Yes, another jetting question

I've read the post on the site but you guys have gone too deep for me on most of them. Hopefully this will be simple for ya'll.

I have a 99YZ with stock exhaust, 14/50 ratio, (0-2000ft)ONLY jetting changes are 180 main, 50 pilot,and 2 turns out on the fuel screw as per question 17 & 18 in the questions section on I broke my ankle right after making the changes so I didn't get to compare the two different settings until now 6 months later. I have the hicup off the bottom and also hicup / flooding feeling (on bottom) when getting on the gas hard under load. It pulls really hard all the way to the top after the stumble. I have been on the WR site and compared my jetting with their chart and seem to be close. The manual just doesn't explain it well to me (former 2-stroker) and can someone explain "breathes hard" to me. Just wondering if stoker had it wrong.

Thanks for any help! And YES it was great to get back on the bike and ride after time off.

Since my 426 is a '00 model, my carb settings won't apply to you. However, I also fought that hesitation....hiccup...when I would blip the throttle hard and suddenly as I tried to launch off of a jump lip. There are times when a burst of power is necessary to make the bike do what you want to do. Some riders have said that you must "roll" the throttle on to keep from experiencing this hiccup. Rolling on the throttle won't always accomplish what you want to do. With that being said, I would think that perhaps your accelerator pump action isn't quite spot on. Assuming that you've tried different main and pilot jets without success, you might try changing the accelerator pump timing. If I remember correctly, the '99 has a type of clip that you can squeeze with a pair of needle nosed pliers to make it quirt sooner....or....widen the clip to squirt later. I might have it backwards.

To correct the stumble on my '00, I installed a Factory R&D P-38 accelerator pump bottom plate and dropped the main jet one size. My carburetion has been perfect since.

Maybe James Dean or Clark Mason will see your post and offer their advice. They know what they are doing when it comes to jetting.

[This message has been edited by Boit (edited 03-07-2001).]


It is hard to say if you can cure the hiccup to your liking with jetting alone because of the accelerator pump. If you really want to jet this bike properly IMO you have to do it with the AP disconnected, especially if you are trying to find the right needle and clip position. If you have the patience to monkey with this I think you will be rewarded.

Failing all of that, since you went richer on pilot and main, you might just try raising your stock needle (lowering the clip) one notch and see what that does.

James Dean on the WR side is some kind of jetting and carb needle idiot savant, I’m certainly glad I found him, any more questions you have you might post on the WR side to get it right from the horse’s mouth. He will probably suggest you try a few different needles. Listen to him.

As for the accelerator pump, a lot of stuff is happening in that area right now. Taffy on the WR side is trying to jet his bike to run without it, other guys (like me) are trying different ways to limit the stroke length, you can adjust when the pump begins to operate (like Boit mentioned), and you can go with the Factory R&D part, this seems to have been an improvement for all who have tried it (but it is pricey).

All the good jetting threads are on the WR side because that is what Taffy, James Dean & Clark Mason (another R&D user) ride, those guys are the ones to watch for and listen to. Believe it or not, they have me drilling holes in my $700 carburetor. :)

This may be too involved for you but IMO there is no simple answer to your problem.


I have a '99YZ and ride at same elevation. I went with 48pj and 178mj with sir screw 2&1/4 out and DVP needle on 4th clip. Bike runs great, no hesitation or hiccup, may be a touch lean at WOT @ top end, but I ride enduros and the 180mj seemed to cause less response coming out of bottom end. Good Luck, Keith.


if you buy a pilot air (jet) screw & spring from sudco for $12. your prayers will be answered.

i put some questions up on this side a couple of months back on the 'carbon airbox thread' i believe, about the relationship of the pilot jet & pilot air jet.

i've owned the PA(J)S since last august & have only started to muck with it when i went to the EKP needle from the euro DXM two months back.

the bike used to 'hover' or vibrate at 1/4 throttle now it's extremely smooth & you can really wind it up at 1/4 throttle & slightly less.

i haven't got definitive figures but what the hell its a big improvement so i'm not complaining. i had 48PJ, PA(J)S out 1 1/2 turns, PS out 1 turn.

if you go to the wr side & look for "at last snap & 3/4 throttle response" you'll see that i've gone a lot further.

for those interested, i ride at tickover & just slam the throttle as hard as i can & it goes!

anyone who has an old pilot air jet & can bore it out to No.125 or 1.25mm or 3/64" which is 1.2mm to go with their 48 or 50 PJ will find an improvement when the PS has been adjusted accordingly.


All - THANKS for your info. I think I will try the 178mj with the 48pj. If that dosen't work, Taffy where do I get the pilot air (jet) screw and spring from or can I drill out the one? I'm not familiar with "sudco". Also do I understand you right when you say to drill out my pilot air jet - is this a fix instead of buying the one with the screw? I also have the stock needle DVP.

One of the biggest problems is this thing runs different that the RM I had ( which I thought was easy to jet just by sound and feel. The manual doesn't really go into jetting runs.

In the tuning section the manual states that the PAJ may need to be changed if the pilot jet is changed. The great thing about the PAJ screw is that it is adjustable, replacing every available fixed-size pilot air jet.

Sudco is a carb part supply company (800-998-3529), ask for an air screw and spring for a 39mm Keihin FCR (PNs are 021-230 for the screw and 021-235 for the spring), for around twelve bucks. I think you’d probably rather do this than drill out your stock air jet.

Don’t forget about needle and clip position, you spend way more time running at least partially on the needle than pilot or main combined. If you are still in clip position # 3 I think you should try # 4. Also, I think some guys are running an EKP needle (more or less an ’01 YZ needle) in their 400s in #3 and #4 positions.

Use Thumpertalk’s search feature and look for “DVP” and “EKP” in posts by James Dean on the WR side if you want some good information and tips.

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