doing BK mod

another SNAFU

[ December 16, 2001: Message edited by: NH Kevin ]


Do you want to gut your Octopus? The YZ250F websight has discussed this in detail. Do a search on their forum. I do not know about the 2K/2001 bikes, but on the 98/99 WR's, after de-octopusing, the YZ pilot AIR jet replaces the WR PAJ. You'll need to replace a total of two/three(?) of the jets.

Check out the parts microfiche, if your bike and the YZF426 are listed to find the differences in jetting:

Please verify this using the de-octopus procedure prior to jumping the gun. Removing the Octopus allows you TOTAL control of your bottom end using your Kouba fuel screw adjuster. Disconnect your grey wire, and your bike can't get any better (w/ the proper jetting).

You also may want to look at the adjustable pilot air screw from SUDCO. In fact, you can get ALL your jetting needs from SUDCO. Their brass needles ARE cheaper than the Yamaha OEM replacement needles:


I wouldn't do the BK mod unless your carby's APJ is pumping gas beyond 2seconds after full throttle is reached.

Get the timing of the squirt correct - yes.

Cut the grey wire - A MUST

Get rid of the throttle stop - A MUST

Yank the snorkel out of your airbox lid - again a must

BTW, if your experimenting with jetting get yourself a new sparky as well. Cause you'll need it down the track after running your bike in a rich condition for too long. But that's how you learn.

I did the BK mod on my bike and found it put my jetting out. I may have needed a richer needle to do it justice. If you notice most people that have done the BK mod also have YZ timing. From memory YZ's don't have a accelertor pump. Just thoughts, don't have the time to confirm this though.

The YZF's DO have the accelerator pump :D .

The carbs are close, but to wreck the performance of the WR, they stuck that gosh-dern octopus on our carbs. Does nothing but foul it up. :)

The CORRECT ballpark time on the accelerator pump squirt is ~ 0.3 seconds.

Mine was > 2 seconds.

Yamaha apparently jetted lean to compensate for the deluge of fuel gushing into the throat of the carb from the AP. Can you say "Drowned Out?!!".

At steady state - low throttle, the WR is too lean due to the bogus settings required by the gusher accelerator pump, which at steady state is doing nothing.

[ December 16, 2001: Message edited by: NH Kevin ]

befor I do the BK mod I would like to go to Yamaha and get an assortment of jets so I can find out what works best for my bike, ( 01 WR426) it is currently stock with 165 mj, 42 pj, OBDRR needle @ #4, I have never touched the pilot screw.I have extra jets, MJ 170, 158, pj 38, any recommendations for additional jets would be appreciated.



I'm considering do do the BK (KL) mod this winter after the big tear down.

So, apparently the WR is virtually jetted to lean to compensate the too long AP squirt

After having done the KL mod should i start with another needle pos or another needle, or with a bigger MJ ? (MJ 172, PJ 42, DXM #5 and PS oops forgot)?

I just did the BK last week. From reading posts about it, I richened my needle up a clip, and it did not help. My mid is kind of sloppy now, and Im in the process of trying to clean it up. I feel better throttle response, and gassing out of turns is alot more fun. But as soon as I hit that mid-upper revs range, I am sputtering. My timing of the squirt is around .5 secs, so leaning the clip will be my next move. Hope that cleans it up. Any suggestions would be appreciated.

I'm considering do do the BK (JK) mod this winter after the big tear down.

So, apparently the WR is virtually jetted to lean to compensate the too long AP squirt

After having done the JK mod should i start with another needle pos or another needle, or with a bigger MJ ? (MJ 172, PJ 42, DXM #5 and PS oops forgot)?

And what about de-octopussing the thing, could I do it all a once ?

Which parts should I order ?

To de-octopus : PAJ #100 an SJ #65

JK mod : ?

Originally posted by Ynahg:

So, apparently the WR is virtually jetted to lean to compensate the too long AP squirt?

As far as I can tell the accelerator pumps are identical among same-year YZ/WRs.

IMO it would be more accurate to say the WR is jetted overly lean to compensate for the ridiculously restrictive exhaust (w/ stock insert) and air box (w/ stock cover).

IMO then if you remove the throttle stop, air box lid and “uncork” the exhaust (or go to aftermarket or a YZ pipe) then your first order of business should be trying YZ jetting, because that is what you are now riding, engine-wise. I don’t think the exhaust cam timing would have nearly the effect on jetting that the exhaust and air box changes have.


In fact I meant that both : WR or YZ had a relatively lean main jet system needle/Mj to compensate the amount of feul injected by the APJ when opening the throttle. So after reducing the squirt it should be logic to enrichen the needle/MJ circuit a BIT. Or not ?


A lot of guys end up one clip richer on the needle after taming the accelerator pump.

For me it was a wash, EJP # 3 and 160 main before and after mod (4k to 5k ft. elev. – dez).


if your bike is rich WITH the APJ why would you lift the needle afterwards?

these bikes run pretty much ok within two or even three needle clips (depending on your MJ) and all this tosh started with someone in north east guatamala saying that he thought it was safer if you kept the richness by lifting the needle.

two people here have said that you don't have to lift the needle so that's that then isn't it! end of story. you don't have to lift the needle.

you don't need a new plug for anything. one per long season or two seasons will be fine. but that's if your jetting is close and for the majority it isn't close. sometimes old plugs are better than new but because i'm not a shop telling you to buy something i can understand your not believing this could possibly be true.

these bikes are jetted for your very restrictive pipes. the euro/aussie/canuck pipe is more open and i would suggest that people look at this jetting for some ideas.

ynahg. you seem to be having real problems understanding this jetting business. you're talking to americans for a start with different exhausts. if you had copied the missile you would have a self-propelled grenade launcher of a bike by now.

on jetting Q's you'll find just about the last post there mentions the pilot ratio and it's from this you can start to jet correctly. especially as we only live 200km apart and at see level.

wayneo go down one then two clips. try to ride flat out, close (1 second) and open the throttle and this is your needle test. you may find that two clips down freshens the midrange but makes it lean when starting and at low revs so you may need a larger PJ.

most folks run too rich anyway so you'll probably find it runs better but may have an off idle blip. contact me if any of this is right.

hick has said the same as i did last month and that is that the inlet (airbox) and exhaust affect jetting far more than cams, valves, pistons and compressions.

i cannot believe that after hick and i have had such good results with 38 and 35 PJ's anybody could think these bikes are lean at low revs.


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