WR400F Starting/Jetting Problems

Hey everybody, I have a 1999 WR400F... I bought the bike used about a year ago. The previous owner said he hadn't done any modifications to the bike, so I am assuming the jets are stock.

Since I bought the bike I have uncorked the exhaust, cut the throttle stop down, and put a UNI airfilter on it. These mods(At least the first two) helped the performance of the bike greatly.

My bike is really hard to start. Takes me 5-10 minutes to get it started if it is cold (Hasnt been ridden in a couple days). If the bike is warmed up, it runs great! Starts first kick everytime (NEVER touched the Hot Start EVER!) It doesnt POP on decel, but I do see my header pipe glow red when I let it idle to long.

I have read that some people try blipping the throttle when starting cold, but when I have tried that it kicks me back!

I usually pull the choke out, kick it through with decomp. five times, and then find TDC and go at it. It will finnally start, but it takes a lot longer than It should. Once I finally get it running, I have to "Half-Choke" it because it sounds like its going half throttle... and without choke it dies.

Is this a jetting problem? I ride at sea level 100% of the time, in Alaska so its usually 50-70 Degrees out when I ride.

Any help would be great, thanks. :)

You may want to look at your starter jet size and move up to a 68 or 72.

This procedure is similar to a prime button on a lawn mower or snow blower: When the bike is cold, try giving the throttle one or two full turns before going through “The starting drill”.

Be careful not to get too throttle happy or it will flood. And only do this when the bike is cold.

So would tweaking the air screw help at all?

Going up in number/size would richen the mixture, correct? I was thinking about doing the Grey wire and Blue wire mods would either of those help?

Sorry, I dont know very much at all about changing jets or messing with carbs. Do I have to take the carb off to change the starter jet?


First off, you NEED to disassemble the carb for a VERY THOROUGH cleaning. The 98’s and 99’s have an inferior accelerator pump design and the AP is notorious for filling up w/ dirt and water resulting with corrosion and SEIZING!

Before you pull the carb off your bike, run the bike dry by shutting off the fuel petcock on your tank. As the bike starts sucking the float bowl dry of fuel keep blipping the throttle. This will drain the fuel out of your accelerator pump reservoir if it not seized.

Work over a VERY CLEAN area. A cardboard box may be ideal. You can very easily drop some o-rings and you will very hard-pressed to find them. Be sure you seal up any areas in the box where tiny o-rings can roll/slide into. EXPECT fuel to still be in the carb. You can rotate the carb upside down/sideways/every which way several times. This will cause most of the residual gas to flow out the carbs vent lines.


You will want to disassemble the carb completely. The AP has two o-rings in it. One ALWAYS falls out.

I recommend doing the following:

ENTIRE carb disassembly, cleaning, and blowing out w/ compresssed air.

Including: both main and pilot air jets – these are under the air horn on the airbox end of the carb. It is common these two jets plug up w/ air filter oil if you do not wait enough time after oiling your air filter.

ALL jets on the underside (inside the float area of the carb).

The accelerator pump in it’s entirety.

Your fuel screw >> WARNING,WARNING WILL ROBINSON!! When you pull your fuel screw out, there is a tiny o-ring and tiny washer, along w/ the spring in the hole. DON'T LOSE THEM!!

Set your float height.

Remove your air cut valve (the octopus), plug the two brass nipples or fill w/ silver soldier

Perform either the Taffy mod or KL mod to your accelerator pump linkage.

Re-jet your main jet, pilot jet (if necessary), needle jet, main air jet, pilot air jet (or adjustable pilot air screw available only from SUDCO)

Hey NH Kevin. Does the AP seal problem exist with the 2000 WR 400's? I have heard of it, but I don't seem to have too much of a problem.

Does the AP seal problem exist with the 2000 WR 400's?

>> No, whole different carb style

Thanks for the replys guys!

Come to think of it, I did get a little oil-happy last time I oiled my filter. The clear tube thing on the bottom of the airbox was full of oil, so I bet those jets you were talking about are plugged NHKevin.

Alright, well since it runs so good when warm... I hoped it was something I could do without taking it all apart. Looks like I'll be finding me a nice cardboard box to work in when I get home from work. :)

Whats a good thread that would give me a good starting place to jet from? I will check all my jets when I take it apart, and write down what I have.

Senoir Moderator Bill out of York, PA jetting specs.


KL Mod to acceleator pump (sets AP stroke and AP Timing)

Factory jeting by Taffy:

EMM #2,






1.5 FS at 500-2,000' Thanks Taffy!

i live at sea leval and have a 99 wr 400f gray wire mod ac taffy uncorked air box lid gone 168main and i dont now witch one is witch but there is a 45 and 88 on either side of the main need good jeting advice for sealeval to 1500ft

Yeah i have the same problem with my 00' 400F, I can get it going (after some swearing) and then it wont idle when the choke is let out and when i do get it going ill get maybe 20 metres and the thing will just die out. Me and my brother think its something to do with air/fuel ratio-not having enough fuel and being drowned by oxygen. Or the carbie needs a good clean out and the idle needs adjusting. Thoughts?

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