carb mods...Brutally Wicked, in a GREAT WAY!!

I wanted the carb upgrades and studied everything from Andy (Oz) the Taffster (Great Britain) and our own JD. I e-mailed JD about what I was looking for performance wise. This is where I am at right now:

- 168 Main jet (JD recommended 170 - didn't have)

- FHP needle, #6

- 48 pilot @ 1.25 turns

- [sudco purchased] Pilot AIR SCREW @ 1.5

turns out

- main air jet port drilled out to 7/64"

- main air jet drilled to 2.3 mm (#43 bit)

- shortened accel pump actuating rod by

3mm. The accel pump stroke was 3 mm.

With the shortened rod, the diaphragm is

not stroking fully like it was.


still need a bit of tweaking but...

The bike breaks traction off of idle ALMOST violently and pulls hard in up to the midrange. The bike cannot rev out and is stumbling pretty bad, the MJ is too lean (172?). Also, there is SEVERE backfiring on deceleration (will be going out on fuel screw). Tell you what, that bottom end is wicked excellent - VERY PUNCHY. Did I say VERY PUNCHY! Talk about a thrill machine! Just from my short test, I DO prefer this power to any other. I raced a YZ250, KX250, YZ465 and CR250 Honda and have learned to be ginger w/ the throttle where need be. In Moab, this bike would have absolutely KICKED ASS in the fun factor. My botttom end was pretty tame, to the point of being boring. Now...what a rush!! :):D:D


'99 WZ with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, forked over by Pro-Action, OEM YZ tank, IMS YZ seat, carb mods by Jim Dean, Sir "Taffy" from Jolly Old England & Andy from OZ, AMA, NESC.

[This message has been edited by NH Kevin (edited 05-06-2001).]

After testing, I slapped a 175 MJ into it. The bike revs a little cleaner, but WILL NOT come even close to the limiter, still stumbles severely. I am going to a 178 MJ. This is the largest I have.

One point I neglected to mention: I am running the DSP airbox w/ Doug Henry velocity stack. The intake tract from the box to the carb is shorter, larger I.D. w/ less bend (less restriction). How much, if any effect will this have on jetting?

My TPS is not working at all. Don't know if it was prior to disassembly.

So I have a full throttle problem. Maybe the enlarged air jet w/ associated air port...???

Any ideas JD, Taff, Andy?

My next step is to look for a vacuum leak on carb or check float height or ...???


'99 WZ with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, forked over by Pro-Action, OEM YZ tank, IMS YZ seat, carb mods by Jim Dean, andy from OZ and Sir "Taffy" from Jolly Old England, AMA, NESC.

[This message has been edited by NH Kevin (edited 05-06-2001).]


This is a tricky combo to get dialed in. Try staying closer to the #168 main, 162-172?? The pump should be stroking about 1.5mm and then the pilot screw can be tweaked richer to reduce the deceleration popping.

The other thought is to use clip #5 instead of #6. This will be more lean yet down low but may be cleaner up higher.

What about the TPS not working, are you sure this is not going to be a factor?


Of course the TPS plays a role !

The TPS determinates the ignition timing.

If the CDI has no TPS signal, your timing can't be right.

The higher the revs, the more pre-ignition.

You say the bike does not rev out = not enough pre ignition.

The fact that the bike pulls hard from from off idle to midrange is logic, less pre-ignition from low to mid revs is normal.

I would first matter about the whole system working correctly.

And the start the settings.

Good luck. :)



'99 WR400 F, YZ exhaust, throttle stop trimmed, WR timing, airbox cover removed

My TPS is bad, not the wires or the multi-meter. I am cutting open the TPS to see what makes it tick. Doubt I'll be able to seal it up even if it can be repaired.

There goes $141 that could've gone for beer!

[This message has been edited by NH Kevin (edited 05-14-2001).]

The throttle position sensor is just a variable resistor

TPS shipped from Ohio last Friday, 5/18. Hopefully here by Wednesday, 5/23. My knee surgery is @ 1130 Thursday, 5/24.

Hopefully I can wrap up this post and these carb mods.

[This message has been edited by NH Kevin (edited 05-22-2001).]

This is a very old post, but I'm curious...

Is anyone still using the FHP, FHN, FMP, FMN needles in their bikes? What are your settings for pilot, screw, needle, clip, main? I am thinking about taking the suggestion over to the YZ250F forum.



