Sea level jetting for a '99 with YZ timing?

Thanks, Taffy, for the jetting advice-problem solved! I rode the bike about 100 miles today and it ran better than ever. Fuel economy went up by about 2 mpg, bottom- and mid-range power are phenomenal compared to before, it starts even easier than it used to, and no more stalling when I chop the throttle coming into a turn. Yes!

Also, today was a very warm day and a lot of the riding was on slow, tight singletrack. No problems at all with overheating (so it's not too lean.)

I lowered the needle 1 clip to #4 and that made a big difference. I'm still waiting for the 160 MAJ, and I think I'm going to play around a little just to see what the best clip position and main jet is, but it's VERY close right now with the 162 and EMM-4.

So here's my jetting as it stands:

162 MJ

200 MAJ (soon to be 160)

35 PJ

55 PA(J) Screw (3/8 turn out)

60 SJ

EMM clip 4

PS 1-1/2 turns out

tiny squirt of the APJ

[ March 10, 2002: Message edited by: Rich in Orlando ]

Originally posted by Rich in Orlando:

So here's my jetting as it stands:

162 MJ

200 MAJ (soon to be 160)

35 PJ

55 PA(J) Screw (3/8 turn out)

60 SJ

EMM clip 4

PS 1-1/2 turns out

tiny squirt of the APJ

[ March 10, 2002: Message edited by: Rich in Orlando ]

Could you post what was stock?

Stock jetting for a '99 WR is:

168 MJ

200 MAJ

45 PJ

75 PAJ

52 SJ

DTM clip 4

PS 1-1/4 turns out

3-4 sec. APJ squirt


it's going to get better!

if you fit that #160MAJ go to 158/160MJ and either keep the needle as is or lift it one.

BTW. YZcourt/yamakazi the SJ was a #62 not a #52 on rich's bike (i hope!).


[ March 10, 2002: Message edited by: Taffy ]

I was guessing it was a 52 SJ because mine came with a 60 and I didn't think it was stock. So, I looked up the carb parts diagram at and it lists the standard SJ for a '99 to be a 52. They/I could be wrong. There's always a first time. :)

this one's for bill.

bill your PM is fullapparently but i know that you'll read this.

i think if you say that it's an FCR carb that should do. the needle is code;N427-OC EMM. the rest they should find one hopes.


:) Stock SJ for my 99 was a #62 not a #52 :D

im at sea level right now and rode for a couple hours yesterday it was sunny temp around 75*.i have 98wr420[yz timed] my bike was running a little on rich side.with 180mj,dvp4thclip,48pj,pilotscrew 1 1\2 out.

The 160 MAJ arrived Wednesday and I replaced the stock 200 MAJ. Along with that, I replaced the 162 MJ with the 160. It is PERFECT!. Throttle response is instant and strong throughout the entire throttle range. And when it comes onto the needle--look out! Just try to keep the front wheel down! :) That's how this beast is supposed to run.

Now, I just have to be a little careful--it got away from me a couple of weeks ago when I was hammering the throttle coming out of a turn. And that was before this latest mod.

Taffy, couldn't have done it without you (and this website). Thanks again!

Could I get the part # for the 160 MAJ.

I am having problems leaning out the top end. Will that help?

I don't know the part number--I just told the guy at Sudco what kind of carb I have and that I needed a 160 MAJ and they sent it to me.

I don't know exactly what the different air jets do to the air/fuel mixture but I think that basically, you have to match the right size jet to the right size air jet. For example, I think that's why it was important to go to the pilot air screw, so it could be adjusted according to the pilot jet. I went down to a 35 PJ and set the PA screw equal to a 55 (from a stock 45PJ/75PAJ). When I still had a 200 MAJ, I had to run a slightly bigger MJ so it would run right. Now that I have the 160 MAJ, I went down to a 160 MJ. I think that MAJ will work for the leaner range of MJ (150-160) that I am now running with.


that's great! speaking personally i think that the balance has now tipped toward these settings and i i think everyone will be doing it now.

the first bloke to do it was eccentric

the next couple were brave

the next few have read the script

now anyone and everyone should do this.

as far as the air jet thingy's go, well it's like this.

the PAJ is what lifts the fuel at low revs, but we only want a small amount of fuel so we only want a small amount of air. the mixed ratio is then controlled by the PS.

with the MJ it's totally different. the air goes through the carb throat and sucks up fuel. we go on MJ at-i dunno but i'd guess- 8,000 revs and the bike hits max power at say 9,000.

from then on power tails off a little but the revs go up and so we suck even more fuel in but no more air. we create an over rich situation.

so we have to bleed in air to lean it off. the question is how much?

what's been happening is that yamaha decided to go for the maximum size of #200, that leans it off a lot.

because you, like me would normally jet the bike for absolute top speed, we were all chucking in large MJ's to fight the MAJ!!

trouble is at max power it was over rich as well but it's not so easy to feel.

so we have to juggle things. find jetting Q's and you'll see that all in one day i went from 235 down to 180 MAJ and it improved the top speed. THEN i brought the MJ down and improved max power!

