Jetting after APJ mod !!HELP!!

24 replies to this topic
  • Ynahg

Posted January 10, 2002 - 11:00 PM


Last week I rebuilt the the engine, new piston rings, new distribution chain, valve clearance etc...
I also modified the AP to 0.75 s squirt. Before the mod my settings where : MJ 170 PJ 45 PS 2 turns DXM #4 (DXM #4 = DTM #3).
First tried with the same settings after the mod, but the engine dies when I touch the throttle. :) Raised the needle to clip 5 to compensate the APJ mod, now I have no bog from idle to 1/8 but a big bluuurp from 1/8 to 1/4 or even 1/2. Also tried a 175 at #3 end #4 but dint seem to work ether. I swear I'm trying to stay cool and try to understand what the **** is happening, too rich too lean, but after having dismounted, changed settings etc... for 10 or 15 times i'm loosing my patience. :D
So any input would be appreciated.
Thanks :D

  • James_Dean

Posted January 11, 2002 - 07:46 AM


The DXM#4=DVM#3=DTM#2.

I don't think you are too rich on the needle yet. The '99 US WR came with DTM#3 or even DTM#4 (DXM#5-6), #45 pilot, 2 1/4 turns, #168 main. Most riders considered the #168 too lean as the YZ used a #175 with open exhaust. The KTM 400/520EXC have #175 mains also.

Try clip DXM#6 , #45, 2 1/4 turns, #175 main.
If the top end is rich, go back to #170, bottom end- adjust the fuel screw.

Hope you are successful,


  • Guy

Posted January 11, 2002 - 08:13 AM


:) However by the same token your climate is the same as mine....europe/UK and I found a 170 toooo rich and a 172 blo*dy daft! I run a 168 and I am considering going SMALLER to see what effect it has!,
My symptons were identical to yours with the bigger jets in and the 168 sorted it!

  • James_Dean

Posted January 11, 2002 - 11:16 AM


Thanks for the added help Guy. Your exhausts are probably the same too. What needle and clip position do you use? (OBDXM?)

  • Guy

Posted January 11, 2002 - 11:56 AM


:) I'm running a DXM clip #6, tried clip#4 and a 170 it didn't like it so 168 DXM clip#6 :D

  • Ynahg

Posted January 13, 2002 - 11:29 PM


I tried MJ 170 #6 yesterday, it runs better. I will try the 175 for more top end. The only symtom that still bothers me is that the engine needs lots of time to come back to idle, it 's kind of hanging at maybe 2000 or 2500 rpm.

BTW Guy, it's strange you use a 168, this was my stock setting and there was really a lack of top end. My dealer also says the stock jetting is often too lean.

Anyway thanks for your input guys.

[ January 14, 2002: Message edited by: Ynahg ]

[ January 14, 2002: Message edited by: Ynahg ]

  • Taffy

Posted January 14, 2002 - 12:09 AM


agree with guy

168 or 170 max MJ
this will allow your lifted needle to work at those low revs and the MJ will stop the richness that sounds like it's killing your mid range.

don't go back to 175 if you can help it. a smaller MAJ woulkd help your top end.

have you got a #75 or a #100 PAJ fitted?


  • Ynahg

Posted January 14, 2002 - 02:23 AM


The PAJ is stock, i think it's a 100. I'll try the 175 MJ to be sure but the the 170 indeed seems to be the best compromise.
About the idle hanging at higher revs.... any idea what this could be. It sounds like a lean spot (like when you run out of gas ?). I have to wait for at least 15 sec till it comes back to idle.

  • Taffy

Posted January 14, 2002 - 05:09 AM


the #75PAJ would cure that straight away. yes you do have lean jetting. some say up the PJ but i think you know by now what i think.

i think you've mistaken your needle for the top end MJ.

the MJ is flat out in fifth
the needle is when you whack it in fourth from, say 45mph etc.

your needle is down two clips on guy or me. the reason it's not lean is because the MJ is chucking fuel in. so;

175MJ and DXMc4 (ok mid, rich top)
168MJ and DXMc6 (ok mid, ok top)

as i've said before the MJ controls the needle completely.


  • Guy

Posted January 14, 2002 - 08:35 AM


[QUOTE]Originally posted by Ynahg:

BTW Guy, it's strange you use a 168, this was my stock setting and there was really a lack of top end. My dealer also says the stock jetting is often too lean.

Anyway thanks for your input guys.

