Help please brains trust!!

G'day and thank you for your time. I have had this problem for a while now but I cannot pinpoint when it started happening or whether it always did. The problem is when you crack the throttle open from low speed/almost idle to over half throttle the engine chokes/hesitates until you ease back on the throttle. If you hold the throttle at this point for long enough the engine stops. It is only noticable in 2nd and 3rd gears. (just when you want to get the front wheel over a log etc.)There is no problem if you only wind the throttle on to 1/4 initially get past the 'hole' in acceleration then wind it on to full throttle. (You can still get the front up very easily and get wheelspin etc. at less than 1/2 throttle)

This is what I have tried so far.(99 wr400, wr timing, Staintune exhaust with and without bung in, Twin Air filter, sea level)

1. 172 main, 48 pilot, 1.5 pilotscrew, dxm #4. I played with the needle with the best results at #7 but still hesitates slightly and used lots of fuel plus ran rich. At #1 it backfired alot and still hesitated. Pilot screw to 2 turns but no difference.

2. Went back to stock main 168, 48 pilot, 1.5 screw, needle at #4. Still the same problem.

Does anyone have any suggestions on what may be the problem and how I can rectify it?

Thank you.

Jim Bob.


I''m certainly no expert on jetting but, looking at your changes vs. results. Your changes need to be made on the jetting at 1/8 to 5/8's throttle and you only changed the Main jet (3/4 to full throttle).

Jetting is very throttle position specific.

I would think turning out the fuel screw and lowering the clip is what you might have done first. If you get the fuel screw out to 2.75 to 3.0 turns, you may want to consider going up one on the PJ.

Here is also a list of "D" needles from lean to rich:

DTM, DTP, DVN, DVP, DVR, not sure where your "DX" needle fits in.

You might also want to consider YZ timing. I waited for a couple of years. I really didn't want any more power for woods riding and after changing. I wished I would have done it sooner. I fell it softened the bottom a little, to make technical riding easier (for me). Yet has the power to loft the front any time.

Also not knowing your altitude and country or if you have the airbox lid off or on makes it harder to recommend jetting or suggest removing the octopuss.

We now return you to your regularly scheduled program smile.gif



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, All YZ Modifications, de-octopussed (Thanks Clark) EKP #4, 50PJ, 175MJ, 100PAJ and 65 SJ at 500-1000' Thanks James Dean!

AMA, Happy Ramblers MXC (Dist #7), Tower City and Rausch Creek


i'm going to beg to differ.

i run the '99\WR also and from your introduction "G'day" i know where you live and at what altitude-i hope.

i have worked with the needle you've got and you were right, the needle is best on clip 7. i've had it at clip 8! by using washers. however your problems are at lower revs and centred around the pilot system.

a 48 PJ needs a 110-125PAJ to correct the 1/4 throttle mixture ratio, BUT that even mixture is TOO MUCH so you need to buy a 45 and 42 PJ and an adjustable PAJ called the Pilot Air Screw. as you come down on one you can balance it with the other. eventually you could go all the way to 38PJ.

i think the hesitation is the sheer richness of the mixture making a flame path all but impossible.

you can discover more by looking at the thread "jetting Q's" which is still a current topic.


[This message has been edited by Taffy (edited October 14, 2001).]

Taffy. I found that with this needle at #7 at constant revs eg. on a transport section, there was a surging sensation. The rest of the time the bike really ripped. The fuel consumption was a bit of a worry though. Is this just a trade off for extra mid/top end?

I have not had the chance to try the smaller PJ yet for the hesitation problem.

Thank you.


jim bob

i had this sensation on my first dyas testing in the spring. unfortunately i moved on to another project at the time and my notes weren't as copious then as they are now.

i did the test to get snap wheelies and not as the ultimate running guide.

if i remember i dropped the needle a clip and lowerd the main jet one.

i moved on to the EKP needle and it was all forgotten.


I went to a 45PJ yesterday and played with the pilot screw from 1 to 3 turns but it made no difference. I have not played with the needle since going to the smaller PJ.

Is this really a problem or am I just too heavy handed with the throttle? Does anyone else have a similar effect when they crack the throttle wide open from low revs?


My hesitation problem has disappeared. I pulled the carby apart and gave it a really good clean, checked all the hoses and joins etc. Now running 172main, 48PJ, DXM #6 1.5 turns on pilot screw. The bike really rips and ran neck and neck with a hot 520 last ride. But..... Can anyone tell me why I feel a miss at constant 1/4 throttle. ie.cruising speeds? I thought it might be a bit lean but the plug is not white. Any thoughts?

Thank you


P.S. EKN is the next job.

Just for a comparison, the '99 US model WR400 had the equivalent of DXM#5 (DTM#3) with #45 pilot and ~2 1/4 turns. Same pilot air jet #75.

At steady 1/4 throttle your bike still might be a little rich in #6, but snapping wheelies would be better than going to #5.

EKN#3 or #4 will add that back and more. wink.gif

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