amount of air like a 2 stroke. On the yzf, turning the screw out will richen the mixture. Most of the time when a yzf is taken off the showroom floor it is on the lean side(hesitation and popping). On my 99 yzf 400 the stock pilot screw position is 1-3/8, your screw is positioned 2-1/2. I would verify where your screw should be, and begin from the stock settings after you replace your plug. I also live/ride at sea level and my settings are pilot jet #48(stock #45),main jet #175(stock jet) and the pilot screw position 1 turn out. :)

Thanks for the replies guys,

I just can't see how it could be running so good then I add a new fuel filter, air filter, new plug, and do the blue wire mod and it goes to runnin erratic and popping and such. I usually only notice on hard throttle though after landing a jump or accelerating hard up a hill to a jump. I thought that the pilot jet was beside the main inside the bowl. the air fuel mixture screw is the one on the bottom of the float bowl (externally not internally) I may be wrong so somebody correct me if so. I might just try putting the stock air filter back in, remove the fuel filter and try another plug. What plugs do you run. I am running the stock #8 plug, was running a #7 ngk for awhile but went back to the one they call for. Also plugged back in the blue wire. Never really had a problem starting anyway. Let me know guys, keep the info. coming. Thanks, Frank :)



How much earl did you put on the new filter and what type. If you had over oiled it this may account for the sluggish / fouling / flooding you ar experiancing.

Plus with you saying it rips at top end, then this would mean you are seriuosly clogged in the bottom and the mains are kicking in on top.....

Just my $4.26 worth


You always manage to make us all laugh, I used K@N spray oil and sprayed it pretty good and greased the inner lip. Now it was a completely dry filter so I sprayed it pretty good. But I did not have much left in the can at all so I don't think I overoiled it. And also it ran better the first ride after I installed it two weeks ago than it did yesterday. The filter has only two rides on it along with the plug, fuel filter etc. What do ya think there Mr. Ego?


I would take the carb off and spray all the passages out with carb cleaner. Just in case some oil or grease seeped into the air passages or jets. Take the bottom off your carb when you clean it. I don't know about using K&N oil on a foam filter, I would use oil made for a foam filter. Also make sure the air filter dries after you oil it. I like to let them sit for a day. If the bike was running good before you messed with it, I would definitely check the carb.

My top three, in order, are:

1. You have a partially fouled plug. It would run fine up to a certain RPM/throttle position and start popping, then cutting out, and finally dying until the RPMs come down. Not only most likely but easiest. DOn't clean old plug, put in new.

2. You developed an air leak between the carb and the motor. That would explain all of the symptoms here, and explain the suddeness.

3. You have junk, probably air filter oil, in your carb. Clean it out with carb cleaner and air.

Good luck,


Mr Cantrell said it for me

You probably have a stuck up air pilot system due to the air oil in the injection subframe of the internal cumbustion apriture that feeds low speed fuel to the intake manifold during low speed operation.

Meaning in Itialina:

La Pilota Jetsa isa plugged upa from to mucha olivea oils.

if the bike sat then this would give the oil in the jet to harden or become more gui.....

As Marcos says Clean the fule passage ways real good. And next time easy on the earl

hi fastfrank, hope u r still fast or just frank now??????????

i think u should go backwards and check every single thing that u had replaced. never change more then one thing without checking it first...

and its still not as good as it was, i would suspect the stator or another electrical problem

If you hold the Uni filter up to a light the foam pores are much larger than the stock filter or twin air filter. The Uni still filters well, but from what I have noticed it is more prone to suck filter oil in (it has done this to me twice). You would be surprised what a little oil in your pilot jet circuit will do. First clean that and then go from there. If you feel any oil on the inside of the airboot...then I think you found your problem!

Also do what Mark said and check for air leaks in the intake manifold and carb boot. I used a bench grinder to grind off about 2-3mm from the little brass spacers that go inbetween the boot clamps (this enables you to clamp the clamps down tighter = no more air leak!) While your at it put in a new plug. Keep us posted...Im sure it is a simple problem.


BTW You did make sure fuel still flows with the fuel filter on right? I have seen one brand of inline fuel filters that can only be installed one way, or it says it could block of fuel flow. Im not sure if this would affect it, but it is worth a check (since it takes 2 secs to see if gas flows or not :) )

Funny how you have this problem after getting on the gas from landings or accelerating up a jump face. It appears to me that gravity is pointing out the problem. Just a small amount of debris or crud on the needle valve is allowing to much fuel in the float bowl. As a test, either clean the needle valve and seat and try again, or bend the float tab upwards to shut off the gas earlier and maintain a lower level in the float bowl. Too much fuel in the bowl causes a rich condition which can lead to a bog upon sudden throttle twists. This is just one thing to check. Mark Cantrell makes a good suggestion about the air leak. Air leaks are not uncommon the YZ426, especially in the intake tract. I had a leak at the air boot adaptor on mine that nearly drove me nuts trying to find what was causing my engine to run so erratcially. The bottom line is not to overlook the simple things first before moving onto to the more complex probabilities.

Thanks Guys,

Don't think I am going to be fortunate enough to ride this weekend. (Have to work) But I will pull the carb off and clean next week and will clean the Air Filter and "lightly" oil it after I am done cleaning. Also new plug and will check the gas flow through the fuel filter. Thanks for all your input!! Motoman ck your pm's. I really appreciate all your input guys!! I will keep you posted. Thanks, Frank :)

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