99 yz 400 jetting problems

I have a 1999 yz 400 that is basically stock. The problem occurs when the engine is at low rmps or idle and the throttle is opened quickly. The engine will die as if it is being starved. Also, when first starting the choke takes a while to work. I have adj. the valves, checked the TPS, used a 173 main jet, went up and down several clips on the needle, atc. Help would be appreciated.



See my comments on wilson's posting on the same topic

James Dean


In the users manual it says to step up the main and lower the clip, then it says if this doesn't work do the oposite. I've tried all of that except for changing to a different needle. I will try this. Can you think of anything else it could be?



The fix I found for that was a 48 pilot and the fuel screw at about 2 1/4 turns.

The pilot also fixed the popping on deaceleration.



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Rubber Chicken Racing


Matt has taken the right approach. The manual reference you gave is not specific about the throttle position. Your problem is at "low rpm's or idle" which means pilot screw and pilot jet adjustment, if you look closely at the manual pages on each component before the trouble - Symptom Chart.

James Dean

I have a 99' that is stock and had the same problem and the quickest fix I found was go out 2 to 2.5 turns on the air screw. I now have that same set up with 180 main and 50 pilot as Per Stoker recommendations. Works Great and doesn't run as hot or pop as much.

Thanks alot everyone. I'm going to try the 48 or 50 pilot. I'll tell everyone how it turns out.



Guys, I'm looking at my service manual on my '98 YZ400, I don't see anything identified as an air screw or a fuel screw. Do you know exactly what the manual refers to these items as? Can you provide a general location where on the carb they are?

Also, what is the black knob just below the choke knob and where should it be positioned?

Another question, should I use the choke or the hot start button or neither the first attempt at starting the bike in the morning? Remember, it's roughly 85-90 degrees outside here in Phoenix when attempting the initial start.

Paul in AZ


The "pilot screw" is on the bottom of the float bowl in front of the drain plug. It is commonly called the fuel screw because turning it out enriches the mixture. (unlike your previously owned 2-stroke)

The black knob is idle speed. Set as desired, not too fast or it may become hard starting.

To choke or not... its never that hot here in western Washington, but probably use it breifly. The hot start should not be used first thing.

James Dean

James Dean,

Thanks for the info. Do you know what the manual calls the airscrew and where it is located?

FYI, I tried starting my bike last night without the choke - no luck. Then I tried it using the choke - almost started. Then I tried using the hot start only, no choke - it started first kick. Outside temp about 90 degrees at the time. Why this worked I don't know???

Paul in AZ

I tried a 48 pilot jet, it helped some, but the problem still exists. I think I will try a lower main and see what that does.

Any other sugestions?




Check that the accelerator pump & feed is working properly. You can do this by removing the carb, from the airbox and manifold mountings, and turning the throttle, there should be a healthy stream/spray of fuel exiting the intake manifold side (make sure you have the float bowl full/fuel connected). I'm not sure if the manual gives the volume per pump ?

Do *NOT* look into the manifold side of the carb while doing this ...

Paul Card

00' 426


I have already checked the acc. pump. It did shoot a steady stream out.

Anything else?



Nick, don't change the main jet for low throttle problems. The main jet jet only affects 2/3 to full throttle. Low speed jetting involves pilot screw, pilot jet, and needle straight diameter. How far out is the pilot screw? The needle clip can also play a role to a lesser degree along with float level. Did you check the float level? What is the needle code and clip position? (DTM?)(top slot is #1, count positions down)The third letter indicates straight diameter, M is richer than P. (P is richer than R) Keep focusing your attention on these components and write down each combination. The hesitation is most likely lean if you haven't fouled a plug yet. The manual is specific about which circuits to look at versus throttle position.

Keep us posted.

Nick, if you are using a DVR needle it may be time to look at using a DVP. Its another option worth checking into.

I have checked the float height. It was good. The clip is in the stock position (I think 4 on the dvr). Used a 48 pilot. Fuel screw is out 2.5 to 3. Helped a little going out further on the screw. The main is a 175 (stock). When I first got it I fouled a few plugs(maybe because I didn't know how to start it).

Should I go to a 50 pilot?

How about lowering the clip position?



Nick, lowering the needle clip will make a small difference down low and it will only cost you some time. Make sure you have tried at least position #4 before buying a DVP needle. Keep in mind it will also affect the mixture from 1/8 to 1/2 throttle more than 0 to 1/8. The DVR is the 'leanest' needle listed in Clark's jetting for altitude tips page. Looking at what you have done, the #50 pilot or DVP#4 is the next incremental step.

I have measured a mis-manufactured needle before and it is a possibility, but probably not likely.

James Dean,

I went to a #50 pilot. It stopped the backfiring while decelerating, but didn't stop the hesitation. I'll try a new needle.




Through reading all the responses, I have one question for you........

The hesitation you are experiencing, is this on full throttle "blips" off of idle? I also experienced similar symptoms before I realized I was trying to make my motor respond like a 2 stroke. If this is the same problem, I would take it as "the nature of the beast". I could not see how this motor could ever react as fast as it's 2 stroke cousins. I found that as long as I am somewhat realistic in my throttle openings,and after my 165 main/48 pilot and 2 1/4 turns out with fuel screw, the motor runs perfect!! Try it a couple of times and post your results. Hope this helps.

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