FCR Carb fine tuning - starting help - WR400

OK....here is the story.

Carb was too rich all round, fouling plugs constantly.

Started with

168 main (but mechanic said drilled bigger)

50 pilot

OBDXM (Aus version of OBDTM)


162 main

45 pilot (1 1/2 out)

EKP needle #2 clip

Runs very nicely with the new setup, no stutters and good power, except starting.

Dead cold it will start on choke

Warmed up won't start unless I hold the Throttle at 1/4 and then starts and idles perfectly.(same happened with 48 pilot)

I have read Williams post on setting the pilot and fuel screw which I am still fiddling with.

If I am reading it right.....if I turn the fuel screw all the way in and it still idles smooth I'm too rich and need to go down?

Just wondering...am I missing tsomething to get it to start with closed throttle?


Yes, you might be rich on the pilot. Or it might simply need the hotstart lever pulled after the engine is warm. That's why Yamaha put one on there. After it's hot started, how's the idle? How's the response off of the bottom?

Yes you are correct, it will usually start on the Hot Start, but mine is worn out so I have to hold it while I kick (not easy !)

My previous WR never needed the Hot Start, so there must be a magic combination there somewhere.

The bike still has a small stumble between 0-1/8th throttle once it is warm. Most noticable if I coast to a stop and then roll on slowly it has a but of a stumble.

There is no stumble if I whack it open and the rest of the range is great, so I guess I'll keep leaning out my pilot until it goes away.

My main problem is Jets are really hard to get where I am (bike shop is hopeless and really expensive), always have to mail order. I really thought the #45 pilot would have done it.

I will try to get the fuel screw set correctly this weekend now that it is close to right and hope I don't have to buy more jets.

I'm thinking I need to search the net for a parted out wr and get a hot start replacement, brand new are a rip-off.


Wound my fuel screw right in as a test an it still idles smoothly.

So I'm guessing I need a #42 pilot ?!

That sounds correct to me.

Fully warmed bike, should stall with the fuel screw gently closed. If not, go next size down on pilot (a 42)

168 still looks rich on the main jet. How does it pull WOT in 3rd and 4th gear up an incline? I'd think you'd want a 160 or at most, a 162

Keep in mind, the FCR jets are all the same for all bikes that use that carb (which is every MX bike).


I am running 162 main

EKP on clip #2

I'll definitely have to try 42 pilot.

I put screw all way in and still idled fine.

I'm suspicious about maybe the needle jet (emulsion tube) could be worn too so I might order a new one as well to be sure.

Glad to see you thinking. The worn emulsion tube has the biggest effect (when worn) in making just off of idle way rich and so on to about 3/4 throttle. It should NOT have any effect at idle. If it does/did, your idle speed is too fast when adjusting the fuel screw.

Latest results :

#45 pilot, fuel screw out 1/4

Went or 1/2hr ride, started to sputter on low end after 5 mins.

Tried fuel screw out 2 nd it died and I couldn't start it for 10 min.

Finally started and got going

While riding I pulled hot start and sputter was gone, clean off bottom.....but big POPs on decel as expected.

Soooooo do I need a smaller Pilot or bigger PAJ ?


The part that confuses me is that with 50 pilot and stock needle on #1 there was a spot where bottom was good but I had mid issues ?! I don't fully understand the matrix yet :/

Over analyzing. Pilot and fuel screw are for idle ONLY. You mess with them to make them do what they are not intended to do and you will have other issues.

If your bike runs better at part throttle wit the hot start, you need to either relace the needle jet emulsion tube because of wear or (...if it is in good shape) try a different needle leaner on the straight diameter (not clip position).

Ok, that makes sense.

I removed PAJ as a test and it started really easily.

PAJ is #75 so I think a #100 or 110 would suit.

So I have EKP needle on #1 which I guess tells me it's still a bit rich.

What Is next leaner one? F..?

I've seen lots of mentions of EMN being successful.

Had a read of the needle lettering system. Seems I had it wrong in my head :-/

Obdtm - stock

Obekp - current

OBEKQ - Try next (when I find a cheap one)

I appreciate you putting up with my ignorance.


Not at all! This way we all learn!!! :thumbsup:

It really pisses me off how Yamaha rip off Aussies.

Needle at yamaha in oz $51

Needle at keihin dealer $21

Needle shipped from USA $18.

Emulsion tube yamaha oz $121

Emulsion tube USA $30

Come on ... Give me a break !!!

Same deal in the UK, parts cheaper from the states than at my local Yamaha dealer.

Problem is it takes 2 weeks for USA mail orders to get to me. Which explains why I've been jetting this bike for so long...

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