BTW, nobody has done the definitive test on those MAJ figures. it could be that a 155 or 165 MAJ could be better.

finally rich, if your still here (ZZZZZZ!) that PAJ may be better at #45 so try it sometime. you'll notice it at about 10-15 mph when you roll on the throttle. if it's lean the bike will "vibrate". #45 will cure this.

good luck!


[ March 18, 2002: Message edited by: Taffy ]

Originally posted by Taffy:

... one day i went from 235 down to 180 MAJ and it improved the top speed. THEN i brought the MJ down and improved max power!

Funny you should mention top speed and power because today I experienced an improvement in both aspects. We were on a long straight gas pipeline easement and we got into a little bit of high-speed drag racing, so to speak, and the little 400 surprised me. A friend of mine with an XR650R likes to ride fast, desert style riding. We were up front and opened up the throttle. We were very close most of the time then one of us would back off and the other would then shoot foreward. In the past he has always beaten me handily but not today. At one point, he and I both had the throttles pinned and for a good ways, I was in front and actually pulled away from him. This was a first! The bike was putting out so much power that the clutch actually started to slip as the bike neared top speed. (My clutch is on its last leg-I had hoped the new friction plates that I ordered from Dennis Kirk would have come in before today.) The clutch hadn't been slipping all day except when it was at max power. So, both top speed and power have been increased as a result of the new jetting.

One more thing: it was hot as hell today and no problems with overheating at all. No boiling or any bad behavior. :)


what gearing do you run.

i ran 15/48 for open country and could have gone to 15/46 to help the road work between trails. the secret i guess is what will it comfortably lug down to!


So Rich... no question the YZ timing with

correct jetting is an impressive improvement.

But is it the type of throttle/rev response

you'd choose for the tight singletrack?

Sometimes I think I get on the gas earlier

with a tamer machine. ?

I ask because I think a good majority of

guys seem to be riding more open areas.

Today I spoke with the friend with the XR650 about yesterday's exploits. He has a GPS and told me he hit 99 mph. He estimates we were going around 90 when I finally backed off during the drag racing.

Taffy, I am using stock gearing- 14/50 and am very happy with it. I don't plan to ride at 90 very often. I had been running with a 49T rear but I really like the snappiness with the 50T. Although my bike is street-legal and plated, we don't get out on the highways very often. (I've got 15/46 gearing sitting in the garage just in case. Last year I did a weekend of street riding on the WR with Avon Gripster tires. That was a hooligan bike if there ever was one!)

Bender, I think the YZ timing is no problem at all in the woods. We ride some seriously tight trails and I've found that I can pop the front tire up and rotate on the back wheel around obstacles a lot easier with the YZ timing. Also, the low end hit is so much fun that I wouldn't want to go back to WR timing.

[ March 18, 2002: Message edited by: Rich in Orlando ]

overall im happy with how my 98yz runs

there is a very slight hesitation off bottom

ocasionally it stalls when i drop from high revs

it starts fine.

and it over heats now and then in snotty enduros

it runs a white brothers e-series exhuast with 12 discs

im no carb genius, and to be honest id be a bit scared experimenting with jetting i recall storys of blown two stroke engines cus of lean jetting. reading this post feels like im learning latin

so my question is,

is there now an agreed 'carb solution'

i guess im after the easy low hassle fix with minimum down time

also taffy where did you buy the jets needle etc?



i think its called a leap of faith :)

go to allens for your bits although don't ask for mr allen coz he died this month aged just 49. god bless him he was a really nice bloke.

anyway his men will help and his women is keeping it all going. it's the knowledge that's gonethough isn't it :):D

the point is mark are you open pipe or euro std?

are yz timing or wr?

what year are you again?


yeah i read about the guy dying

infact i saw him not so long ago racing supermoto.

the bike im using is a 98 YZF 400

obviously YZF timed

The pipes a White brothers E-series with 12 discs

it doesnt pop or bang on the over run although it did with the standard YZF pipe.

the thing feels pretty fast,,, but i just dont have anything to compare it with.

i should hual it down to you so you can see how it feals

[no doubt cheap and nasty next to the husaberg] :)

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