[ January 14, 2002: Message edited by: Ynahg ]
with a 168 on clip #4 the top was a little "breathless" but I'm running clip#6 or even #7 which loosened up the top without flooding the bottom!. A 170 was crap,everywhere!
just try it...168 clip#6, 10 mins work and see what you think, you have nothing to loose :)

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  • Taffy

Posted January 14, 2002 - 12:06 PM


guy i'm sending you a #170 MAJ. you have no choice but to trey it and that's an order!

it'll increase your top end by five mph


  • Guy

Posted January 14, 2002 - 01:32 PM


:) OK Taffy, I daren't argue with ya! :D I'll give it a whirl and report my findings on the usual Bat channel! LOL :D I checked, and I'm running the stock #200 MAJ at the mo, You've been "cock on" with everything else :D :D

  • Guy

Posted January 14, 2002 - 01:58 PM


:) Taffy, I'm going to drop the DMM in at clip#4 to start with, does this sound somewhere near my DXM #6? or do I need to be lower...higher... lower, higher hmmnn, I feel that could be a game show somehow! :D

  • Taffy

Posted January 14, 2002 - 04:27 PM



the middle suffix is what you go by and there are two letters to each clip change, ok?

so there are ten letters between M and X ok? and that is 5 clip positions. BTW we don't count the letter 'O'.
DXMc6 is the same as DMMc1
so start on the minimum position. clip 1
then try the 170MAJ
then come down ten on the MJ and then you can go up 2 or 3 clips on the needle

don't panic, as long as you take it nice and steady each change will be an improvement and a step in the right direction. look at the missile!

you won't lack for motivation whilst it's going well. just don't try it at 3pm in the afternoon and expect to have valid result in an hour.


  • Ynahg

Posted January 15, 2002 - 12:06 AM


Ok ok I'll try Mj 168 DXM#6 this evening. Just afraid the top end will suffer. Concerning the richer (smaller) MAJ, will this only help the top end or could this cure the hanginhg idle thing ?
I keep the faith :)

  • Taffy

Posted January 15, 2002 - 12:48 AM


they are seperate. the MAJ helps get the the curve of fuel at the top end right. the PAJ is there for the bottom end.

at the risk of saying it again and again, you need the amount of fuel to tail off when you get past max torque. if it bleeds lots of air in through a #200MAJ that means the curve of fuel dives down.

with the largest jet-the #200-it dives down the most. now on a piece of paper draw a curve (#1)gently up and then slightly down so that the peak is 3/4 the way along. this is the curve you want.

now starting from the furthest point to the right at the bottom of the "drop-off" you draw (#2) steeply up and then down to the "drop-off" on the left hand end.

why? well your jetting is tested by you for "top speed" and therefore jetted for max revs so it's only right your two lines should join here and it's only right they should start together at the left where the jetting is unaffected,ok?

so you should have one low curve and one high curve with their peaks 3/4 along.

now the big hump in the middle is the fuel you're chucking in at max torque which is 7-9,000 revs to you. it's pouring it in and it dulls the throttle.

SO WHAT YOU DO IS PUT IN A #160MAJ AND THIS BLEEDS IN LESS AIR AT THE TOP. SO NOW YOU GET MORE FUEL. so now i want you to draw a third line (#3). this one follows the upper path to the top of the hump and then stays high exactly the same distance above the lower line and therefore for this purpose looks like a railway track with these two lines (# 1 & 3). ok with that?

what does it mean? well you have been getting too much fuel at max torque and now your getting too much at the red line with the #160MAJ as well and it's rich at max torque and rich at max revs.

so what the phuq i hear you say "what did i do this for?" well what you do now is come down 10-15 on the MJ and the upper line drops neatly onto the lower line and the bike goes like PHUQ.

now i could add more and fry your brain but just try and take that in first ok!

on another subject, definately change the PAJ to #75. but ynahg you're starting to sound like our cousins over there so don't talk-WALK!!!!


  • MN_Kevin

Posted January 15, 2002 - 07:59 AM


Sounds like you should "RoadTrip" to Belgium! Maybe meet Guy, Missle and Ynahg and have some brews and rides Ynahg's bike!

  • Guy

Posted January 15, 2002 - 08:58 AM


:) Ah have seeen the light...... That explains a lot...thanks Taffy, I feel a trip to my Yam shop to order a "Boat load "more jets..he he!
Taff, I'll let you know how I get on, cheers m8 :D

  • Guy

Posted January 19, 2002 - 08:26 AM


:) Ynahg, Have yo tried that 168 clip#4 yet, if so how did it go??? :D

  • Ynahg

Posted January 21, 2002 - 06:21 AM


I tried 168 #6. It works quiete fine. But still tends to bog after 1/8. Will try 168 #5 and #4. Ordered a 65 SJ and 100 PAJ (to de-octopus the thing) and a 160 MAJ for the top end.
Guy do you still have a stock exhaust ? (Mine is a YZ exhaust with home made spark arrestor for the noise)


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