I went to the E series needle after trying the DVP. My bike is still hideously low on power, comparitively speaking (to Bill's bike). My feeling is I had waaaay to much air going in through the air screw. I was contemplating coming up with a liquid based test in order to come up w/ settings on the airscrew compared to the 100 air jet. Some sort of tube, filled w/ H2O, to check turns out and flow rates. As of yet, I have done nothing. This very likely will be a winter project. I am seriously considering sticking the bike on a dyno for some jetting test runs (with the accel pump disconnected). So far, the closest I can find a dyno is CT, about 2.5 hrs from me...

My bike will not rev out to proper rev limiter setpoint. I have checked and adjusted valves, checked neutral switch cutout, check TPS, re-installed the stock airbox, checked thr. stop, checked YZ valve timing. I could check the ignition timing today, come to think of it.

I don't know where to go from here...Maybe sell the bike and buy a tricked out Z50 Honda...or a 450 Honda?

[This message has been edited by NH Kevin (edited 07-29-2001).]


Have you tried a smaller main jet, 162 or 165? This would be close to what the YZ426 runs.

your over jetted NHK

go down. the more power you produce the smaller the main jet, so congratulations!!!!



How are you?

Are you able to ride much these days and anything new on your bike's jetting?

I've been distracted with a second bike to play with, an orange 250 smoker. It's light but can't match the 400's torque. Haven't been riding much really. Too many other summer activities.



This post does not have some vital info on this carb matter. I had been dealing w/ James directly at his home e-mail. The Sudco airscrew was too far out allowing excessive air in and creating a severe lean condition. After two days (and 15 hrs) of troubleshooting the problem, at Bill's (moderator Bill) recomendation, we went w/ a proven combination for carb settings (DVP, 175 MJ). The bike became ridable whereas before, it essentially was not. Several TTalk members rode my bike to confirm it's condition. I rode Bill's bike and was AMAZED at the potential power of this bike. I am running the FMF PB w/ Stroker silencer. Bill is running a White Bros E-series.

With the new carb settings the bike is still substantially down on power.

I am wondering how much difference the exhaust system is making. That is the only difference between our two bikes...

I am at a total loss of where to go from here.


If your interested in trying a YZ pipe, I have one sitting in the basement. I traded my Dub-R pipe for it, so I could tinker around with it but, haven't used it yet.

Let me know :)



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, EKP, 48PJ, 175MJ at 500-1000' Thanks James Dean!


If you are coming up this direction any time in the future, I would really like to bolt it up for the day.

It is too bad, my Wife JUST came back from PA last week. She was in Harrisburg.

sorry lads, i needed a break!

it was becoming very addictive here and i do enjoy it. but the enforced lay-off when i couldn't rejoin was what i needed.

i was starting work at 9am by looking at my E-mail's then everything from this site. the UK is the first place to log-on after you boys had gone to beddie-byes and so there was always something to chat about.

i felt that i was getting more trivial (did i mean i or it?) and i wasn't getting my work done. you want to be an authority on everything but you can't in the end.

so it's nice to be back. i had done about 770 posts before the crash but never mind!

as far as the bike goes? well, i as you know wanted to try a different path by not using the APJ. i was unable to tell you of my findings but i did do one more days testing and i got the jetting set up nicely-or so i thought.

i'm trying to remember as i didn't even record my findings this time.

i took the tailpipe that you saw in my photo's off and went back to the european std pipe with the PB header. i went from 48 to 50 and then 52 on the PJ and at 52 my low end hesitation was worse than ever. i decided that i had misinterpreted the feeling the bike was giving and in the next few hours i went down through 50, through 48 and ended up at 45. the bike was excellent, superb. on the night before my only enduro of the year i was happy.

the next day though as i asked the bike to grunt out of bogs at 15mph in second it wouldn't have any of it. i was very disappointed as this sat right where you wanted the power on a wet muddy day.

it was only after a couple of days that i remembered that i hadn't been reducing the the PAJ SCREW to go with it all! i think it still sits at 2 1/4 turns out and should be back nearer to 1 turn. never mind, i'm just so busy that i don't have time to worry.

one of the lads gets married this weekend but will continue to enduro with me. one has lost his job and another has suddenly gone back to roadracing. infact a month ago he did the same enduro and now it's sold and he's got a single cylinder and did a round in sweden at the weekend! there are certainly no flies on him.

i look forward to riding the bike in about a month when the school holidays are at an end. the manx gp is off due to the foot and mouth so i will work right through to early november when i hope to take a holiday for the first time in 15 months.

a friend of mine lives in housten and has invited me over to the states to go skiing and i may well take up the offer. i've never done it before, so i should end up on my arse a few times. a bit like biking really!

sorry i haven't kept up with you Kev but i'm sure JD has tried most things to help so good luck. if i think of anything new, i'll let you know.


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