400 That coughs sputters and pops when started

I have a 99 YZ400f that is very hard to get running . It dies as soon as you give it any gas. And if you are able to give it any gas it sputters and backfires. It also backfires when you kick it sometimes. It has a very irratic idle as well. Once the bike is warmed up it runs great. The current jetting is 48 pilot, needle clip moved one down from stock, and the rest is stock. Any Help Would be appreciated. THANKS JIM

I forget to include. My elevation is around 2500'

dont know if this helps any but i had the same problem when i took 2 discs out of my white bros e-series pipe, put the 2 back in and it runs fine now, maybe try a leaner pilot jet

can i have al;l your jetting settings please.



needle code



PJ = 48



Also, make sure the slide plate is not upside down in the carb.

Dave S

Taffy and others, Here are my settings PJ=48 , MJ=175, NJ=OBDVR , Clip pos #4, Pilot Screw 1 3/4 out, Idle screw 1 1/2 out. I also have a WB E series exhaust system 12 disks.

Jimmy K

i don't know your PAJ but i'll presume it's a #100. i also presume you're not flying too high-under 2,500ft.

try a 45 or preferably a 42 PJ to go with another needle. the _ _R suffix means your not getting the fuel down the side of the needle at tickover.

this forces riders into ridiculously high PJ's to get some form of help. sadly 15% (let's say for arguments sake) of that extra fuel helps tickover but the other 85% has now gone and dumped itself in the 2nd gear low revs area.

but no! we don't want to stop it there we now want to dump a load of neat fuel in on top of that called the APJ!

if you want to carry on with a 'D' series needle for tractable but fast rides go for the DMM at sea level or the DMN for higher.

if you want the latest 'hang on for grim death etc' get the EMM or EMN for altitude. better still why don't you go over to the WR side, find "jetting Q's" and read it and print it.



EMN clip 3

MAJ 200

PAJ 100

PJ 42

APJ disconnected completely. the above are conservative #'s so it could come down a clip and down a PJ etc.

get yourself the variable PAJ screw.


Taffy, Thanks for all the info. I have a couple of questions though (I'm brand new to the 4 stroke game, as well as this board). First of all you are correct in the elev. below 2500'. When I got the bike it had a very bad hessitaion just off idle, as well as running poor when cold. So I switched from the 45 pilot to the 48 and lowered the clip on the needle to richen it up. These modifications took care of the hessitation. The bike runs good after warmed up. It even seems like the poor starting and running when cold is alot better if the bike has just been taken out of the garage 65-70 degrees. I hope this info helps. My questions are what is the MAJ? and what do you mean by APJ dissconnected?also what is a variable PAJ screw and where do I get one? What is this OBEJN needle I have heard about and will it help my problem? THANK you so much Jim K



don't worry about what the MAJ is unless you're prepared to look up a web site by a man named patrick burns. print it off and read it and read it until you understand it.

at low revs you have a pilot jet and a pilot air jet. they should work as kissing cousins but if we go up one or down one on the other the lean/rich is changed at roughly low revs/2nd gear ok?

you can buy a screw from sudco that has a drilled passage down the middle and out the nose of it's taper and you can open or close the screw the same as buying the jets themselves. if you wanna play (w/ jets) you gotta pay!

you fell straight into the trap that everyone falls into above, you noticed a hesitation in the first 1 yard and someone shouted "lean on the pilot" well it wasn't. it was lean on the needle. i told you about the 15% BEFORE you gave me this info, yes?

in the meantime one of the hardest things to diagnose on a bike is a rich mixture at low revs because all that happens is that the engine just can't "light up". this is when you short shift into 2nd gear and the engine just goes bwooooooaaaaaaarrr instead of bwooaar!!!ok!!!

the APJ is the Accelerator Pump Jet (APJ) it is a neolithic piece of shyte that doesn't belong on any bike and pumps enough neat fuel in for four buicks!!

it operates mainly right where you just dumped a dirty great 48PJ in. getting the picture?

the 98/99 model is unadjustable although you can buy a mod called a P38 which stops it still squirting when you're in bed and tucked up.

i've had mine disconnected by pulling that silver steel rod you can see on the right -out.

go to the WR side and read about "jetting Q's" ok.

hopefully next time we talk you will have tried the above settings i gave you.

the ELP needle will richen your tickover which is what you need, so an ELM, ELN or ok, ELP if you insist.

on the WR side some new boys have got their needles down to 2nd clip and ideally it should be on it's fourth clip.

so now ELP becomes EQP (mNpQ = 4 letters and there are two letters per clip).

the important bit is do you want an animal? yes or no?

E = is wild

D = is mud etc

i'm running EMM clip 2 so if i want to be at clip 4 i have to go EPM for clip 3 and ERM for clip 4.

you're at higher altitude so we can go ERN to make the tickover a tadge leaner.

so ERN, EPN, or ERP, EPP if they do any of those four. if you take my advice go for one of the first two (ERN, EPN)

i'd be running them (ERM for me) but i'm close to finishing some MJ tests which will have a knock on effect on my needle, so i'm not going to buy a new needle each weekend, you know.


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